102 THE BIOLOGY OF A PLANT. 



brown plates (sclerotic prosenchyma) lie between the inner and outer 

 systems of bundles, and are only separated from one another at the sides 

 by a narrow band of parenchyma. They are often joined on one side 

 or even on both, in the latter case forming a tube which separates the 

 two systems of bundles. (Hofmeister.) 



Apogamy. Apospory. In rare cases, e.g., in Pteris cretica, the ordi- 

 nary alternation of generations in the life-cycle of ferns is abbreviated by 

 the omission of the sexual process and the immediate vegetative outgrowth 

 of the sporophore from the prothallium (apogamy). In other cases there is 

 an omission of the spore stage, and immediate vegetative development of 

 the oophore from the frond (apospory). (Of. Farlow, Quart. Journ. Hie. 

 Science, 1874; Do Bary, Botan. Zeitung, 1878. Druery, etc.; Journ. 

 Royal Mic. Soc. 1885, pp. 99 and 491.) 



GENERAL PHYSIOLOGY. 



The brake is a limited portion of matter occupying a definite 

 position in space and time. It is bounded on all sides by mate- 

 rial 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 atmosphere, 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 immediate 

 environment. But in a wider and truer sense the environment 

 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 environ- 

 ment the means of subsistence ; death and decay mark its final 

 and unconditional surrender. 



In its relations to the environment the fern illustrates an im- 



