BOTANY AND MORPHOGENESIS 12 7 



in various ways. What this reaction will be is determined by the genetic 

 constitution of the plant itself. A gene does not control a specific trait but a 

 specific reaction to a specific environment. We thus must approach the prob- 

 lems of morphogenesis from a study of the inner genetic mechanisms as well 

 as the outer factors to which it reacts. 



The role of genes in the control of various metabolic processes is beginning 

 to be understood. Each seems to mediate a particular process — perhaps by 

 the production of a particular enzyme — in a successive series of processes. 

 How genes control development, however, is far more difficult to understand 

 and is the least explored field of morphogenesis. Gene action must evidently 

 control relationships between parts or processes. There is good evidence that 

 the inheritance of form is independent of that of size and that what is in- 

 herited is not particular dimensions but particular growth ratios or patterns. 

 For studies of form inheritance the rather rigid structures of plants, notably 

 leaf, flower, and fruit, provide particularly favorable material. 



There are other points of contact between genetics and morphogenesis. 

 Multiplying the chromosome complement through ploidy has various effects 

 on development. It tends not only to increase cell size but often organ and 

 body size. It also characteristically increases transverse dimensions more than 

 axial (polar) ones and thus affects the shape of leaves and fruits. Heterosis, 

 though its genetic basis is still not fully understood, also affects development 

 profoundly. Both polyploidy and heterosis are commoner and easier to study 

 in plants than in animals. 



Plants have the further advantage that tissue mixtures of various sorts 

 can be made easily between diverse forms which are sometimes rather widely 

 separated taxonomically. The mutual effects of stock and scion have long been 

 studied. It is chimeras, however, and especially periclinal ones, that are of the 

 most importance here. In the latter, tissues from plant tj^es differing in the 

 size and form of their organs may be brought together in a most intimate 

 fashion, and in varying proportions, to form essentially normal plants. The 

 morphogenetic effect of each component can here be readily observed. 



Such are some of the more important of the problems of morphogenesis 

 as these are approached through studies with plants. It is clear that many 

 of them are simpler and more readily open to investigation with botanical 

 materials than by other means. We can be proud of the accomplishments of 

 botanists in this field in the past, and there are prospects of much fruitful 

 research in the years to come. 



With whatever material one works, the problems of morphogenesis present 

 extraordinary difficulties. One deals here with the central and most distinc- 

 tive feature of living things — organization and self-regulation. He must 

 never underestimate the magnitude of the task that confronts all explorers 

 in this difficult realm. The great advances that have been made in many 

 aspects of biology in recent years, particularly in metabolism, should not 



