274 PLANT PHYSIOLOGY 



water, etc., sufficient to ensure the perfect accomplishment 

 of its vegetative functions, will favor the accomplishment 

 of sexual functions also. More than enough warmth, 

 food, water, etc., for these purposes will not act as a 

 stimulus to the plant to engage in the processes pre- 

 liminary to and composing the sexual functions. The 

 special stimulus of light is necessary, and in this re- 

 spect light differs from the other forces and from the mat- 

 ters upon which the plant depends and to which it irrita- 

 bly responds. 



We are told that Arctic and Alpine flowers are especially 

 brilliant, and in certain parts of the temperate zone the 

 brilliancy of the flowers, their large size, perfection, and 

 number is a matter of local pride and of general admira- 

 tion. According to the hypothesis of natural selection, 

 Arctic and Alpine plants produce especially flne flowers be- 

 cause, in order to secure cross-pollination from the com- 

 paratively few insects of those regions, the flowers must be 

 especially conspicuous; that less conspicuous flowers, less 

 attractive to insects, set fewer or no seed, and that finally 

 the plants which bear them become extinct. This may be 

 the case, but Vochting's work suggests an explanation more 

 susceptible of experimental test. During the Arctic summer, 

 brief though it is, the hours of darkness are few, the hours 

 of light many, and the light often intense. Alpine regions 

 are like Arctic regions in that the summer is brief and the 

 light intense. Certainly the high mountain side receives 

 more light per hour of clear sunshine than does the valley 

 below, and in clear weather the mountain side is illuminated 

 for more hours per day than is the valley. This factor, 

 light, may therefore play a very important part in connec- 

 tion with the sexual reproduction of plants of high altitudes 

 and latitudes. 



One more point should be mentioned before we leave the 

 subject of the influence of light on blooming. Vochting 

 found that he could convert all the flowers of Viola odorata 

 into cleistogamous ones by diminishing the light. He could 

 not, however, by the opposite course, bring about the 

 development of all the flowers into chasmogamous ones. 



