390 Morpho genetic Factors 



apical-bud dominance. Active growth of lateral roots may also inhibit 

 apical growth (Street and Roberts, 1952). 



Correlations of position may be the result of auxin activity in many 

 cases. Thus where an upright terminal shoot of a coniferous tree is re- 

 moved, one of the lateral branches will swing up from its nearly horizontal 

 position to a vertical one and replace the lost leader, evidently in re- 

 sponse to the absence of auxin previously produced by the apical bud. In 

 woody plants the orientation of branches with reference to the main 

 axis and to gravity also seems to be due to auxin action since it is regu- 

 lated by the production of reaction wood ( p. 356 ) , which seems to result 

 from the presence of auxin. The precise amount of auxin (and thus of 

 reaction wood formed) determines the angle that a given branch 

 will assume and thus the branching pattern and form of the whole tree. 



The form of individual organs, ultimately the result of dimensional 

 correlations, may be affected by growth substances, particularly in leaves 

 (von Denffer, 1951; Linser and others, 1955; Wenck, 1952; Fig. 18-15). 



Such correlating activities are doubtless present in the lower plants as 

 well. Moner (1954) describes the action of a substance, as yet unidenti- 

 fied, which is concerned with the development of the precisely formed 

 colonies of the alga Pediastrum. 



Much evidence is therefore available that the correlated and integrated 

 character of the plant, whatever its final cause may be, is the immediate 

 result of specific amounts of growth substances at specific places and 

 times. What controls this precise production and distribution of these 

 substances is a more difficult problem. 



GROWTH SUBSTANCES AND THE DETERMINATION 



OF STRUCTURE 



The effect of growth substances on the specific form and structure of 

 plants has attracted more attention than any other of their morpho- 

 genetic activities. 



Tropisms and other auxin-mediated orientations of plant parts to cer- 

 tain factors in the environment, notably gravity and light, account for 

 many features of external form, though the familiar patterns of plant 

 growth are produced by interaction between these tropisms and certain 

 specific inner factors. Sometimes a simple tropism may produce such 

 a profound change in plant form as to be significant morphogenetically. 

 In "lazy" maize (van Overbeek, 1936), for example, the stalk grows 

 flat on the ground, not through mechanical weakness but because of the 

 characteristic distribution of auxin in it. Tropisms are primarily the 

 concern of physiology, however, and there is no room here to consider 

 the extensive literature in which they are discussed. 



