INTRODUCTION. 5 



plants as to their minute internal structure. Both alike consist essen- 

 tially of minute solid particles (molecules or granules), of cells, or of 

 fibres. 



Thirdly, as regards chemical composition, there are some decided, though 

 not universal, differences between plants and animals. As a general rule, it 

 may be stated that plants exhibit a decided predominance of what are 

 known to chemists as " ternary compounds " that is to say, compounds 

 which, like sugar, starch, and cellulose, are composed of the three elements, 

 carbon, hydrogen, and oxygen. They are, comparatively speaking, poorly 

 supplied with " quaternary " compounds, which contain the fourth element, 

 nitrogen, in addition to the three first mentioned (e. g., gluten and legumin). 

 Animals, on the other hand, are rich in quaternary nitrogenized compounds, 

 such as albumen or fibrine. Still in both kingdoms we find nitrogenized 

 and non-nitrogenized compounds, and it is only in the proportion which 

 these bear to one another in the organism that animals differ in any way 

 from plants. The most characteristic of all vegetable compounds is the one 

 known as cellulose, very nearly allied in its chemical composition to ordinary 

 starch. As a general rule it may be stated that the presence of an external 

 envelope of cellulose in any organism raises a strong presumption as to its 

 vegetable nature. Still cellulose is not exclusively confined to plants, as 

 was at one time believed. It is now well known that the outer covering 

 of the so-called sea-squirts or Ascidian Mollusks contains a large quantity 

 of cellulose (as much as 60 per cent, in some cases) ; and recent researches 

 seem to prove that this substance is present also in some of the lower forms 

 of animal life (coccospheres). Another highly characteristic vegetable prod- 

 uct is chlorophyll, the green coloring-matter of plants. Any organism 

 which exhibits chlorophyll in any quantity as a proper element of its tissues 

 is most probably vegetable. In this case also, however, the presence of 

 chlorophyll cannot be regarded as a certain test, since it occurs regularly in 

 some undoubted animals (e. g., Stentor among the Infusoria, and the Hydra 

 viridis, or green fresh-water polype, among the Cozlenterata). 



Fourthly, as regards locomotive power, or the ability to effect changes of 

 place at will, the results of observation are singularly at variance with our 

 preconceived notions. Before the invention of the microscope, no instances 

 of independent voluntary movements were known in plants, if we except the 

 voluntary opening and closure of flowers and their turning toward the sun, 

 the drooping of the leaves of sensitive plants under irritation, and some 

 other phenomena of a like nature. Now, however, we know of many plants 

 which are endowed, either when young or throughout life, with the power 

 of effecting voluntary movements apparently as spontaneous and independent 

 as those exhibited by the lower animals. In some cases the movements are 

 brought about by means of little vibrating hairs or cilia with which a part 

 or the whole of the surface is furnished. In other cases the movements 

 seem to be certainly not produced by cilia, but their exact cause is obscure 

 (e. g., in the Diatomacece and Desmidice, two of the lower orders of plants, all 

 of which are microscopic in size). When it is added that many animals are 

 permanently fixed and rooted to solid objects in their fully-grown condition, 

 it will be seen that no absolute distinction can be drawn between animals 

 and plants merely on the ground of the presence or absence of independent 

 locomotive power. 



FiftJdy, we have shortly to consider one of the most reliable of all the 

 tests by which an animal may be separated from a plant namely, the nature 

 of the food, and the products which are formed out of the food within the 

 body. 



