The simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded by phloem. Together, xylem and phloem tissues form the vascular system of plants. Xylem and phloem : Xylem and phloem tissue make up the transport cells of stems. The direction of water and sugar transportation through each tissue is shown by the arrows.
Xylem is the tissue responsible for supporting the plant as well as for the storage and long-distance transport of water and nutrients, including the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The tissue consists of vessel elements, conducting cells, known as tracheids, and supportive filler tissue, called parenchyma.
These cells are joined end-to-end to form long tubes. Vessels and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. It is the thick walls of the tracheids that provide support for the plant and allow it to achieve impressive heights.
Tall plants have a selective advantage by being able to reach unfiltered sunlight and disperse their spores or seeds further away, thus expanding their range. By growing higher than other plants, tall trees cast their shadow on shorter plants and limit competition for water and precious nutrients in the soil.
The tracheids do not have end openings like the vessels do, but their ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass horizontally from cell to cell. Tracheids and vessel elements : Tracheids top and vessel elements bottom are the water conducting cells of xylem tissue. Phloem tissue is responsible for translocation, which is the transport of soluble organic substances, for example, sugar.
The substances travel along sieve elements, but other types of cells are also present: the companion cells, parenchyma cells, and fibers. The end walls, unlike vessel members in xylem, do not have large openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates.
Despite the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. The activity of the sieve tubes is controlled by companion cells through plasmadesmata. Roots support plants by anchoring them to soil, absorbing water and minerals, and storing products of photosynthesis.
Roots are not well preserved in the fossil record. Nevertheless, it seems that roots appeared later in evolution than vascular tissue. The development of an extensive network of roots represented a significant new feature of vascular plants. Roots provided seed plants with three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis.
Importantly, roots are modified to absorb moisture and exchange gases. In addition, while most roots are underground, some plants have adventitious roots, which emerge above the ground from the shoot. There are mainly two types of root systems.
Dicots flowering plants with two embryonic seed leaves have a tap root system while monocots flowering plants with one embryonic seed leaf have a fibrous root system. A tap root system has a main root that grows down vertically from which many smaller lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these weeds; they can regrow another shoot from the remaining root. Root types : a Tap root systems have a main root that grows down, while b fibrous root systems consist of many small roots.
A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, forming a dense network of roots that also helps prevent soil erosion lawn grasses are a good example, as are wheat, rice, and corn.
In addition, some plants actually have a combination of tap root and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water tend to have shallower root systems.
Zones on a root tip : A longitudinal view of the root reveals the zones of cell division, elongation, and maturation. Cell division occurs in the apical meristem.
Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through soil.
The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation. The zone of cell division is closest to the root tip; it is made up of the actively-dividing cells of the root meristem.
The zone of elongation is where the newly-formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three zones are in the first centimeter or so of the root tip. In bryophytes, the sporophyte remains physically attached to the maternal gametophyte throughout its lifespan. Young sporophytes do photosynthesize; however, they are dependent on nutrients and water from the maternal plant Ligrone and Gambardella, This presents a conflict over resources between the offspring and maternal plants Haig and Wilczek, ; Haig, , especially for species with perennial gametophytes that will reproduce in subsequent years.
The cuticle on the maternal calyptra may not only play a protective role in dehydration, but this layer may concurrently decrease sporophyte transpiration; reducing the resources taken by the offspring sporophyte from the maternal plant. On the opposite side of this conflict, the offspring potentially increases transpiration, and thus its pull of resources from the maternal plant, by increasing the number of stomata on the capsule or by increasing seta length, elevating the capsule further above the boundary layer.
Our data on the calyptra cuticle directly align with the predictions of this conflict. We observed that the calyptra cuticle is thicker for species with taller sporophytes, which may enable them to limit the transpirational pull of resources by the offspring from the maternal plant Figure 4A.
The evolution of sporophyte morphology across the Funariaceae is widely thought to occur via the process of reduction McDaniel et al. Parallel losses in structures that facilitate spore dispersal, such as peristome teeth, the operculum, and the seta, are observed across the family.
The morphological reductions observed in both the maternal calyptra and offspring sporophyte of the Funariaceae could have occurred under several alternative scenarios; driven initially by morphological evolution of either the offspring sporophyte or the maternal calyptra or alternatively evolving in concert. In one scenario the evolution of a shorter sporophyte results in lower levels of dehydration stress, enabling the maternal gametophyte to invest fewer resources in the protective calyptra, by thinning the cuticle and ultimately developing a smaller calyptra.
In an alternative scenario, the evolution of a smaller calyptra with a thinner calyptra cuticle results in higher levels of sporophyte dehydration stress, constraining and ultimately reducing sporophyte height. A well-resolved phylogeny combined with comparative methods may enable us to determine the most likely scenario Felsenstein, In either case, the cuticle represents a costly structural investment, the lipids of which may require more than double the glucose for a plant to build compared to cell wall polysaccharides Poorter and Villar, Thus, any decrease in cuticle investment frees up resources to be allocated to other developmental, reproductive, or physiological processes.
Many maternal organisms provide protection for their offspring and this study highlights a unique example of maternal protection in plants. The maternal calyptra is not a vestigial structure, but has been retained and elaborated across the 12, species of mosses Crosby et al. Using a comparative developmental framework we have expanded our knowledge of moss cuticle development to a broader number of taxa.
This study lays the groundwork for future studies of morphologically and ecologically diverse species to ultimately further our understanding of the connections between maternal structures and their functional importance for offspring plants.
Our observations broaden our knowledge of the plant cuticle and highlight the functionally important role the cuticle plays in preventing dehydration even in the relatively diminutive bryophytes. JB designed, performed, and analyzed the experiments. JB and BG conceived the study and wrote the manuscript. All authors read and approved the final version of the manuscript to be published.
Assistance from the faculty and staff of the University of Connecticut Electron Microscopy Facility was appreciated. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We also thank members of the Goffinet laboratory and reviewers for their helpful comments on earlier versions of this manuscript.
Barthlott, W. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta , 1—8. Beike, A. Molecular evidence for convergent evolution and allopolyploid speciation within the Physcomitrium-Physcomitrella species complex.
BMC Evol. Bopp, M. Die wirkung von maleinhydrazid und kalyptraextrakt auf die verdickung von laubmoossporogonen. Naturwissenschaften 41, — Buda, G. An ATP binding cassette transporter is required for cuticular wax deposition and desiccation tolerance in the moss Physcomitrella patens.
Plant Cell 25, — Budke, J. A hundred-year-old question: is the moss calyptra covered by a cuticle? A case study of Funaria hygrometrica. The cuticle on the gametophyte calyptra matures before the sporophyte cuticle in the moss Funaria hygrometrica Funaria ceae. Dehydration protection provided by a maternal cuticle improves offspring fitness in the moss Funaria hygrometrica.
Busta, L. Phytochemistry , 38— Campbell, C. V, eds J. Horsfall and E. Google Scholar. Clarke, J. Establishing a time-scale for plant evolution.
New Phytol. Crosby, M. A Checklist of the Mosses. The biophysical design of plant cuticles: an overview. Felsenstein, J. Phylogenies and the comparative method. Fife, A. French, J. Jr On the role of the calyptra in permitting expansion of capsules in the moss Funaria. Bryologist 78, — Frost-Christensen, H. Species specificity of resistance to oxygen diffusion in thin cuticular membranes from amphibious plants. Plant Cell Environ. Goffinet, B. Micromitriaceae: a new family of highly reduced mosses.
Taxon 60, — Graham, L. Origin of Land Plants. New York, NY: Wiley. Chemical and structural analysis of Eucalyptus globulus and E. Plant Sci. Haig, D. Filial mistletoes: the functional morphology of moss sporophytes. Sexual conflict and the alternation of haploid and diploid generations. B , — Hallam, N. Growth and regeneration of waxes on the leaves of Eucalyptus.
Planta 93, — Holmes, M. Effects of pubescence and waxes on the reflectance of leaves in the ultraviolet and photosynthetic wavebands: a comparison of a range of species. The same innovation appeared independently in eusporangiate ferns Johnson and Renzaglia, In the scenario proposed here, further elaboration of the sporophyte body plan and underpinning embryo organization in the polysporangiophyte lineage involved suppression of the sporangium primordium, meristem apicalization and a temporal splitting of the developmental programmes for the vegetative epidermis with stomata and photosynthetic tissue and reproductive part of the sporangium archesporium and associated tissues.
In a broad sense, homeotic genes also embrace genes encoding small RNAs associated with the timing of developmental transitions Moss, ; Poethig, The hypothesis that the ancestral SAM arose from an embryo area pre-determined to produce sporangial tissue implies interpretation of the sporophyte shoot as a sterilized sporangial axis intercalated between the early embryo and the fertile sporangium.
Sterilization and diverted development of reproductive structures is a recurrent mechanism of morphological innovation in plants. Striking examples are the modification of marginal flowers in the inflorescences of Asteraceae into sterile structures for attraction of pollinators, the conversion of stamens into staminodes and even the origin of petals from parts of the androecium reviewed by Crane and Kenrick, Other likely examples are microphylls in lycophytes and interseminal scales in Bennettitales, both suggested to be derived from sterilized sporangia, for the former as an alternative to the enation model Crane and Kenrick, Indeed, partial sterilization of the sporangium is also the likely mechanism at the origin of the capsule neck or apophysis in peristomate mosses, that is a specialized sporangium segment containing stomata and chlorenchyma but lacking archesporial tissue Goffinet et al.
It may also be observed that sterilization of potential sporogenous cells is at the very origin of the sporophyte vegetative tissue in embryophytes Hemsley, With reference to the putatively ancestral condition found in extant mosses, the sporophyte body plans in hornworts and polysporangiophytes may be viewed as the expression of opposite heterochronic events, i.
In hornworts, the sporophyte starts producing archesporial tissue in the embryo and young sporophyte Renzaglia et al.
In either case, the result is a pedomorphic sporophyte permanently retaining juvenile characters: an active meristem and the potential to produce spores. The evolutionary model presented here assumes that a moss-like spindle-shaped embryo is plesiomorphic in stomatophytes and that the embryo and sporophyte body plans in hornworts and polysporangiophytes arose by sequential elaboration of this ancestral pattern.
This model is consistent with molecular phylogenies as well as with substantial similarity in the sporophyte body plan of mosses and liverworts, the latter being the earliest diverging extant embryophyte lineage Qiu et al. An alternative scenario assuming the ancestral stomatophyte embryo to be similar to the globular embryo of hornworts and the spindle-shaped embryo to be an apomorphy of mosses would be less consistent with phylogenetic evidence but would still be compatible with our model, its main implication being, in our opinion, that the seta of mosses should be interpreted as an innovation derived from a sterilized sporangial segment.
An independent origin of the moss seta by partial sterilization of the sporangial axis appears to be a plausible hypothesis, although less parsimonious than our model because it implies two separate events of sporangial sterilization in stomatophytes and rules out homology with the liverwort seta. A polysporangiophyte-like plesiomorphic embryo would not only be at sharp variance with phylogeny but would also imply a sequence of events far less parsimonious than the scenarios outlined above.
Unlike determinate sporophyte development in liverworts and mosses, sporophyte development in hornworts and polysporangiophytes is essentially a stochastic process involving an unpredictable number of cell divisions although producing highly regular forms. Arguably, the evolution of an indeterminate body increased the photosynthetic and reproductive potential of the sporophyte but possibly also amplified a conflict of interest with the parental gametophyte, at least in terms of allocation of water and mineral nutrients Haig and Wilczek, Probably this was the major factor driving sporophyte evolution towards autonomy in polysporangiophytes.
Because of the absence of vascular tissue, stomatal transpiration in the hornwort sporophyte presumably is lower than might be expected in a comparable vascularized system, and this may have worked in reducing potential conflict with the gametophyte; moreover, the basal position of the meristem in the hornwort sporophyte is probably incompatible with branching, which presumably was an essential prerequisite for evolving root-like structures and gaining autonomy Ligrone et al.
Taken together, these two factors may account for the retention of a bryophytic life cycle in hornworts. The root apical meristem RAM and leaf primordium present in the embryo of extant polysporangiophytes Johnson and Renzaglia, , are additions that followed the evolution of branching and were associated with roots and leaves; each appeared at least twice independently, in lycophytes and euphyllophytes Raven and Edwards, ; Pires and Dolan, The leaf primordium most probably resulted from further segregation from the SAM Johnson and Renzaglia, , whereas the origin of the root is more uncertain Raven and Edwards, ; Pires and Dolan, In the last two decades, molecular research has identified several classes of genes involved in the control of the SAM.
The functioning of the SAM in arabidopsis and other angiosperms also depends on a regulatory loop between clavata and wuschel genes Schoof et al.
The LEAFY gene is involved in the control of the transition from vegetative to reproductive growth in angiosperms by regulating the transcription of ABC genes; AP2 genes, a gene family associated with floral development in angiosperms, include sequences involved in stem cell maintenance and transition from vegetative to reproductive growth in both early diverging and more derived tracheophytes reviewed by Floyd and Bowman, The recent addition of the near complete genome sequence of the moss Physcomitrella patens Rensing et al.
The overall evidence indicates that the evolution of the complex gene networks underpinning sporophyte development in angiosperms entailed repeated events of duplication, functional co-option and neo-functionalization of genes already present in the ancestral genotype. A similar picture is produced by comparative analysis of growth-promoting hormones in early diverging and more derived land plants Ross and Reid, So, it is increasingly evident that understanding the molecular bases of sporophyte evolution and development requires filling the knowledge gap between angiosperms and bryophytes.
Essential to this purpose is nuclear genome sequencing in liverworts and hornworts. It is anticipated that the present analysis will inspire future research and provide a framework for data testing and interpretation. Even within the boundaries of limited current knowledge, the hypotheses presented are amenable to experimental testing.
Research on shared genetic signatures of sporophyte meristems in mosses, hornworts and polysporangiophytes not only might permit assessment of homology but also is a promising line of enquiry for genetic markers of the transition from embryonic to post-embryonic development. The same applies for testing the hypothesis of a common origin of indeterminate sporophyte development in hornworts and polysporangiophytes. In Physcomitrella the former control formative cell division in the sporangium primordium and basal meristem whereas they are not expressed in the gametophyte Sakakibara et al.
Our model points to differences in the timing of spore production as a major character distinguishing mosses, hornworts and polysporangiophytes. Expression and functional analysis of genes controlling the transition from vegetative to reproductive growth, notably EP2 genes Floyd and Bowman, , is likely to produce data useful to test our model and, in particular, our heterochronic interpretation of sporophyte evolution in hornworts and polysporangiophytes.
Google Scholar. Google Preview. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. The origin of the sporophyte shoot in land plants: a bryological perspective.
Its filamentous form is remarkably similar to green algae. This photosynthetic colonizer lies flat against its substrate, making it seem as if the rock or tree it grows on is painted green. The protonema will eventually produce leafy shoots called gametophores. The gametophores are the most conspicuous part of the moss. Although these shoots seem to vary from moss to moss, there are many morphological characteristics that are common to most of the mosses. For instance, most mosses have spirally arranged leaves that are one cell layer thick unistratose.
Generally most mosses have multicellular stems and rhizoids associated with these stems. Of course there are always exceptions to these norms, but they are rare. Rhizoids in the mosses are multicellular, but uniseriate exception: Andreaeidae mosses have biseriate rhizoids.
This means that more than one cell is needed to make a rhizoid and that these cells are aligned end to end, forming a filament.
Rhizoids usually arise from the cortical cells of the stem, but can occasionally grow out of leaves. In mosses, the rhizoids have oblique crosswalls and are non-photosynthetic.
Unlike the roots in plants, rhizoids do not absorb water or nutrients from the substrate; instead, their main function is to attach the plant to its substrate. It is thought rhizoids also play a role in water retention and conduction by capillary action. Stems in mosses are multicellular and can show a surprising amount of tissue differentiation depending on the species of moss. Typically, most mosses have cortical cells also called parenchyma cells that compose much of the stem, and thick walled cells called stereids for structural support.
Some mosses have internal conducting strands that guide water and even metabolites through the stem. In general the cells that conduct water are called hydroids and cells that conduct metabolites are called leptoids. Most hair cap mosses Polytrichidae have highly differentiated stem cells.
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