CHAPTER X. 



THE BIOLOGY OF A PLANT (Continued). 



The Physiology of the Fern. 



THE brake, like the earthworm, is a limited portion of organ- 

 ized matter occupying a definite position in space and time. It 

 is bounded on all sides by material particles, some of which may 

 be living, but most of which are lifeless. The aerial portion is 

 immersed in and pressed upon by an invisible fluid, the atmos- 

 phere, while the underground portion is sunk in a denser 

 medium, the earth, which likewise acts upon it. At the same 

 time the fern reacts upon the air and the earth, maintaining 

 during its life an equilibrium which is disturbed and finally gives 

 way as the life of the plant draws to a close. 



The Fern and its Environment. Those portions of space, 

 earth, and air which are nearest to the brake constitute its imme- 

 diate environment. But in a wider and truer sense the environ- 

 ment includes the whole universe outside the plant. To perceive 

 the truth of this it is only necessary to observe how profoundly 

 and directly the plant is affected by rays of light which travel to 

 it from the sun over a distance of many millions of miles, or 

 how extremely sensitive it is to the alternations of day and night 

 or of summer and winter. The plant is fitted to make certain 

 exchanges with its environment, drawing from it certain forms 

 of matter and energy, and returning to it matter and energy in 

 other forms. Its whole life is an unconscious struggle to wrest 

 from the environment the means of subsistence ; death and decay 

 mark its final and unconditional surrender. 



Adaptation of the Organism to its Environment. We can dis- 

 tinguish in Pteris as clearly as in Lumbricus the adaptation of 

 the organism to its environment. The aerial part of Pteris 

 must be fitted to make exchanges with, and maintain its life in, 

 the atmosphere, while the underground part must be similarly 

 " adapted ' to the soil in which it lives. 



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