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Registration's totally free, of course, and makes snowHeads easier to use and to understand, gives better searching, filtering etc. When you register, you get our free weekly -ish snow report by email. It's rather good and not made up by tourist offices or people that love the tourist office and want to marry it either We don't share your email address with anyone and we never send out any of those cheesy 'message from our partners' emails either. Anyway, snowHeads really is MUCH better when you're logged in - not least because you get to post your own messages complaining about things that annoy you like perhaps this banner which, incidentally, disappears when you log in Username:- Password:. Or: Register to be a proper snow-head, all official-like! Prev topic :: Next topic. Poster: A snowHead. Grateful some advice for a beginner boarder. It seems standard practice to ride chairlifts with only the front foot in the binding, but if I have room, I generally try to get my second foot in either when waiting for the chair, or on the lift Flow bindings help. The only time I have ever felt that a snowboarding fall injured me as opposed to just hurting like hell was when I fell getting off a chair lift, with just my front left foot in, and the board really dug in the snow and put far too much leverage on my knee. I have fallen other times without the same result - but all told, I feel that my legs are much less vulnerable when I have two feet in. What do more experienced riders recommend? I have no problem clipping up the second binding waiting for a button lift - but should I bite the bullet and learn to ride better with just the one foot in? Obviously A snowHead isn't a real person. I'm not very experienced, but when I've had lessons the instructors say it's better to have just one foot in. Not entriely sure why, but I think you should get used to doing it. If you do have a spill at the top it's quicker to get out of the way if you've only got one boot in. Well, the person's real but it's just a made up name, see? You should indeed bite the bullet and learn one foot - it gets easier quite quickly, and soon you will be at the point where you never fall getting off the lift. This is a useful skill to have, because often on flats you need to unstrap a foot and get around. Furthermore, you may find that after getting off certain lifts, if you are strapped in you will get stuck on a flat, have to unstrap, get to the top of the hill, and then strap in again. An uneccessary pain in the ass, even with flows. Also the being able to get out of the way if you have a spill at the top is a good thing too. Side note - tips for riding with one foot: Begginers often put too much weight on their back foot. While you can get away with this on easy slopes with both feet strapped in, in any other situation this is terrible for your riding. Try putting more weight on your front foot when getting off the lift, and at all other times for that matter. Also, when getting off the lift, stomp the board down and stand up hard and fast. Once you are on the board it is easy to ride away, but you can easily put yourself off balance if you try to get off the lift too tentatively and slowly. You need to Login to know who's really who. Similar question, but regarding drag lifts I've seen people with one foot loose, others with both in their bindings Any recommendations? Anyway, snowHeads is much more fun if you do. Quite a few resorts now insist that boarders ride on drag lifts with the back foot out of the bindings. The reasoning is so that you can get out of the way if you fall off of the drag. I found drags knackering when I first tried them on a board, not to mention the cut off circulation on lond drags, but you soon toughen up and get used to it. At many resorts it opens up a lot more options for you if you can comfotably do drag lifts on a board, so get on those pomas and tough it out! You'll need to Register first of course. I refuse to use any kind of lift that does not invlove some sort of chair. Then you can post your own questions or snow reports In my view you are a potential liability to others if you ride a chairlift with both feet clipped in because you have no means of propelling yourself, and it can be difficult to maintain your balance if you have no momentum. If someone cuts across your path while getting off a chair and you have both feet strapped in you will have no option - you will have to fall over or take them out or both. With one foot loose you can scramble out of the way and then shout your abuse at them safe in the knowledge that you are entirely blameless. I sometimes ride drags with both feet in but usually i still prefer to have my back foot loose just incase i do happen to get out of sorts on the way up. As ponder says, it gets easier with practice. Chairs will soon become no problem, although drags will always inspire a certain element of uncertainty. After all it is free. Thanks very much for the advice - will try to bite the bullet! I don't care about falling over, in the normal course of events, it was just that bad fall when the board dug in which made me wonder. Stomp down and stand up sounds exactly right - I will say that to myself just before we arrive! However, I'm not sure I agree about being able to get out of the way quicker if I fall off a drag with one foot out. I find that even being elderly, I can lift both legs and sling the board over pretty quickly with both feet in, if I'm scared enough of whoever is coming up behind. You'll get to see more forums and be part of the best ski club on the net. When you ride with only one foot slide your back foot so it is tight against the back binding. This will give you a lot more control also fit a stomp pad if you board does not have one.. Ski the Net with snowHeads. Always use one foot, a lot of resorts will not let you on with two feet anyway, but it is highly dangerous getting onto any lifts with two feet esp drag lifts. This is also a useful Technique when riding around on Flat Parts! I always keep both feet in the bindings. I know I am in the minority on this and many places do not like you doing it. I undertsand their point of view if you fall over cus you stuff the whole system up but if you are competant enough to stay upright it is the only way to go on both drags and chairs. This is because you are so much more in control, you can shoot straight off at the top, carving easily around the other riders sitting on their back bottoms making the whole place messy and tripping up skiers. If you know your resort, you can usually gide straight to the lift queue and onto the lift with minimum effort. And love to help out and answer questions and of course, read each other's snow reports. Just a quick tip for t-bars if you must take them, as you sometimes have to - face inward, with your strapped in foot uphill, the t-bar between your legs, and your hands holding on wherever is most comfortable. Stay loose. As for rope tows, I refuse to take them, as they wreck my gloves, costing me money. So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much. I have step-in bindings and can step-in on the lift itself but I find that once you get used to riding drag lifts, it is actually more comfortable to ride with the back foot free. You can have a narrower stance, standing more upright and get a bit of a breather. You know it makes sense. Otherwise you'll just go on seeing the one name:. Don't ride them alone would be my top tip, I find riding a t-bar with another boarder much easier than alone, this isn't really the case on skis. I can ride moderate distance and moderate speed with only one foot bound and the other weighting the board via the stomp plate just as I can ski on one ski over a moderate distance and moderate speed. Having that sort of balance, pressure and edge control are reasonably basic skills that help your technique all over the mountain anyway. Though I did get almost castrated by a young lady boarder who used me as a prop on the way up and refused to let go when we got to the bull wheel. Perhaps a large L strapped to my back maybe in order! Masque wrote: ise , Watching a pair of boarders getting on a T-bar is like witnessing two monkeys fight over a stick. Well, I recall being at Soelden a couple of years back in October with a guy who was working for me as an intern, he was from the US and hadn't been sliding in Europe, so I took him for some glacier stuff this is just the kind of guy I am. It was busy at the first lift so we stood queuing while a a local girl tried to chat him up Then, we arrived at the front with the usual 30secs to departure when he mentioned he'd never seen a T-bar before and I gave him the crash course, we rode up no problem at all and I was on a aboard as well that day. I reckon that proves my point really, he could balance and control a board he'd rented the night before onto a lift he'd never even seen before. It's precisely because I am all too aware of not yet being 'in control' that I asked the question in the first place. The standard ise describes might be a reasonably basic level of skill for him, but not for me, and apparently not for IncoqSkiSno either. But we will keep trying to reach that masterly level If it's a basic balance skill, you should be practising it And not just in the 10sec's before you get on the lift. I don't think I've ever been up a t-bar on the wrong i. IMHO the best t-bar partner is a shorter skier. Armed with a tame skier, I have been up a t-bar in a tiny ski lift in Rothenthurm that was so steep that the t bent alarmingly under our weight. Mind you boarding t-bars seems fairly tame now compared to skiing up one with 2 small children. Did he cop? IncogSkiSno , when you're learning to use a button or Tbar, it's best to start on your own, that way only one person has to clear the track before the lifty has to restart. You see lots of people leaning back and pulling against the tow. This just puts weight into the back unsecured foot and makes the whole plot unstable. Masque , Thanks - I will certainly give it a go at xmas; can't wait to try boarding on 'real snow'. Not sure if I'll be more of a danger to myself or to others! Pretty nice resort - will have to head back there one day. Pretty little resort though. New Topic Post Reply. Snow Snow Snow! Solo Skiers v Groups - Orga Archives Lost and Found Ski Club of Great Britain To one side secret Mountain Hideout snowShops You cannot post to forums until you login You cannot read some forums until you login Read about snow conditions : snow conditions And leave your own snow report : snow report Find advice to help plan your ski holidays : ski holidays The snowHeads Ski Club : Ski Club 2. Terms and conditions Privacy Policy. Snow Reports. After all it is free After all it is free. So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much. Otherwise you'll just go on seeing the one name: Otherwise you'll just go on seeing the one name:. Masque wrote:.

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A review of the fossil record coupled with insights gained from molecular and developmental biology reveal a series of body plan transformations that gave rise to the first land plants. The colonization of land involved increasing body size and associated cell specialization, including cells capable of hydraulic transport. The evolution of the life-cycle that characterizes all known land plant species involved a divergence in body plan phenotypes between the haploid and diploid generations, one adapted to facilitate sexual reproduction a free-water dependent gametophyte and another adapted to the dissemination of spores a more water-independent sporophyte. The amplification of this phenotypic divergence, combined with indeterminate growth in body size, resulted in a desiccation-adapted branched sporophyte with a cuticularized epidermis, stomates, and vascular tissues. If it is true that the present is a key to understanding the past Gitzendanner et al. The conceptual reciprocity obtained by studying organisms from the perspective of molecular biology and the perspective of the fossil record is critical to understanding evolution e. The goal of this paper is to apply this strategy to review the evolution of plant body plans using information drawn from the fossil record and molecular biology. Plants are broadly defined here to include any photosynthetic eukaryote, thereby including the polyphyletic algae Graham et al. However, we focus primarily on the monophyletic land plants i. Our goal is motivated by the fact that little has been written about the evolution of plant body plans, and the very little that has been written has focused largely on the vascular land plants e. This inattention is particularly striking because molecular phylogenetic analyses clearly indicate that the embryophytes are the direct descendants of one lineage within the Chlorophyta, justifying the inclusion of the green algae and the land plants in a monophyletic clade, the Viridiplantae Liu et al. In addition, molecular phylogenetic analyses of the Chlorophyta indicate that the unicellular body plan is the ancestral condition, as is likely for all very ancient lineages, and that multicellularity is a highly derived condition Fig. Consequently, any attempt to reconstruct the evolution of plant body plans requires a broad assessment of how multicellularity and the diverse traits characterizing the Viridiplantae emerged from something as seemingly simple as a unicellular photosynthetic cell. Redacted phylogenetic tree showing the relationships among the lineages within the Viridiplantae. This schematic also diagrams some of the shared features between the major lineages within the clade i. The latter include the charophytes and the embryophytes, embracing the evolution of a diplobiontic life cycle see insert at lower right. Some of the relationships are problematic, as indicated by broken lines. For example, some molecular analyses place the Zygnematophyceae closer to the land plants the embryophytes than either the Charophyceae or the Coleochaetophyceae adapted from Lewis and McCourt, The following sections 1 present the developmental processes required for a unicellular-to-multicellular transition, 2 consider a scenario for the transition from simple to complex multicellularity, and 3 review the evolution of the land plants based on the fossil record and molecular data in light of the information arising from 1 and 2. A second theme is the phenotypic convergence on axiation seen between the two multicellular generations in the land plant life cycle, i. A main thesis is that axiation was a central adaptation, starting with the first elongate cells and culminating in some of the largest multicellular autotrophs such as kelps and sequoias. Setting aside the similarities among unicellular or colonial species, which are arguably trivial owing to their simple morphologies, extensive phenotypic diversification and convergence are evident within the Viridiplantae. Consequently, it is often impossible to distinguish between species drawn from different lineages within the same clade based on their general appearance, size, or internal structure Bold, ; Bierhorst, ; Bold and Wynne, ; Graham and Wilcox, ; Graham et al. Indeed, the degree to which internal structure i. The perspective taken here is that body plans are more profitably discussed in terms of how they are achieved developmentally rather than discussing them in terms of their resulting phenotypes, given the extensive morphological and anatomical convergence and divergence among the various plant lineages e. This approach identifies four body plans distinguished on the basis of a few simple developmental motifs: the unicellular, colonial, coenocytic, and multicellular body plans Fig. Each body plan requires the interaction of four processes Niklas, : 1 the presence or absence of synchronous cyto- and karyokinesis, which determines whether the body plan consists of uni- or multinucleate cells e. The multicellular body plan in this scheme has three variants based on the planes of cell division with respect to the body axis Fig. A schematic of the developmental motifs that result in four plant body plans i. The unicellular body plan is achieved by the separation of cell division products after cytokinesis. The colonial body plan is a collection of uni- or multinucleate cells aggregated by extracellular adhesives but lacking intercellular continuity among cells. The unicellular and colonial body plans are determinate in cell size, although the overall size of a colony may increase by the addition of cells. The coenocyte body plan is indeterminate in its growth in size e. The multicellular body plan consists of uni- or multinucleate cells that maintain intercellular continuity after cytokinesis e. The scheme shown in Fig. However, land plants have a dimorphic life cycle in which haploid and diploid generations differ in size, morphology, or anatomy. Consequently, Fig. For example, unicellular, colonial, coenocyte, and multicellular body plans occur in the Chlorophyta and the golden-brown algae, the Chrysophyta. Likewise, the three variants of the multicellular body plan i. However, unlike the charophycean algae, which have unicellular and colonial representative species e. Likewise, the coenocytic body plan is expressed transiently among some land plants e. The ability of cells to adhere is necessary but not sufficient to achieve multicellularity. Adhesion occurs among all unicellular and colonial species, whereas the control of the planes of cell division is required to achieve an organized multicellular body plan. Multicellularity requires a systemic spatial reference system SSRS. This appears to be ancient among prokaryotes. Precambrian filamentous prokaryotes exhibit simple multicellularity Schopf and Barghoorn, ; Knoll, Complex multicellularity appears to have evolved only within the eukaryotes. The distinction between simple and complex multicellularity may seem trivial Fig. However, when viewed through the lens of physiology, the distinction becomes important. It increases with increasing cell volume and decreases with increasing cell surface area. The same holds true for an amorphous mass of randomly dividing cells. With continued cellular proliferation, passive diffusion becomes increasingly unable to meet the metabolic demands of cells progressively deeper within the aggregate for nutrients available only from the external environment, and conversely, for the expulsion of potentially toxic metabolites into the external environment. Schematic of how simple and complex multicellularity may have evolved. The proximal condition is assumed to have had a colonial body plan, a descendant of a unicellular ancestor. In the absence of an organismic reference system, the colonial body plan lacks the capacity to organize systemic planes of cell divisions shown to the left. By evolving an organismic reference system possibly by means of cell-to-cell cytoplasmic connections , a colonial body plan can evolve simple 1-D and 2-D and complex 3-D multicellularity. There are different adaptive solutions to the metabolic and physical constraints imposed by passive diffusion. For example, aggregated cells can fragment once a colony reaches a critical size, possibly fostered by the death of internalized starved cells e. Cells could also achieve greater control over the orientation of their planes of division to achieve simple multicellularity Fig. Prokaryotes have gone down both pathways. Some have followed what is arguably the simpler path and produced amorphous cell masses e. Other prokaryotes produce filaments of cells e. The fragmentation of colonies can be advantageous for species living in hydrodynamically active environments. Fragmentation provides a simple method for long-distance dispersal and colonization. Simple multicellularity provides the same potential while simultaneously bypassing the constraints of passive diffusion. Cellular congeries and filaments are preserved as Precambrian microfossils estimated to be 3. A simple scenario provides a model for how complex multicellularity evolved Fig. It begins with prokaryotes producing extracellular adhesives but being incapable of systemically controlling the orientations of their planes of division. These organisms subsequently evolved and diverged in their ability to control and orient cell division. Some failed to do so, whereas others achieved the capacity to produce unbranched or branched filaments denoted as 1—D and 2—D, respectively , thereby achieving simple multicellularity. How this integration of cell division was achieved remains problematic, although it is noteworthy that physical contact can serve as a cue between adjoining cells for aligning the plane of division. Likewise, diffusion of a metabolite between cells can create a reaction—diffusion R—D morphogenic system e. Heterocyst formation in Anabaena and Nostoc filaments is an example. Heterocysts produce a diffusible inhibitor that prevents heterocyst formation unless its concentration falls below a specific threshold. As the distance between two heterocysts increases due to the division of intervening vegetative cells, the first undifferentiated cell to be triggered by the drop in the concentration gradient develops into a new heterocyst. This newly formed heterocyst releases the inhibitor and reiterates the process. Although this example deals with the morphogenesis of a specialized cell i. Interestingly, the mechanism achieving the heterocyst involves diffusion through intercellular cytoplasmic connections sometimes called micro-plasmodesmata. These structures facilitate intercellular coordination. They also help to establish or at the very least contribute to body plan polarity Niklas et al. Importantly, these intercellular connections have analogs in eukaryotic multicellular organisms, as, for example, the gap and tight junctions in metazoans, plasmodesmata in land plants, and septal pores in filamentous fungi. The convergent evolution of diverse modes of intercellular connections in disparate lineages and clades indicates the profound importance of cell-to-cell communication and coordination. Importantly, the systemic control of planes of cell division need not reflect the immediate consequences of natural selection. Evolutionary innovations can also arise via phenotypic plasticity or from the inherent physical properties of tissues and condition-dependent developmental systems. For example, phenotypic plasticity may help to explain why the cells within colonial life forms can achieve different metastable functionalities reflecting different phases in the life cycle of unicellular ancestors that presage the appearance of complex multicellularity in some lineages, e. Divergence in cell function among genetically identical cells contributes to fitness, particularly when some functions cannot be performed simultaneously e. Likewise, the colonial body plan has advantages over the unicellular body plan, e. Importantly, organisms can evolve along different pathways even within closely related groups of organisms. Consider the volvocine algae, in which multicellularity likely evolved by differential modifications of cell wall layers in a unicellular Chlamydomonas -like progenitor Kirk, , Specifically, the walls of unicellular Chlamydomonas are composed primarily of hydroxyproline-rich glycoproteins and are separated into a structured outer layer and a more amorphous inner layer. Among the colonial volvocines e. Yet, among multicellular volvocine algae e. High degrees of similarity in protein sequences among unicellular and multicellular volvocines support this scenario; e. In contrast, cell adhesion in the evolutionarily related land plants involves a middle lamella enriched with pectins, a functionally and structurally diverse class of galacturonic acid-rich polysaccharides. Molecular phylogenetic analyses identify the charophycean algae as the sister group to the land plants Fig. The three candidates are the Charophyceae, Coleochaetophyceae, and Zygnematophyceae, all of which have an unbranched or branched filamentous body plan and a life cycle in which the only multicellular generation is haploid. Therefore, the multicellular land plant sporophyte generation is presumed to be a derived condition, as is parenchymatous tissue construction. One scenario for the evolution of the land plant sporophyte is the co-option of the gametophyte developmental program to produce a diploid generation with comparable developmental capacities. Delaying zygotic meiosis can confer an immediate benefit——the amplification of reproductive output——because a zygote can yield four spores following meiosis and thus only four new gametophytes. However, if a zygote delays meiosis and divides mitotically only once, each derivative cell can divide meiotically, resulting in eight, not four, haploid cells that can develop into gametophytes. Thus, delaying meiosis confers a significant reproductive advantage. If the co-option scenario is accepted, it is reasonable to assume that the most ancient land plant life-cycle was isomorphic, i. Based on extant multicellular charophycean algae, the body plan of these sporophytes would have been an unbranched or branched filament. It is noteworthy that a filament consisting of cells sharing the same general shape and size has many physiological advantages. Consider, for example, the relationship between cell surface area S which affects the ability of a cell to exchange mass and energy with its external environment and volume V which serves as a proxy for the metabolic demands of a cell. Delayed zygotic meiosis a priori requires mitotic cell divisions within a multicellular body plan. Hypothetically, new cells could be added at a variety of locations e. However, although excellent in an aquatic environment, a filamentous body plan is incompatible with a terrestrial existence because of its high surface area with respect to volume, which makes it susceptible to dehydration. This incompatibility can be resolved by a simple-to-complex multicellular transition, which could have been achieved with the evolution of a parenchymatous tissue construction. As shown in Fig. A multicellular cylindrical geometry is particularly adaptable to physiological and mechanical demands. As noted, its surface area to volume ratio can be adjusted. In addition, the cells located at the center of each transection are equidistant from the perimeter of each section, which gives them equal access to external resources e. Likewise, if some centrally located cells are specialized for hydraulic conduction, they can deliver, remove, or receive materials from all other cells with equal efficiency e. Mechanical advantages are equally evident. Centrally located cells experience little or no tensile or compressive bending stresses induced by laterally moving water or wind relative to peripheral cells. A cylinder fixed at one end and free at the other has the potential to resist gravity by virtue of a high flexural stiffness while retaining the ability to twist or bend if laterally loaded e. Likewise, a flexible buoyant cylindrical axis can deflect and twist to resist drag see Koehl, All the aforementioned advantages also occur in a branched cylindrical architecture, a phenotype easily achieved if the meristems at the growing free ends of cylinders multiple. Indeed, the most ancient known land plant sporophytes are composed of branched cylindrical sometimes tapering axes bearing terminal or lateral sporangia, e. Nor is it surprising that an unbranched or branched cylindrical architecture is seen in the growth habits of the vast majority of extant vascular land plants e. The principal attribute of the cylinder is its ability to establish polarity in a simple and economical way. Polarity is essential on land because gravity pulls down, wind pushes sideways, and often in a preferred direction, light generally comes from above and water and nutrients come from below. In addition, a vertical cylinder can elevate reproductive organs above ground, thereby facilitating the capture and release of gametes and propagules in the air stream. The de novo evolution of a diploid sporophyte raises the issue of the effects of ploidy level on plant functional traits and morphology. Indeed, the first sporophytes can be thought of as the first autopolyploids. Polyploidy is reported to decouple variation among functional quantitative traits and is hypothesized to provide an evolutionary advantage in some lineages. In addition, polyploidy is reported to affect bryophyte mating systems and the associated evolution and maintenance of reproductive traits Jesson et al. Each of these effects may have contributed to the evolution of sporophyte novelties. However, the effects of polyploidy, even among closely related species, can differ in a species-dependent manner e. In addition, inferences about ploidy levels in fossil materials are problematic. The role played by polyploidy during the early evolution of the land plants therefore remains highly conjectural, albeit eminently worthy of future investigations. It is reasonable to argue that a cylindrical body plan provided advantages for the successful colonization of the land or, more accurately, the air. But this body plan was not sufficient to control the size and shape of a multicellular land plant involving a cylindrical axis e. The mechanism seen among extant land plants is phenotypically manifest in the form of meristems, which are here defined as any cell or group of cells to which one or more lineages of derivative cells can be traced. The meristems of most extant multicellular charophycean algae consist of apical, lateral, or intercalary cells that allow unbranched and branched filamentous body plans to increase in size in an organized often reiterative manner. By extrapolation, the first complex multicellular land plants are likely to have had similar meristematic capabilities Fig. Indeed, the fossil sporophytes of the most ancient known vascular plant consist of simple, branched axes, some of which bear either apical or lateral sporangia. The fact that the majority of these fossils have indeterminate growth in size as seen in the form of continued branching indicates that meristems had become specialized to produce vegetative and reproductive body parts. Indeed, mosses and liverworts are among the very few known land plants to have sporophytes with determinate growth as a result of producing terminal sporangia on unbranched axes. The sporophytes of liverworts, mosses, and hornworts elongate by virtue of an intercalary meristem, creating the seta in the case of the former two that elevates sporangia, thereby facilitating spore dispersal. Scenarios for the evolution of the first land plant sporophyte resulting from delayed zygotic meiosis. The first multicellular land plant sporophyte could have been nothing more than a sporangium s , i. Continued mitotic cellular divisions after the formation of spores in meristematic m regions would have produced additional vegetative tissues, increasing the size and elevation of the sporophyte structures to the right. Subsequent specialization of meristematic zones into apical, lateral, and intercalary meristems am, lm, and im, respectively; shown in the lower left would have led to a systemic control of vegetative growth and the location of sporangia. The evolution of dedicated apical meristems that continue to produce vegetative axes would have resulted in indeterminate vegetative growth and the production of an indeterminate number of either lateral or terminal sporangia observed in the fossil record diagrammed on the upper and lower right. Just as divergent genetic changes can give rise to convergent phenotypes e. A review of the literature shows that the meristematic ability to produce a branched sporophyte is not the same across all vascular plant lineages. For example, among non-seed vascular plants pteridophytes , at least three mechanisms for dichotomous branching have been reported. Bierhorst and others defined the dichotomy of an apex with a single apical cell as the establishment of two new apical cells by the division of a single apical cell, a phenomenon he observed in some filicalean ferns. In the second case, Roth reported that the apical cell of the aerial axes of Psilotum persists to become the apical cell of one of a pair of axes, while a new apical cell derived from a lineage of cells derived from the original apical cell becomes the apical meristem of the other axis. In the third mode, Gottlieb and Steeves observed that the main stem of the fern Pteridium ceased to grow and was replaced by the establishment of two new apical cells, a phenomenon that has also been reported for the apical meristems of the lycophyte Selaginella Jernstedt et al. A treatment of the genomic and physiological mechanisms responsible for the appearance and organization of meristems is well beyond the scope of this paper. Although the specification of the plane of cell division is a consequence, or at least a correlate, of mechanisms that rely on some form of cellular polarity, comparative analyses of diverse organisms with rigid cell walls i. POL can be evoked by internal cytoplasmic asymmetries and by external stimuli, e. Among multicellular organisms, orderly cell division typically takes place in one or more directions with respect to the body axis. Therefore, POL must establish different spatial reference systems among different plant lineages even if the mechanism responsible for POL is invariant at the cellular level. Across the Viridiplantae, IAA mobility is driven mainly by active transport into and out of cells, but auxin influx can also result from the passive diffusion of the protonated form of IAA across the plasma membrane. Finally, we note that the gene regulatory networks underlying the development of sporophyte axiation and branching have been the subject of intense scrutiny. However, the evolution of the genetic changes required to achieve the polysporangiate vascular plant remains an open question e. Wang et al. PIN-mediated polar auxin transport is conserved between moss sporophytes and Arabidopsis Fujita et al. Disruption of PIN function at low penetrance induces a branched phenotype in moss sporophytes Harrison, ; Harrison and Morris, The effects of PIN and PpTCP5 disruption on moss sporophyte development appear provocative until one realizes that they shed little direct light on the gene regulatory networks controlling cell competency and differentiation and meristematic responses. For example, auxin is a signaling molecule with manifold effects across all land plant lineages and the green algae Cook et al. Until such time that the gene regulatory networks underlying shoot development are dissected more completely, it is premature to speculate about the genetic basis for sporophyte axiation and branching, particularly early in the history of the land plants. Although the last common ancestor to the land plants may have been aquatic or semi-aquatic, the first multicellular plants exposed to the air experienced a number of challenges, such as coping with the biomechanical stresses produced by gravity, significant temperature fluctuations, higher light intensities particularly in the UV range , and potentially lethal water loss to the atmosphere. These challenges required metabolic adaptations from the ancestral condition, including the acquisition of structurally rigid cell walls, UV protection, and hydrophobic extracellular polymers that could impede the rapid loss of water molecules. Among extant land plants, these functional tasks are achieved primarily by four hydrophobic biopolymers sporopollenin, lignin, cutin, and suberin , all four of which are produced by a very few metabolic pathways, some of which serve as the foundation to other critical biosynthetic pathways, as illustrated by the shikimate pathway that provides the precursors to auxin, glucosinolates, tannins, suberin, lignin, and many other compounds Haslam, ; Hermann and Weaver, Importantly, the shikimate pathway is highly conserved across bacteria, fungi, algae, and land plants. For example, homologs of the phenylpropanoid biosynthetic pathway genes have been identified in the moss Physcomitrium patens formerly Physcomitrella patens Ye and Zhong, , whereas the Arabidopsis orthologues controlling the first enzymatic step in the shikimate pathway are found in Chlamydomonas and Volvox Tohge et al. Similarly, cytochrome P, which catalyzes the first irreversible step committed to the biosynthesis of monolignols in angiosperms, is involved with the synthesis of phenolic components in the cuticular membrane of Physcomitrium patens Renault et al. Further, both the capacity to produce sporopollenin, which protects spores and pollen from mechanical damage and desiccation, and the biosynthetic ability to produce lignin-like molecular moieties occur in green algae. For example, the cell walls of the zygote wall and placental-like transfer cells of the green alga Coleochaete contain sporopollenin and material similar to lignin, respectively Delwiche et al. These and other examples of polymers similar or identical to sporopollenin, lignin, cutin, and suberin in the green algae testify to the comparative ease with which ancient biosynthetic pathways could have been co-opted to produce polymers that structurally reinforce and chemically protect cell walls and tissues, thereby permitting the first land plants to live and reproduce in the air. This pre-existing biosynthetic versatility was paired with an increasingly specialized range of cell-types that evolved concomitantly with the diversification of indeterminate plant growth forms. Specifically, as terrestrial plants grew larger, passive diffusion could no longer accommodate the nutrient and water demands of living tissue, and new cell types and associated polymers were needed to address mass transport of fluids, the effects of desiccation, and the pull of gravity. Indeed, up to a limit, a statistically robust correlation between body size and the number of specialized cell-types across diverse animal and plant species has long been observed Fig. Although involving a wide range of cell-types, we focus here on one subset, the hydraulically specialized cells in the land plants, including the hydroids and leptoids seen in the sporophytic axes i. Therefore, for very small terrestrial plants e. Indeed, the water within hydroids can be thought of as an internal reservoir that can be drawn upon to stave off dehydration. However, for taller plants, the rate at which water passes from one cell into another is limiting, particularly when water must pass through cytoplasm. Hydroids and xylem cell-types are dead cells lacking cytoplasm. Because the storage and transport of water through these cells is expedited by the removal of cytoplasm during cell differentiation and maturation, their evolution required the capacity for controlled cell death i. Lignification rigidified cell walls in two ways. It provided a bulking agent that increases cell wall stiffness and hydrophobicity. Although not found in bryophytes e. Such lignified walls are largely impervious to the passage of water molecules, necessitating the presence of cell wall perforations pits permitting the passage of water molecules. A bivariate plot of the number of specialized cell-types identified in plants, animals, and fungi versus organism cell number as gauged by the number of cells per organism. However, it must not escape attention that xylem is only one of two vascular tissue systems. Therefore, it would be incorrect to taxonomically exclude an organism possessing phloem but lacking xylem from being a vascular plant. This caveat is important because it is possible that the evolution of phloem predated the evolution of xylem. Phloem can conduct large quantities of water as well as dissolved metabolites and nutrients. It is also potentially energetically advantageous to lose the capacity to form xylem, provided phloem is able to transport nutrients and water in sufficient quantities. In the absence of high evapotranspiration rates, a phloem-like tissue system functionally analogous to moss leptoids can provide the bulk transport of water as well as metabolites and nutrients in comparatively short plants. Such living transport cells would not involve the acquisition of the developmental controls allowing apoptosis, required by xylem. The fossil record makes testing this hypothesis difficult because phloem is a delicate tissue and typically evades preservation much more than xylem cell-types. Nevertheless, an illustrative example is provided by the Devonian plants called Rhynia major and R. Rhynia major was originally described as possessing xylem and phloem Kidston and Lang, However, a re-examination demonstrated that it lacked xylem, resulting in its removal from the genus Rhynia and assignation to the new genus Aglaophyton Edwards, Whether this absence of xylem reflects the pleiotropic loss of the ability to produce xylem or the appearance of phloem before xylem, describing Aglaophyton as a pre-vascular plant would be unwarranted if it possessed phloem. A similar argument can be made in the case of Horneophyton , which is purported to lack conventional tracheids Kenrick and Crane, This review of plant body plans concludes by considering branched variants within a simple morphospace based on three variables: 1 whether axes are determinate or indeterminate in growth; 2 whether individual axes branch; and 3 whether the system providing anchorage to vertical axes is rhizomatous or rhizomatous-like, or root or root-like Fig. These three variables yield 12 architectures grouped into two classes based on their mode of anchorage i. Each of the 12 architectures provides a scaffold to which vegetative or reproductive organs can be attached e. The morphospace does not consider variants of the geometric arrangement of vertical or horizontal axes e. In addition, the morphospace does not consider the presence or absence of secondary growth, which would be required to maintain the mechanical stability of vertical axes with indeterminate growth e. Nor does it purport to provide insights into evolutionary changes in ontogeny, many of which are carefully discussed by Stein and Boyer Schematics of a highly redacted morphospace depicting twelve hypothetical land plant architectures based on the permutations of only three variables: 1 the presence or absence of rhizomatous or rhizomatous-like, e. Simple dichotomous and overtopped branching patterns are subsumed in Figs. The presence of reproductive organs or of leaves or leaf-like structures e. Although all 12 permutations are theoretically possible, eight have terminal or lateral branches that are mechanically unstable unless axes have the capacity to produce secondary growth producing wood or wood-like tissues such as sclerenchyma i. Examples of extant and extinct taxa known to the authors are provided when possible in the text note: the absence of an example does not indicate that the architecture was not achieved during the evolution of plants. Despite its simplicity, the morphospace identifies branching patterns that mimic those observed among extinct and extant terrestrial plants as well as bryophytes and algae. For example, arguably the simplest architectures consist of a rhizome-like anchorage system bearing indeterminate or determinate unbranched or dichotomously vertical axes supported by rhizomes Figs. Likewise, Figs. Turning to the architectures anchored by roots or root-like systems Figs. This determinate growth model also occurs in some branched angiosperms Fig. Finally, the vast majority of angiosperm and gymnosperm trees and shrubs conform with Fig. Nevertheless, as in the case of many other morphospaces see Raup, ; Raup and Michaelson, ; Mitteroecker and Hutteger, ; Gerber, , not all of the architectures depicted in Fig. For example, we find it difficult to identify any example of the architectures shown in Figs. The morphospace is also bedeviled by the fact that plants exhibiting outwardly similar morphologies might have arrived at the same architecture via very different developmental pathways. For example, the architectures depicted in Figs. By example, architecture Fig. Similarly, in the absence of secondary growth, Fig. Clearly, a greater complexity of form than depicted in Fig. Nor does the morphospace consider construction costs or the optimization of transport distances e. Importantly, the architectural affinities of their herbaceous precursors in the earlier Devonian and Silurian floras have not been explored. Nor has the potential for the application of this approach to identify models for different rooting structures been explored. Although many roots arise endogenously, others exhibit features observed in aerial axes, such as overtopping, which can exhibit complex behavior. A transition from a simple unicellular body plan into a multicellular body plan that has undergone subsequent evolutionary elaboration has been observed or at least postulated to occur many times in the history of eukaryotes. A broad review of the fossil record of the land plants likewise reveals a history of axiation and its subsequent elaboration, facilitated by the elaboration of meristematic activity and the exploitation of ancient biosynthetic pathways and organographic toolkits as revealed by molecular studies of representative extant green algae. The co-option of ancestral traits and their subsequent deployment to achieve new adaptive solutions, as illustrated by the embellishment of the shikimate pathway and the evolution of increasingly elaborate branching architectures, is a theme repeatedly seen across all major eukaryotic photosynthetic and non-photosynthetic clades. 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Corrected and typeset:. Select Format Select format. Permissions Icon Permissions. Abstract A review of the fossil record coupled with insights gained from molecular and developmental biology reveal a series of body plan transformations that gave rise to the first land plants. Open in new tab Download slide. Google Scholar Crossref. Search ADS. Lignin-like compounds and sporopollenin in Coleochaete , an algal model for land plant ancestry. Van dePeer. Issue Section:. Download all slides. Views 1, More metrics information. Total Views 1, Email alerts Article activity alert. Advance article alerts. New issue alert. Receive exclusive offers and updates from Oxford Academic. Citing articles via Web of Science 1. Hypoxia Tolerance of Two Killifish Species. Motherhood and Academia: Tradeoffs. More from Oxford Academic. Biological Sciences. Science and Mathematics. Authoring Open access Purchasing Institutional account management Rights and permissions. 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