448 PLANT GROWTH AND PLANT COMMUNITIES 



material. Since pectic material appears to constitute the glue that binds 

 together the cellulose microfibrils and other components of the cell 

 wall, and since short-chain pectin molecules are less effective in this 

 function than long-chain molecules, one can feel intuitively that this 

 biochemical alteration should lead to cell wall softening. Exactly how 

 this is so is yet to be understood. So, too, is the enzymology by which 

 indoleacetic acid influences pectin metabolism. With what enzyme or 

 metabolic process does indoleacetic acid interact in pectin metabolism? 

 Although we are getting closer, we do not know. 



The second group of hormones with which I wish briefly to con- 

 cern myself are those having to do with flowering. A plant grows vege- 

 tatively for awhile, its bud producing leaf after leaf, and then suddenly, 

 upon receipt of an appropriate signal, the bud alters its behavior and 

 produces an entirely new structure: a flower and fruit. It becomes a 

 reproductive bud. The initiation of reproduction is elicited by different 

 kinds of signals in different kinds of plants. In the simplest case it may 

 consist merely in notice that an appropriate number of leaves has been 

 produced. This requisite number is promptly followed by production 

 of a flower bud. In other species the reproductive signal may consist in 

 cold treatment of the bud, as in the biennials and winter annuals, or in 

 appropriate day-length treatment of the leaf, as in the photoperiodi- 

 cally sensitive plants. In the photoperiodically sensitive plants, ex- 

 posure of a leaf or leaves to a long night, if the plant is a long-night 

 plant, or to a long day, if the plant is a long-day plant, results in the 

 sending to the bud of a specific evocator of floral differentiation. The 

 leaf, as the result of treatment with appropriate interplay of light and 

 dark, sends to the bud a hormone whose nature we do not know. This 

 hormone causes the bud to differentiate into a floral bud, and, as a mat- 

 ter of fact, in many species the bud becomes "induced"; that is, it 

 grows as a floral bud forever after, even though the leaves no longer 

 continue to transmit the floriferous signal. 



Although we do not yet know the nature of this hormone which 

 travels from leaf to bud to bring about floral development, we do 

 know a little bit about the processes that take place within the bud it- 

 self (Salisbury and Bonner, I960; Bonner and Zeevaart, 1960). We 

 know, for example, that the bud responds to the signal received from 

 the leaf by the production of some specific kind of ribonucleic acid, and 

 that if ribonucleic-acid synthesis in the bud is suspended, the bud can- 

 not perceive or act upon the stimulus sent by the leaf. Here again we 

 have to do with the timing of genie activity. The genetic material of a 

 plant contains all of the information requisite to the formation of the 

 floral structure, yet in the vegetative plant this information is not used. 

 The genes that contain it are inert— turned off. I suppose that when the 



