1/21/2024 0 Comments Plant stem tissueStructures associated with asexual reproduction (gemmae cups), and structures associated with sexual reproduction ( antheridiophores and archegoniophores ) are sometimes observed extending from the upper surface, their structure and function will be discussed in a later chapter. The lower epidermis often produces rhizoids, i.e., some of the cells have thread-like extensions that anchor the organism to the substrate. Lower layers of the thallus are less porous and have cells that lack chlorophyll. The porous nature of the upper thallus is a feature that is also be present in most vascular plant leaves. Generally, the largest air spaces are below the por es. Cells of this layer have abundant chloroplasts. Below the epidermis is a porous layer of cells, i.e., cells are not tightly packed and have air spaces in between them. The pores are formed by barrel shaped clusters of cells that span the epidermis and, at least in some forms, are capable of closing the pore under dry conditions. There is an upper 'skin' (epidermis) that is coated with a cuticle and often is regularly perforated by pores. The thallus is often over 20 cells thick and has discernible layers. The thallus consists of flattened sheets that spread over the substrate (soil, rocks or tree trunks and branches, leaves) and commonly bifurcates, splitting in two. Like the previous form there are no obvious organs except those associated with sexual or asexual reproduction. This form is present solely in the liverwort group. Non-vascular plant gametophyte structureĬomplex thalloid forms Figure 2 Cross section through a complex thalloid liverwort.For vascular plants it is the sporophyte plant that is dominant (much larger on longer lived) and the gametophyte stage will only be considered briefly here but will be described when considering sex and reproduction. When present, sporophytes often appear to be an appendage of the gametophyte which, in a structural sense, they are. In all of the non-vascular plants the g ametophytes are much more likely to be encountered because of their greater size and longevity. Non-vascular plants, and all plants, alternate between two stages: a haploid, gamete producing form (gametophyte) and a diploid, spore producing form (sporophyte) (Chapter 11). They do vary in form and exhibit three basic body plans, that do not follow the phylogenetic groupings. M osses, liverworts and hornworts are all small, often less than a centimeter in height, but they can extend over a considerable area. Thus, the plant group is best separated into four units (generally put at the phylum level): mosses, liverworts, hornworts and vascular plants. Moreover, none of the three groups appears to be more closely affiliated with vascular plants than the others. However, grouping organisms on the basis of what they lack is generally not useful in a phylogenetic sense and it certainly is not in this case: non-vascular plants do not form a 'natural grouping'. The remaining plants, non-vascular plants (mosses, liverworts and hornworts), are often put together in a group. The majority of plants, and the ones that most people observe, are 'vascular plants', a monophyletic group whose name refers to particular cell types and tissues that they possess. All plants are considered 'truly multicellular' although there are some members with very little cellular specialization and only very rudimentary tissue and organ structure (Figure 1). Most workers consider plants to be a monophyletic group derived from green algae. \)įigure 1 A hornwort, a plant with thallus, a very rudimentary structure.
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