PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 413 



For a long time, it was supposed that the cycle of development 

 is determined exclusively by internal hereditary periodicity and 

 that this cycle or rhjrthm of development being predetermined 

 is strictly constant and does not change readily under the 

 influence of external agencies. The age of the plant was assumed 

 to be the chief factor determining the transition to reproduction. 

 Thus, some varieties are precocious and mature much earlier 

 than other later varieties. Among the trees and shrubs, there 

 are species that bloom at the age of 5 to 10 years, as for instance 

 many fruit trees; those hke the maple, the linden, and the larch 

 that flower at the age of 25 to 30 years; and finally, those like 

 the oak, the elm, and the chestnut that are mature only at the 

 age of 40 to 60 years. Though in horticulture wide appUcation 

 is being made of methods of accelerating the time of fruiting, 

 chiefly by means of grafting on dwarfing stocks, nevertheless 

 up to the present there have been no serious attempts at syste- 

 matically elaborating methods of controlling the length of the 

 vegetative period. 



Klebs was the pioneer in the field of the physiology of plant 

 development. In his investigations, which were carried out 

 at the end of the nineteenth and at the beginning of the twentieth 

 century, Klebs was the first to affirm that the cycle of develop- 

 ment of the plant can be changed at will by the interference of 

 man. According to his conception, the development of the 

 plant, as well as its other vital processes, are greatly influenced 

 by environmental conditions; and if these conditions are changed, 

 development can be changed, retarded or accelerated. Alter- 

 ations in environmental conditions act upon the processes of 

 reproduction not directly but through changes brought about in 

 the internal state of the plant. 



According to Klebs, one of the most important factors con- 

 tributing to the transition to reproduction is a sufficient light 

 intensity. If a plant is grown under conditions favorable in 

 all respects except for an insufficiency of illumination, blooming 

 will be considerably delayed and sometimes does not occur at 

 all. Klebs, for instance, grew, in diffused light, specimens of 

 Glechoma hederacea, which remained in a vegetative condition 

 for several years. But when the plants were placed in a stronger 

 light, they soon began to bloom. Such experiments have been 

 successful with other plants, also. 



