1 6 MANUAL OF ZOOLOGY. 



doubted animals (e.g., Stentor, amongst the Infusoria, and the 

 Hydra viridis, or the green Fresh-water Polype, amongst the 

 Ccelenterata). 



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 perma- 

 nently fixed, or attached to some foreign object; and the em- 

 bryos 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 are so char- 

 acteristic of many of the lower forms of animal life. Not only 

 is this the case, but large numbers 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 researches have hitherto failed to 

 show the mechanism whereby these movements are brought 

 about. 



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

 failed to yield a means of invariably separating animals from 

 plants, a distinction which holds good almost without excep- 

 tion is to be found in the nature of the food taken respectively 

 by each, and in the results of the conversion of the same. 

 The unsatisfactory feature, however, in this distinction is this, 

 that even if it could be shown to be, theoretically, invariably 

 true, it would nevertheless be practically impossible to apply 

 it to the greater number of those minute organisms concern- 

 ing which alone there can be any dispute. 



As a broad rule, all plants are endowed with the power of 

 converting inorganic into organic matter. The food of plants 

 consists of the inorganic compounds, carbonic acid, ammonia, 

 and water, along with small quantities of certain mineral salts. 

 From these, and from these only, plants are capable of elabo- 

 rating the proteinaceous matter or proroplasm which consti- 

 stutes the physical basis of life. Plants, therefore, take as food 

 very simple bodies, and manufacture them into much more 

 complex substances. In other words, by a process of deoxida- 

 tion or unburning, rendered possible by the influence of sun- 

 light only, plants convert the inorganic or stable elements 

 ammonia, carbonic acid, water, and certain mineral salts into 

 the organic or unstable elements of food. The whole problem 

 of nutrition may be narrowed to the question as to the modes 

 and laws by which these stable elements are raised by the vital 

 chemistry of the plant to the height of unstable compounds. 



