Correlation 103 



is well shown in the formation of reaction wood (p. 356). This wood (in 

 conifers) elongates faster than normal wood and thus tends to bend a 

 branch away from the side on which it occurs. The branches have a spe- 

 cific angle of orientation to the main axis, or to gravity, which is main- 

 tained by the development of reaction wood on one side of the branch. 

 If this normal orientation is experimentally altered, reaction wood will 

 appear at the precise place and in the precise amount elsewhere which 

 will tend to restore the normal branch pattern. The origin and character 

 of this pattern are the essential problem. What happens to any com- 

 ponent of it depends on the place that this occupies. 



There are many other examples of the operation of such developmental 

 patterns in the plant body. Among these are the studies of Dormer ( 1950) 

 on the development of xylem in different internodes of the young plant of 

 Vicia; of Friesner and Jones ( 1952 ) on the relation of primary and sec- 

 ondary branches in length growth; and of various workers on the struc- 

 ture of leaves borne at different levels on the stalk. Ashby and his 

 colleagues (1948) have emphasized the structural and physiological dif- 

 ferences among successive leaves along the axis and have related this to 

 the problem of aging. Instances of positional differences shown in topo- 

 physis (p. 212) are particularly clear and may become irreversible. 



The control that the organized whole exercises over its parts is some- 

 times termed "correlative inhibition." This term explains nothing but it 

 emphasizes the fact that inhibitory action is certainly involved. In the 

 physiology of development both inhibition and stimulation are important. 

 A number of workers, among them Libbert (1954, 1955), have discussed 

 the various interactions between substances which promote and those 

 which check the growth of buds. Thimann (1956) has called attention to 

 the fact that in most physiological processes there is a balance between 

 reactions tending to promote the process and others tending to inhibit it. 

 No single factor is solely responsible, but physiological activities, includ- 

 ing those of development, are often under multiple control. Furthermore, 

 certain factors such as auxin may stimulate under certain conditions and 

 inhibit under others. 



Some students of development are therefore inclined to look on the 

 growing organism as the seat of constant competition between different 

 and distinct processes, a state of equilibrium between opposing forces. 

 This resembles the concept of the organism as a balance between distinct 

 cellular individuals each with specific tendencies of its own. It also calls 

 to mind the older idea of the "battle between the parts" as the basic fact 

 in development. Analysis of the structures and the activities that go to 

 make up an organism gives some support to this interpretation of devel- 

 opment. The close correlations that are everywhere present in develop- 

 ment, however, and particularly the persistent tendency toward regula- 



