Plants are multicellular eukaryotes. Nearly all are photosynthetic, and most are terrestrial (some are secondarily aquatic). Plants contain a variety of cell types that are specialized to perform different functions: the aerial parts generally have a waxy cuticle to prevent water loss, but the roots don’t so they can absorb water. The photosynthetic pigments include chlorophylls A and B plus various carotenoids. Plant cells have cell walls which are made of cellulose. Plants grow by mitosis, and reproduce by meiosis and alternation of generations with special adaptations of the gametes and embryos to survive in a non-aquatic environment. The gametes are produced in gametangia. The male gametangium is called an antheridium (anthe = flower), while the female gametangium is an archegonium (arche = first, beginning; goni = seed). The eggs are fertilized within the female archegonium to reduce desiccation, and often stay within the archegonium to continue their development. In all plant groups except Division Bryophyta, the 2n sporophyte is the dominant generation.
|The various Divisions (remember botanists use Division = Phylum) of plants are classified based on presence/absence of a vascular system and/or seeds. If seeds are present, the location of the seeds (whether on the surface of a reproductive
structure or within some type of ovary) becomes significant.|
It is worth noting here that many plant names end in “wort” (NOT “wart”), such as liverwort, spleenwort, toothwort, etc. The rootword wort is an old Anglo-Saxon or Celtic word meaning “herb” or “plant”. Back in the Middle Ages in Europe, people believed in the Doctrine of Signatures which said that God purposely created some plants to look like certain body parts (at least in the eyes of the Medieval herbalists) as a sign that those plants were meant to help or cure the body parts they resembled. Thus, while we have plants with names like liverwort or spleenwort, we now know that most of these plants don’t appear to help the organs after which they’re named, and some are even too toxic to safely consume. “Wort” was also used in other plant names. For example, St. Johnswort blooms (at least in Europe) around 24 June, when the Catholics celebrate the festival of the Nativity of St. John the Baptist, Pewterwort is another name for Scouring Rush because of its use in cleaning pewter, and a sudsing juice can be extracted from Soapwort.
In the vascular plants, soil minerals and water are absorbed by the roots (which also anchor the plant and have no cuticle so water can be absorbed). Air, light, and CO2 are absorbed by the leaves, while the O2 “waste” from photosynthesis is released from the leaves. The stem functions in the support of the plant, and its vascular system transports water and nutrients. The diploid sporophyte is the dominant generation. Some gametophytes are monoecious and some are dioecious. “Monoecious” means that the male and female reproductive organs are in/on the same individual plant, while “dioecious” refers to having separate male and female plants. The vascular system (transportation system) is composed of xylem (xylo = wood) which transports water and nutrients up from the roots and must be dead to function (this consists of hollow tubes where cells have died) and phloem (phloeo = the bark of a tree), which distribute sugars around the plant and consists of live cells. In general, in the vascular bundles in the stems, xylem is found “inside” the bundle, surrounded by phloem, and in leaves, xylem is found on “top” of the bundles with phloem underneath.
Note that the ancestors of these primitive plants were present back in “dinosaur times,” and the structures of the modern plants have changed little since then.
For more information on plant taxonomy, check out this site on Systematic Botany at the University of Maryland.
Copyright © 1997 by J. Stein Carter. All rights reserved.
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