450 PLANT GROWTH AND PLANT COMMUNITIES 



ment before it becomes effective. Here again, then, response to climate 

 is written in the genetic language. The very control of form and shape, 

 mediated as it is by the plant hormones, is mediated in accordance with 

 the instructions contained in the plant's genetic material. As another 

 example, take the dwarf mutants of corn, of peas and beans, and of 

 other species. These are dwarfs because they possess genetic informa- 

 tion telling them to make less gibberellin and thus be less august in 

 stature than their normal sibs. When we get right down to the nub of 

 the matter, the biology of plant growth and the control of plant growth 

 and development centers in the cell— in fact, the nucleus. 



The plant cell, like other cells, contains mitochondria to conduct its 

 respiration and to assure a supply of available metabolic energy; it 

 contains ribosomal particles made of RNA and protein for the syn- 

 thesis of the manifold species of enzyme molecules required to produce 

 the many different metabolites essential to plant well-being; and, most 

 importantly, it contains a nucleus full of DNA. The genetic material 

 consists of genes, each gene bearing information on how to make a 

 specific kind of enzyme molecule. The genes made of DNA can repli- 

 cate in the manner about which we have heard so much in recent years. 

 The nucleus, too, and apparently the genetic material of the nucleus, 

 can produce ribonucleic-acid particles which contain bits of the genetic 

 information printed off in RNA form— information on how to make this 

 or that specific species of enzyme molecule. We sum up our knowledge 

 and our guesses by saying that cells behave as though the DNA makes 

 the RNA and the RNA makes the enzymes. Some enzymes make the 

 building blocks for making more enzymes. Other enzymes make build- 

 ing blocks for making more RNA. And still other enzymes make the 

 building blocks that must be present if the DNA is to replicate itself 

 and thus to permit of cell division and growth. In the operation of the 

 cell we glimpse the logic of life. 



But as we get our first glimmering of the manner in which cells 

 operate, we are at once confounded with a new and puzzling question. 

 This has to do \\ith the programming of the use of the genetic informa- 

 tion. We have seen clearly that not all of the genes of our plant are 

 turned on all of the time. The genes for making flowers, for example, 

 are turned off until our plant receives the signal for flowering. The 

 genes responsible for making some particular hormone are turned off in 

 all cells except those charged with making hormone. The inescapable 

 conclusion is that not all genes make their characteristic RNA, their 

 characteristic ribosomes all of the time. There would appear to be 

 some further part of the cellular system which controls the activity of 

 the gene within the nucleus and which determines whether or not a 

 given gene may produce its characteristic RNA, its characteristic ri- 



