352 TEXTBOOK OF PLANT PHYSIOLOGY 



duce blossoms before the age of 40 to 60 years is attained, and some 

 lindens, maples and larch trees will flower at the age of 20 to 30 

 years. What causes prevent them from fruiting at an earlier 

 age is not yet clear. But it is possible to accelerate flowering 

 considerably by various methods. When, for instance, plants are 

 grown frcm cuttings taken from mature trees, they will start 

 many years earlier. The same is true in cases of grafting, espe- 

 cially on dwarf stocks. Flowering and fruiting of trees growing 

 in open spaces likewise occur several years earlier than in those 

 growing in a dense forest. In this case, it is undoubtedly due to the 

 effect of a more intensive illumination and a greater dryness of 

 the air. 



The fact that great changes occur in the different parts of a 

 plant at the time of blossoming and that the nature of its further 

 development is also somewhat altered, is of great interest and 

 importance. Sachs (1883) observed that cuttings obtained from 

 blooming specimens of the Begonia and other plants blos- 

 somed very soon, even before the plant had time to develop suf- 

 ficiently; while similar cuttings taken from plants that were not in 

 bloom produced for a long time nothing but vegetative growth. 

 This phenomenon he explained by the fact that a certain "flower- 

 forming substance" accumulates in the plant during the period of 

 blooming. This substance then spreads throughout the organs of 

 the plant. Consequently, when a leaf from such a plant is used 

 for propagation purposes, flowering will occur very early. 



Sachs's results have been repeatedly confirmed by other investi- 

 gators. His hypothesis with regard to the special flower-forming 

 substances has not been readily acknowledged. Since it has been 

 established lately that many growth processes both in plants and 

 animals are governed by specific chemical regulators, a number of 

 investigators are falling back to the old belief advanced by Sachs 

 and now seem to accept the idea that the essential cause, which 

 leads to blooming, should be attributed to an accumulation of a 

 specific hormone, all the other external and internal factors merely 

 either contributing to or delaying this accumulation. 



102. Physiological Processes Accompanying Fertilization and 

 Embryo Development. — The process of fertilization and the 

 subsequent development of the embryo have been studied in great 

 detail, both morphologically and cytologically. The physiology of 

 the process, on the other hand, has been very little investigated. 



