416 Morphogenetic Factors 



actively investigated. The role of genes in the synthesis of enzymes and 

 other substances and thus in the determination of successive steps in 

 metabolic processes is yielding much information as to the relations 

 between genetics and physiology. Increasing knowledge of those re- 

 markable compounds, the nucleic acids, is leading to an understanding 

 of the chemistry of the gene and of the manner in which it reproduces 

 itself. Indeed, desoxyribonucleic acid ( DNA ) has such significant proper- 

 ties that some biologists hopefully believe that it will finally produce an- 

 swers for most of the basic problems of their science. All this, however, 

 has as yet thrown little light on how it is that genetic factors affect the size, 

 shape, and structure of plants and their parts. This is a much more diffi- 

 cult problem than working out the biochemical steps in the synthesis of an 

 organic compound produced in plant metabolism. How a single pair of 

 genes can determine, for example, whether a tomato plant will have the 

 familiar deeply lobed leaves or the unlobed "potato-leaf" type is very 

 difficult to see. Here something more than a series of chemical steps seems 

 to be concerned. Growth relationships are being controlled, and at present 

 we must admit that very little is known about how such control is exer- 

 cised. A solution of this problem must start with a knowledge of what 

 actually happens in the inheritance of form and structure. To make such 

 a descriptive analysis and at the same time seek hopefully for clues that 

 will lead to a knowledge of the mechanisms involved is the chief task at 

 present of the student of morphogenesis who is interested in the genetic 

 aspects of his science. 



Genes and Growth. The underlying problem in growth is the increase 

 of living substance, due ultimately to the reduplication of genes. How this 

 is accomplished and how the DNA molecule divides into two new ones 

 like itself are now beginning to be understood. Traits of size, either of body 

 or organ, are markedly affected by environmental factors, but there is also 

 a genetic basis for most of them. Since the pioneer work of East and 

 Nilsson-Ehle it has been recognized that most quantitative traits depend 

 not on single genes but on a series of multiple factors or polygenes, cumu- 

 lative in their effect and in most cases without dominance. Such traits are 

 difficult to analyze genetically since it is rare that the effect of individual 

 genes can be followed, though there are statistical methods for determin- 

 ing the number of genes by which two individuals differ for a given trait. 

 That polygenes are operative in quantitative inheritance is indicated by 

 the fact that the variability of the F 2 is markedly higher than that of the 

 parents or the Fi, as would happen if segregation were taking place. 

 There is now a substantial body of evidence that confirms the multiple- 

 factor hypothesis. 



In a few cases the inheritance of size is not so complex, and the effect 

 of individual genes can be traced. One of these, vine height in peas, was 



