Moreover, none of the three groups appears to be more closely affiliated with vascular plants than the others. Thus, the plant group is best separated into four units generally put at the phylum level : mosses, liverworts, hornworts and vascular plants. M osses, liverworts and hornworts are all small, often less than a centimeter in height, but they can extend over a considerable area.
They do vary in form and exhibit three basic body plans, that do not follow the phylogenetic groupings. These are delineated below. Non-vascular plants, and all plants, alternate between two stages: a haploid, gamete producing form gametophyte and a diploid, spore producing form sporophyte Chapter In all of the non-vascular plants the g ametophytes are much more likely to be encountered because of their greater size and longevity.
When present, sporophytes often appear to be an appendage of the gametophyte which, in a structural sense, they are. 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. A thallus refers to a body form that lacks organs and tissues and is relatively amorphous, often occurring as a flattened sheet.
In all of the hornworts Fig. There is no cellular specialization within the thallus, although the lower surface produces unicellular rhizoids that attach the thallus to the substrate. These cells lack chlorophyll and therefore must acquire nutrition from the photosynthetic cells above. This form is present solely in the liverwort group. Like the previous form there are no obvious organs except those associated with sexual or asexual reproduction. 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.
The thallus is often over 20 cells thick and has discernible layers. 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. Below the epidermis is a porous layer of cells, i. Cells of this layer have abundant chloroplasts. Generally, the largest air spaces are below the por es.
The porous nature of the upper thallus is a feature that is also be present in most vascular plant leaves.
Lower layers of the thallus are less porous and have cells that lack chlorophyll. The lower epidermis often produces rhizoids, i. Structures 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.
This is the form found in most mosses and many liverworts. Secondary tissues are either simple composed of similar cell types or complex composed of different cell types. Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes three different cell types: vessel elements and tracheids both of which conduct water , and xylem parenchyma.
Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells which conduct photosynthates , companion cells, phloem parenchyma, and phloem fibers.
Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other Figure 1.
In stems, the xylem and the phloem form a structure called a vascular bundle ; in roots, this is termed the vascular stele or vascular cylinder. Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. The dermal tissue of the stem consists primarily of epidermis , a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark , which further protects the plant from damage.
Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place Figure 2.
Two cells, known as guard cells , surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface.
They help to reduce transpiration the loss of water by aboveground plant parts , increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.
Figure 2. Openings called stomata singular: stoma allow a plant to take up carbon dioxide and release oxygen and water vapor.
The a colorized scanning-electron micrograph shows a closed stoma of a dicot. Each stoma is flanked by two guard cells that regulate its b opening and closing. The c guard cells sit within the layer of epidermal cells credit a: modification of work by Louisa Howard, Rippel Electron Microscope Facility, Dartmouth College; credit b: modification of work by June Kwak, University of Maryland; scale-bar data from Matt Russell.
The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring.
In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue Figure 3. Figure 3. In a dicot stems, vascular bundles are arranged around the periphery of the ground tissue.
The xylem tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. Sclerenchyma fibers cap the vascular bundles. The main function of collenchyma cells is to provide additional support to the plant, especially in areas of continued growth. License Info. Image Use. Custom Photos. Site Info. Contact Us. The Galleries:. Photo Gallery.
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