. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. PLANTS AND ANIMALS COMPARED 193 organisms, and consequently the capacity for movement has come to be regarded as an essentially animal characteristic. But the motile habit has been seen to occur in quite a consider- able number of unicellular and colonial aquatic plants, and is by no means confined to them, being observable also among higher forms ( Mosses, Liverworts, Ferns, etc.), although here restricted to certain reproductive cells. The forms studied in this chapter force us to recognise that the pow
. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. PLANTS AND ANIMALS COMPARED 193 organisms, and consequently the capacity for movement has come to be regarded as an essentially animal characteristic. But the motile habit has been seen to occur in quite a consider- able number of unicellular and colonial aquatic plants, and is by no means confined to them, being observable also among higher forms ( Mosses, Liverworts, Ferns, etc.), although here restricted to certain reproductive cells. The forms studied in this chapter force us to recognise that the power of movement cannot serve as an invariable distinction between the two Kingdoms. Nevertheless, viewing \ them as a whole, the Vege- table Kingdom may be described as essentially sedentary, and the Animal Kingdom as essentially motile. This distinction may be related to the necessity for animals to move from place to place in search of food, whilst plants, depending as they do almost solely on simple chemical compounds, can best obtain these by being stationary. It will, for instance, be clear that, for the terrestrial plant, the intimate contact which is necessary between root and soil is totally inconsistent with a motile habit. Another dis- tinction between higher plants and animals is the possession by the latter of highly developed sense-organs. This too can probably be related to the motile habit, with the concomitant necessity for rapid response to the everchanging conditions of the environment. It is significant that in animals like the Hydrazoon Obelia, which have a motile and sedentary phase in their life- history, the more specialised organs of sense occur in the former. Contractile vacuoles and eye-spots are found alike in many lowly plants and animals, and cannot be said to be characteristic. Fig 103.—Photograph of freshwater Diatom-Plankton ; the two princi- pal forms present are the filament- ous Melosira, and the star-shaped colonies of Asterionella (from Wes
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