104 The Phenomena of Morphogenesis 



tory action by which a specific norm or pattern of form and function is 

 restored if disturbed, are difficult to explain on the basis of independent 

 action by many variables. The organism more closely resembles an or- 

 ganized army under disciplined control than it does a mob where each 

 individual acts competitively for himself. 



The balance between stimulation and inhibition, however, is worth 

 careful study by students of morphogenesis. In a few cases it has been 

 investigated in the lower plants. In the coenocytic alga Caulerpa, for ex- 

 ample, the "assimilators" (leaves) produce strong growth inhibition but 

 the rhizoids have the opposite tendency (Dostal, 1945). The balance 

 between the influences of these two sets of organs has an important effect 

 on the character of the plant as a whole. 



More favorable material for a study of this aspect of correlation is 

 found in the over-all form of the plant body, especially in such higher 

 plants as trees. A tree is a rather loose aggregation of axes which usually 

 does not show as precise a form as does an individual organ such as a 

 leaf or a flower but which, nevertheless, is characteristic and recognizable. 

 This has been found to result from an interaction of factors in the ter- 

 minal buds and in the growing tips of the branches. Some of these factors 

 tend to push the branches down, in relation to the main axis, and others 

 tend to lift them up. The relative length of branches and main axis is also 

 similarly controlled, evidently by domination of the terminal bud over 

 those of the lateral branches. Munch (1938) has discussed the diverse 

 tendencies that govern such tree form in conifers and interprets these in 

 terms of hormonal action, but he emphasizes the harmony and balance 

 that exist among them. Others (Snow, 1945) have considered the problem. 

 It is a basic one for morphogenesis since the form of the plant body as a 

 whole, although relatively variable, is nevertheless a true organic one. 

 Presumably the factors that govern it resemble those that bring about the 

 much more constant and specific forms of the separate organs. The body 

 is an aggregation of these parts, less tightly organized than are its indi- 

 vidual organs, but clearly showing organization. The beginnings of organ- 

 ization and of the emergence of those correlations that determine form 

 may profitably be studied in these plant bodies, which in a sense are in- 

 termediate between colonies of semi-independent parts and true organic 

 individuals. 



GENETIC CORRELATIONS 



Organized bodily patterns, with their localized differences and specific 

 characters, are examples of physiological correlation though the mecha- 

 nisms involved are obscure. They doubtless have some genetic basis. Many 

 other growth correlations, including those concerned in the form of plant 



