FLOWERING HORMONE MOVEMENT AND ACTION 197 



in structure which are so evident to us? Virtually all living things 

 begin their lives as a single cell. This cell divides, and the daughter 

 cells produced at each division are usually almost identical. Such a 

 process is marvellous in itself, but if this were the end of the story, 

 only a mass of cells might be produced (which is what happens in 

 the case of many tumors including cancer). 



The other phase of the growth process is specialization, or differen- 

 tiation, or morphogenesis. Cells specialize individually and in groups. 

 Tissues become discernible, and soon we can see the organs which 

 make up the organism itself. In the case of indeterminate plant 

 growth this cell division and differentiation continues indefinitely. 

 Cells at the tips of the stems remain embryonic, continuing to divide. 

 Farther down the stem, cells are specializing into specific tissues such 

 as pith, vascular elements, cortex, and epidermis. The epidermal and 

 cortical cells further divide, producing small swellings which will 

 eventually grow into leaves and axillary buds. 



When the plant becomes reproductive, all of this changes. Leaves 

 are no longer produced, but the reproductive organs begin to form. 

 The flower parts appear to be modified leaves, but they are modified 

 very extensively and their position on the stem is changed quite 

 radically. The parts of the flower are all compressed together rather 

 than being strung out at the nodes as are the leaves on caulescent 

 plants. A new set of genes seems to be in control of things. All of 

 this seems to occur in response to the arrival of the flowering hormone. 

 How does it all work? How can a chemical entity (the flowering 

 hormone) control the shape and structure of a three-dimensional 

 organ such as a petal or stamen or pistil ? 



After years in which virtually no interest was shown in this 

 fundamental phase of the flowering process, a few researchers are 

 finally beginning to address themselves to the problem of what is 

 happening in a morphological and in a biochemical way when the 

 meristem is transformed from the vegetative to the reproductive 

 condition. 



1 . Work with Applied Antimetabolites 



As mentioned in the last chapter, the effects of certain anti-nucleic 

 acid compounds seem to be in the bud rather than in the leaf. 

 Zeevaart and James Bonner (38, 42) at Pasadena have shown that 

 5-FU is effective only when it is applied to the cocklebur bud during 



