22 ELEMENTS OF BIOLOGY. 



ever, it may be stated that the presence in any organism of 

 an external envelope of cellulose raises a strong presumption 

 of its vegetable nature. In the face, however, of the facts 

 above stated, the presence of cellulose cannot be looked upon 

 as absolutely conclusive. Another highly characteristic vege- 

 table compound is chlorophyll, the green colouring-matter of 

 plants. Any organism which exhibits chlorophyll in any 

 quantity, as a proper element of its tissues, is most pro- 

 bably vegetable. As in the case of cellulose, however, the 

 presence of chlorophyll cannot be looked upon as a certain 

 test, since it occurs normally in certain undoubted ani- 

 mals (e.g-, Stentor, amongst the Infusoria, and the Hydra 

 mridis, or the green Fresh-water Polype, amongst the Ccelen- 

 terata). 



d. Motor Power. This, though broadly distinctive of ani- 

 mals, can by no means be said to be characteristic of them. 

 Thus, many animals in their mature condition are per- 

 manently fixed, or attached to some foreign object; and the 

 embryos of many plants, together with not a few adult 

 forms, are endowed with locomotive power by means of 

 those vibratile, hair-like processes which are called " cilia," 

 and which are so characteristic of many of the lower forms 

 of animal life. Not only is this the case, but large num- 

 bers of the lower plants, such as the Diatoms and Desmids, 

 exhibit throughout life an amount and kind of locomotive 

 power which does not admit of being rigidly separated from 

 the movements executed by animals, though the closest re- 

 searches have hitherto failed to show the mechanism where- 

 by these movements are brought about. 



e. Nature of the Food. Whilst all the preceding points 

 have failed to yield a means of invariably separating ani- 

 mals from plants, a distinction which holds good almost 

 without exception is to be found in the nature of the food 

 taken respectively by each, and in the results of the conver- 

 sion of the same. The unsatisfactory feature, however, in 

 this distinction is this, that even if it could be shown to be, 



