Plants are distinguished from Protista in their organ and organ-system levels of organization, in having reproductive structures that are at least tissues and in most cases organs, and in passing through distinct embryonic stages during development. Also, plants are exclusively photosynthetic and no motile.
The evolutionary derivation of plants from sporine green algae is suggested strongly be the presence of chlorophylls a and b, by carotenoid and xanthophyll pigments nearly identical with those of chlorophytes, by cell walls made of cellulose, by the deposition of food stores in the form of starch, by the presence of usually two whiplash flagella in most motile cells; and by a body composed very largely of sporine cells.
Most distinguishing characteristics of plants are adaptations to terrestrial ways of life developed during the evolution from aquatic protistan ancestors. For example, in the absence of the buoyant action of water, plants contain antigravity skeletal tissues. These reach their most advanced form in the sclerenchymas and woods of the tracheophytes.
The requirement of mechanical support is met also by the generally upright, radial structure, though this is not an invariable feature. Such a construction distributes the weight equally around the vertical axis and permits lower body portions to support upper ones directly.
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In the absence of open water around all surfaces, plants have specialized absorbing tissues that project into soil. These are either rhizoids or more elaborate roots. Since such structures occur only in specific body regions, plants also contain nutrient-distributing or conducting tissues; the most highly developed being those of vascular plants.
Exposed to air, plants minimize the danger of drying through waxy cuticles on free surfaces, which let light pass but not water or atmospheric gases. But gases must be exchanged with the environment, and plants actually contain gas transmitting surface pores, or stomata. The structures and processes of reproduction likewise represent adaptations of life in an aerial environment.
Reproductive structures are organs containing at least two tissues: an exterior sterile tissue that protects again drying out and an interior tissue that produces the actual spores or sex cells. Spores themselves are always encapsulated, and sex cells either are released only during wet or rainy periods or remain surrounded by protective tissues.
Moreover, the life cycle of plants always includes an embryonic phase, which provides the developmental time necessary for the elaboration of the many specialized tissues of the mature plant. Since this internal elaboration is an adaptation to terrestrial conditions, the plant embryo too can be regarded as an evolutionary response to the requirements of land life.
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Fin all}-, the life cycle of plants always consists of two successive adult generations. One, the gametophyte, produces sex sells only, from which new adults develop. Such an adult, the sporophvte. Then produces spores only, which grow into new gametophytes.
The mature plants of these two generations are structured quite differently; they are excellent examples of polymorphism. Life cycles characterized in this way by an alteration of generations originated among the Protista, in adaptation to their own problems of aquatic life. Plants then inherited such life cycles and adapted them to the requirement of terrestrial life.
The ancestors of plants probably had to adapt to terrestrial conditions if they were to survive at all. For the algal ancestors occasionally must have experienced prolonged droughts, not an unusual hazard in freshwater habitats. Various evolutionary responses to these hazards that permitted them to survive as basically aquatic organisms. Almost incidentally, however, gradual perfection of the adaptations to temporary terrestrial living must eventually have produced plants able to survive without open water. Such organisms could then be permanently terrestrial.
Two phylum groups of plants have evolved the moss plants, or bryophytes, and the vascular plants, or tracheophytes. The latter are far more abundant and important and will occupy most of our attention.