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 (e.g. 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 

 Wesenberg-Lund) . 



