8 THE FLOWERING PROCESS 



5. Morphogenesis or the Origin of Form 



The final aspect of the flowering process which we will consider is 

 the transformation of the meristems from the vegetative to the 

 reproductive condition. In the cocklebur and perhaps most plants 

 which are sensitive to photoperiod, the flowering hormone is trans- 

 located from the leaf to the shoot tips, where it causes this redirection 

 of growth. The change seems to begin essentially at the moment 

 when the hormone arrives, and the subsequent rate of development 

 of the flower buds is proportional to the amount of hormone which 

 reaches the meristems. 



It could well be that this aspect of the flowering process has the 

 most fundamental biological significance. When we think of the 

 nearly infinite variety of biological structures, the origin of form takes 

 on considerable interest. Here is the real essence of the relationship 

 between diversity and uniformity in biology. Our observations have 

 convinced us that morphogenesis follows essentially the same pattern 

 in all living things : cells divide, enlarge, and then speciaHze (differen- 

 tiate). The secret of diversity in the resulting tissues, organs, and 

 organisms must lie in the differentiation step. During growth the 

 cells are specializing in specific ways that will result in special final 

 organized forms or structures. The degree of coordination of this 

 process is truly fantastic. Only cancerous growth and the occasional 

 monster seem to have escaped this coordination. Since morphology 

 is an inherited trait, all of this coordination and final structure is 

 under control of the genes. 



In flowering we have an excellent situation for study of this 

 phenomenon. The shoot tip carries out the intricate steps of morpho- 

 genesis which produce stem and leaves, with branches and their shoot 

 tips in the leaf axils. Upon arrival of the flowering hormone all of 

 this changes. The complex flower, with a highly specific form for 

 each kind of plant, is now produced. It appears that the genes which 

 ultimately control the production of leaves and elongated stem are 

 turned off, and the genes for flowers are turned on. Since the flower 

 parts may be thought of as modified leaves, it seems likely that only 

 some of the first set of genes are turned off", but obviouslv some new 

 ones are turned on. And all of this takes place in response to our 

 chemical substance, the flowering hormone. If morphogenesis in 

 general is a response to chemical substances, study of the flowering 



