414 Morpho genetic Factors 



standing of the whole protoplasmic system. This is a far goal, but, as 

 Thimann (1957) has remarked, "It begins to look as though the whole 

 cell were necessary to auxin activity." 



But part of the living system in a plant evidently includes other 

 biologically active substances. A good deal is known about several of these, 

 and although they may not be "organ-forming" in the earlier sense of the 

 word, their share in the control of developmental processes is more 

 important than that of most other chemical compounds. A hopeful 

 method of attack on morphogenetic problems is to study the relationship 

 between these substances. It is now well known that there are com- 

 pounds which enhance or which inhibit the effects of auxin. Still more 

 promising are results such as those of Skoog and Miller ( 1957 ) on the 

 relationships between auxin and adenine (or its derivative, kinetin). The 

 presence of both seems to be necessary for vigorous growth, at least of 

 tobacco callus in culture. If the proportions of the two are changed, how- 

 ever, the character of the growth is different. Relatively high levels of 

 auxin and low ones of kinetin, either in cultures of tobacco callus or in 

 cuttings, will tend to produce good root growth but little bud develop- 

 ment, whereas high kinetin and low auxin levels favor growth of buds 

 instead of roots. To be sure, kinetin is a substance which has not yet 

 been found in the living plant, and its balance with auxin has been 

 demonstrated in only a few cases, but the picture this balance pre- 

 sents of the possible control of differentiation through alteration in pro- 

 portions within a relatively simple chemical situation opens up encourag- 

 ing possibilities. 



Other factors are doubtless concerned in these cases, and the problem 

 must involve more than a simple two-compound interaction, but the 

 idea that there may be a relatively small number of active but non- 

 specific substances, with the possibilities for complex interactions among 

 them that this offers, makes understandable how an essentially infinite 

 number of different structures might be produced without the necessity 

 of postulating the activity of substances specific for each of them. Only 

 12 different kinds of chessmen can produce, by their various relation- 

 ships, an almost limitless number of combinations on the board. 



The study of plant growth substances has been of great significance 

 and stimulation for morphogenesis, but it has done little more as yet than 

 pose a series of deeper problems. Chief among these are three: 



1. What is it that controls the distribution of growth substances as to 

 space, time, and concentration? 



2. What is it that determines the specific response which a given cell 

 or tissue makes to them? 



3. How do they interact in their control of development? 



These problems are part of the deeper one of biological organization. 



