Growth Substances 413 



with these qualities. Northen (1942) observed that auxin usually de- 

 creases viscosity of protoplasm. He regards protoplasm as a "reversibly 

 dissociable-associable system," in which auxin (and other agents) 

 cause dissociation of cellular proteins and increased swelling pressure. As 

 the result of such action, it is thought that components of the fine struc- 

 ture of the cell may undergo reorientation and the reactivity of the 

 cell may be changed. 



Of greater morphogenetic interest is the direct relation of growth sub- 

 stances to the development of organs or structures. The situation here 

 is even less clear than in the control of growth. In a number of cases one 

 is faced with a curious antithesis between the action of auxin in dif- 

 ferent situations. For example, at a given concentration it stimulates the 

 growth of stem tissue but inhibits that of the primary root, with the 

 result that differential geotropic bending occurs. It stimulates the de- 

 velopment of root primordia and hence is useful in the rooting of cut- 

 tings, but it checks the elongation of the roots after their emergence. In 

 some cases its effect is to accelerate flowering and in others to inhibit it. 

 Sometimes it prevents bud growth and sometimes it stimulates this. In 

 certain cases its effect is to stimulate the growth of roots rather than buds 

 but in other cases it has just the opposite influence. 



In this confusing situation the hypothesis of specific organ-forming sub- 

 stances appeals to many, especially those who seek direct and primarily 

 chemical solutions to morphogenetic problems. To be sure, development 

 often does seem to be the result of the action of such substances, as in the 

 formation of roots, flowers, and abscission layers. But where, one may 

 ask, does this specificity end? In the flower, are there separate substances 

 for sepals, petals, stamens, and ovaries, for anther and filament, style and 

 stigma? Does each tissue and each type of cell have its appropriate 

 "caline"? It is easy to reduce to absurdity the more naive statements of 

 this hypothesis. 



To what, then, can one attribute the highly specific results of plant 

 development? One answer is that the specificity lies in the protoplasmic 

 system rather than in the growth substance and that the latter serves 

 primarily as a trigger or evocator that calls out a specific response. We 

 should remember that auxin, the substance about which most is known, 

 is markedly nonspecific. A few such biologically active substances, 

 stimulating responses from a highly organized protoplasmic system, 

 might account for development. A dime, it has been said, will open 

 a turnstile, activate a dial telephone, or bring a tune from a juke box, but 

 the dime, like a molecule of auxin, is identical in every case. The dif- 

 ference lies in the complexities of the responding mechanism. The answer 

 to morphogenetic problems is more difficult to come by on this conception 

 than on that of specific formative substances since it involves an under- 



