ADAPTATION TO THE ENVIRONMENT. 145 



The aerial part displays admirable adaptation in its stalk, which 

 rises to a point of vantage for procuring air and light, and in its 

 broadly spreading top, which is covered by a skin, tough and 

 impervious, to prevent undue evaporation and consequent desic- 

 cation, yet translucent, to allow the sun's rays to reach the 

 starch-making tissue within. The rhizome also, with its pointed 

 terminal buds, its elongated roots, armed with boring tips, and 

 its thick, fleshy parenchyma for the storage of food, is admirably 

 adapted to its own special surroundings. In order to realize 

 this, we have only to imagine the fern to be inverted, the aerial 

 portion being planted in the earth, and the underground portion 

 lifted into the air and exposed to the winds and sunshine. Under 

 these circumstances the want of adaptation of the parts to their 

 respective environments would speedily become apparent. 



Yet different as these parts now are, they have originally 

 sprung from the same cell. More recently they were barely dis- 

 tinguishable in a mass of tissue, part of which turned upwards 

 into the air, while another part turned downwards into the earth. 

 But as development went on, the aerial and underground parts 

 were progressively differentiated, thus becoming more and more 

 perfectly adapted to the peculiar conditions by which each is 

 surrounded. 



Thus it appears that the harmony between every part of the 

 plant and its environment is brought about, as in the animal, by a 

 gradual process in the history of each individual. AVe can here 

 clearly see also the functional adaptation of the plant to chang- 

 ing external conditions. The environment of Pteris changes 

 periodically with the regular alternation of summer and winter, 

 and the plant also undergoes a corresponding periodic change of 

 structure in order to maintain its adaptation to the environment. 

 During the summer the aerial part is fully developed, and, as a 

 result of its activity, starch is accumulated in the rhizome. At 

 the approach of winter the aerial part dies, and the plant is re- 

 duced to the underground part safely buried in the soil. During 

 the winter and spring the starch is gradually consumed, and the 

 aerial part is put forth again as the aerial environment becomes 

 once more favorable to it. The plant, therefore, like the animal, 

 possesses a certain plasticity which enables it to adapt itself to 

 gradually changing conditions of the environment. 



