HIGHER ANIMALS AND PLANTS CONTRASTED 109 



like that of animals, has been essentially in the service of 

 nutrition. Great trunks and branches have been evolved appar- 

 ently in response to the need of presenting maximum chloro- 

 phyll bearing leafy surfaces to the air and light, while great 

 subterranean roots absorb water and salts from the earth. 

 Strong frameworks of lifeless wood, giving resistance to winds 

 and weather, have been evolved for the support of the heavy 

 aerial structures while complicated canal systems for the 

 transportation of salts and foods in solution penetrate the entire 

 plant organism from the tiniest rootlet to the tips of the highest 

 leaves. The reproductive organs finally are equally well 

 developed in plants and animals, the maintenance of species 

 being a universal biological need. 



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 Pteris aquilina. 



C. PTERIS AQUILINA 



The common brake or fern Pteris aquilina is widely dis- 

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

 places and resisting all 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 Pteris may be regarded as com- 

 posed 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. Pteris aquilina. 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 

 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 Sedgwick and Wilson.) 



