110 PLANTS, THE FOOD OF ANIMALS 



freed are ultimately recombined into sugars and starch, from 

 which, by a series of changes which can best be described, in 

 connection with a higher type of plant, they are finally made 

 into protoplasm; this, when life is extinct, becomes protein, the 

 main food of animals. 



The plant forms serving as food for the higher animals are far 

 more complicated than Pleurococcus and Sphaerella, and just 

 as the higher animals become progressively differentiated with 

 complicated organs for the performance of the functions of food 

 getting, digestion, assimilation, excretion, nervous response 

 and reproduction, so do the higher plants become progress- 

 ively differentiated with complicated organs for the perform- 

 ance of their metabolic and reproductive functions. 



To trace the food of animals, therefore, it is necessary to ex- 

 amine the structure and functions of the higher plants, a good 

 example of which is the common fern or brake, Pteridium aquili- 

 num, formerly called Pteris aquilina. 



C. PTERIDIUM AQUILINUM 



The common brake or fern, Pteridium aquilinum, is widely dis- 

 tributed upon the earth's surface, growing in damp or shady 

 places and resisting various kinds of unfavorable conditions of 

 the environment. At one time in the earth's history the age 

 of Pteridophytes ferns formed the chief type of vegetation, 

 and some of them grew to an enormous size (up to sixty feet), 

 while even today some ferns are tree-like in size and mode of 

 growth (tree-ferns). Others, like the maiden hair, are extremely 

 delicate, growing only in the most favorable localities. 



For purposes of description, Pteridium may be regarded as 

 composed of two distinct parts; the one, aerial or above ground, 

 is termed the frond or leaf and consists of the chlorophyll-bear- 

 ing parts and their supporting and nutritive organs; the other, 

 underground, is termed the rhizome and consists of a stem 



FIG. 42. Pteridium aquilinum. The underground stem or rhizome (rh.), one 

 frond (I 1 ) of the present year in full leaf, the other (I 2 ) of the past year; ab, apical 

 bud bearing apical cell at the extremity of a branch bearing stumps of leaves of 

 previous seasons; I 1 , mature active leaf; I 2 , dead leaf of the preceding year; l.m., 

 lamina of leaf p, pinna; x, younger pinna shown enlarged at B. (After Sedg- 

 wick and Wilson.) 



