448 THE VASCULAR PLANTS 



dences of success, in producing strains of crop plants superior 

 to the present ones with respect to yield, drouth resistance, 

 ability to withstand diseases, etc. 



You have learned enough of the nature of plants to realize 

 something of what the problems and the possibilities of plant 

 breeding are. You know that for many kinds of places 

 there are many kinds of plants (page 149), you know that 

 these plants vary in their power to vary (page 206), and you 

 know that heredity and environment are two forces always 

 at work in determining what the outcome shall be (page 

 135). The whole nature of any living thing is due to its 

 heredity plus its environment. Its heredity is the sum of 

 the characteristics it inherits. Its environment is the sum 

 of the stimuli to which it responds, and these largely 

 determine the ways in which its hereditary possibilities are 

 fulfilled. 



All this is true of ourselves as well as of plants. It sug- 

 gests wonderful and yet unmeasured possibilities of develop- 

 ment in ourselves as well as in plants, and it urges men on 

 to reveal yet hidden possibilities in plant life, possibilities 

 which, once realized, may solve forever the great food 

 problem of the human race. He who reveals such possi- 

 bilities ranks as a benefactor of mankind along with the 

 greatest of inventors or of statesmen. 



QUESTIONS AND SUGGESTIONS 



SECTION 75. i. What are the two great divisions of vascular 

 plants? 2. Describe the functions of the vascular system. 3. What 

 were the ancestors of seed plants ? Describe them. 



SECTION 76. i. Describe the structure of a true fern, denning 

 frond and venation. 2. Describe the reproduction of a true fern, 

 denning sporangium, sorus, indusium, annulus, and prothalKum. 



