ADAPTATIONS 257 



is wholly inexplicable by any present knowledge of the science of 

 plant growth. 



PHYSIOLOGICAL ADAPTATIONS 



The type of adjustment to environmental conditions which 

 does not result in any recognizable alteration in the structure of the 

 plant, but simply permits it to grow under new conditions, man- 

 ifests itself in many ways. These adjustments are usually asso- 

 ciated with differences in temperature during the growing season, 

 and for this reason, most such examples of adaptation have been 

 studied in connection with possible temperature reactions upon the 

 growing organism. 



However, recent investigations seem to point strongly to the 

 conclusion that the amount of light rather than the temperature 

 of the new surroundings is the most important influence in deter- 

 mining the physiological processes known as the " acclimatiza- 

 tion " of plants. For example, a very elaborate series of inves- 

 tigations has shown that the flowering stage in the development of 

 plants is determined by the length of the daylight period per day, 

 irrespective of the actual amount of vegetative growth which the 

 plant has made. Thus, tobacco plants, which during a period of 

 long days grow to the height of 8 or 10 feet before blossoming, if 

 grown at the same temperature in periods of short days (or if kept 

 in the dark during a portion of the longer days) will blossom when 

 less than 3 feet in height and when the total mass of vegetative 

 material which has been produced is less than one-third of that of 

 the " gigantic " plants of the same variety grown with longer 

 periods of illumination per day. This same principle has been 

 found to hold good for many widely different types of plants. 

 In some species, however, flowering is favored by long days, and 

 vegetative growth by short daylight illumination. But in all 

 species which have been studied, there seems to be a direct relation 

 between the length of day, or the total illumination per day, 

 and the normal or abnormal functioning of the plant. It is 

 apparent that at least the physiological function of sexual repro- 

 duction (flowering and seed-production) is determined by the 

 length of daylight illumination. The duration of daylight per day 

 which is necessary to induce the blossoming of the plants varies 

 for different species, but it is constant for individuals of the same 



