332 PLANT GROWTH SUBSTANCES 



to be in order (78). Attention should be called here to the reduction of 

 boron deficiency symptoms, necrosis of the apical meristems of plants 

 that have been made reproductive by short photoperiods (91,82). 

 Perhaps it may be considered as circumstantial evidence that, with the 

 onset of reproduction, terminal meristematic growth is not only cur- 

 tailed but usually inhibited. 



Flower Development 



Flowers having been initiated, their further development and function 

 would seem to depend upon many internal and external environmental 

 factors. Anyone who has made a study of the growth of flowers is aware 

 of the fact that drastic changes in the environment, such as, for example, 

 marked shift in the photoperiod or temperature, may cause conspicuous 

 changes or even abortion of flower buds, flowers, or parts thereof. 



The following stages in flower inception, development, and function 

 may be recognized (55) : i) Terminal meristems or genetically determined 

 loci where the floral hormone is received or synthesized. This is a stage of 

 "ripeness to flower" established physiologically. Commonly there are 

 far more such meristematic points than there are available hormone or 

 other indispensable substances. Many terminal meristems thereby are 

 eliminated early as flower producers. Devernalization and dedifl'erentia- 

 tion are well-established phenomena (72,63). 2) Early floral development. 

 If a relatively large number of flowers have started to develop, as seems 

 to be frequently the case, most of them may be eliminated because of 

 competition for supply of building material or specific catalytic sub- 

 stances or hormones. 3) Late floral development. Though reaching 

 anthesis, not all flowers are normal or can function properly (that is, 

 participate successfully in fertilization). Though appearing well de- 

 veloped, they may be abnormal in essential morphological and histo- 

 logical structures. Usually plants produce an enormous number of flowers 

 of which only a fraction form seeds (23,47,65). 



There is practically nothing known at present about the function 

 of hormones during various stages of flower development. Though their 

 multifarious forms and structures are determined genetically, most likely 

 hormones of various kinds participate in the production of specific 

 floral organs and tissues. 



