12 



THE PHENOMENA OF LIFE 



Differences between Animals and Plants. Having considered the 

 vital properties of protoplasm, as shown in cells of animal as well as of vege- 

 table organisms, we are now in a position to discuss the question of the differ- 

 ences between plants and animals. It might at the outset of our inquiry have 

 seemed an unnecessary thing to recount the distinctions which exist between 

 an animal and a vegetable organism as they are in many cases so obvious, but, 

 however great the differences may be between the higher animals and plants, 

 in the lowest of them the distinctions are much less plain. 



In the first place, it is important to lay stress upon the differences between 

 vegetable and animal cells, first as regards their structures and next as re- 

 gards their functions. 



It has been already mentioned that in animal cells an envelope or cell wall 

 is by no means always present. In adult vegetable cells, on the other hand, 

 a well-defined wall is highly characteristic; this is composed of cellulose, 

 is non-nitrogenous, and thus differs chemically as well as structurally from 

 the contained protoplasmic mass. Moreover, in vegetable cells, figure n, 

 B, the protoplasmic contents of the cell fall into two subdivisions: i, a con- 

 tinuous film which lines the interior of the cellulose wall; and, 2, a reticulate 



FIG. ii. A. Young Vegetable Cells, Showing Cell Cavity Entirely Filled with Gran- 

 ular Protoplasm Enclosing a Large Oval Nucleus, with one or more Nucleoli. B. Older 

 cells from same plant, showing distinct cellulose wall and vacuolation of protoplasm. 



mass containing the nucleus and occupying the cell cavity. The inter- 

 stices are filled with fluid. In young vegetable cells such a distinction does 

 not exist; a finely granular protoplasm occupies the whole cell cavity, figure 

 n, A. As regards the respective functions of animal and vegetable cells, 

 one of the most important differences consists in the power which vegetable 

 cells possess of being able to build up new complicated nitrogenous and 

 non-nitrogenous bodies out of very simple chemical substances obtained 

 from the air and from the soil. They obtain from the air oxygen, carbon 

 dioxide, and water, as well as traces of ammonia gas; and from the soil they 

 obtain water, ammonium salts, nitrates, sulphates, and phosphates in com- 

 bination with such bases as potassium, calcium, magnesium, sodium, iron, 

 and others. The majority of plants are able to work up these elementary 

 compounds into other and more complicated bodies. This they are able 



