68 



PLANT GROWTH SUBSTANCES 



a synthesis could be attempted. Yet a number of investigators, among 

 whom Sachs, Beijerinck, and Darwin should be mentioned, tried to view 

 the plant not only as an agglomeration of diverse reactions but as an 

 organism. Thus they came to an appreciation of interrelationships be- 

 tween different parts. Darwin (7), for example, found that in grass 

 seedlings and in roots the response to light and gravity is produced by a 

 bending of the zones several millimeters below the tip of the organ, yet 

 that the tip itself is essential to the execution of the curvature. His 

 clear mind saw immediately the implications of this behavior, namely, 

 that some sort of connection between tip and responding region of the 

 organ existed. To make this behavior clear to his readers, he drew some 

 parallels with animal behavior, which caused a storm of protests. Thus the 

 essence of the phenomenon itself was overlooked by fighting about its 

 wording. Certainly his tentative suggestion that a substance brought 

 about this correlation was never taken seriously. 



Whereas Darwin came upon the idea of links between different parts 

 of a plant through a study of tropisms, Sachs (18,19,20,21) hit upon the 

 existence of chemical messengers through his studies of the flowering 

 behavior of begonias and squashes, and the rooting responses of cuttings. 

 He clearly showed that in addition to ordinary nutrition, which makes 

 nongreen parts dependent upon leaves, more specific responses were 

 evoked by leaves, and through clear reasoning he came to the conclusion 

 that minute amounts of chemicals, moving polarly through the plant, 

 were responsible for differentiation of roots and flower parts. 



Beijerinck (2) came to a similar conclusion based on entirely different 

 facts. He had observed that galls were produced by extremely minute 

 quantities of substances given off by the developing larvae of gall insects, 

 or in one specific instance by the mother insect while it laid its egg. He 

 coined the name "growth enzymes" for such substances, because their 

 effect did not seem to depend on their quantity but rather their quality. 

 Both Sachs and Beijerinck made some abortive experiments to extract 

 their correlation carriers or growth enzymes. But these arguments were 

 sufficient by themselves to indicate their existence, according to our 

 present-day views. In those days, more than half a century ago, their 

 arguments were not considered vaUd, and even in the present century 

 much experimental effort was expended in the disproval of Darwin's, 

 Sachs', and Beijerinck's views. 



