ANIMALS AKD PLANTS: DIFFERENCES -y-y 



selves like plants, their body is compact, shaped like an egg or 

 a spindle, and possesses one or more hne lashes of protoplasm 

 (flagella), by the working of which it is rowed or dra^^-n t" :h 

 the water. Many of them, including the exa: CkUi nas 



sho^^-n in Fig. 8, have a pigment spot which is a sense organ for 

 the necessary- appreciation of hght, whose rays it absorbs. While 

 some of these unicellular creatures are undoubtedly plants, and 

 others, in spite of their chlorophyll, are best regarded as animals, 

 some, such as Euglena (p. 39;, are difficult to place. The line 

 between animals and plants, like that between li\-ing and non- 

 living, can only be arbitrarily drawn. 



4. The necessity for a large surface leads to a fourth character in 

 plants. An extensive surface needs strong support. In correspon- 

 dence \^ith this need we find in plants a massive skeleton which 

 forms a strong wall to each cell, so that the protoplasm is upheld 

 by an intricate framework of compartments whose walls are 

 thickest in the most woody parts of the body. Owiug, no doubt, 

 to the ample supply of starch at the command of the plant, this 

 skeleton usually consists of a modified form of starch kno^/^-n as 

 cellulose. Some groups of plants use other substances, and though 

 cellulose is rare amongst animals it is present in some Protozoa 

 and in tunicates (p. 310). 



THE BALANCE OF NATURE 



The difference in nutrition between animals and plants has the 

 important result that in their action upon the inorganic world 

 these two kinds of organisms bring about precisely opposite 

 changes, and do so in such a way that each sets up conditions 

 favourable to the activity of the other. The plant, absorbing 

 the energy of the sun's rays, builds up complex organic compounds 

 from simple inorganic substances and in so doing stores chemical 

 potential energy*. Though it destroys some organic substances 

 in respiration, the net result of its activity is to increase the stock 

 of them in the world. At the same time it sets free oxygen. The 

 animal, on the other hand, uses as food the substances manufac- 

 tured by plants, taking them either directly from plant bodies 

 or after thev have been incorporated in a somewhat altered 

 form into the protoplasm of other animals, A rabbit feeding on 

 grass and a stoat feeding on the rabbit and a parasite feeding on 

 the stoat are equally dependent on the plant for their organic 



