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one of the most direct means of attack on the problem of differentiation. These 

 studies have told us much not only about the relative development of repro- 

 ductive as opposed to vegetative structures, but about the differentiation of 

 other characters, as well. The direct effect of differences in the intensity and 

 the quality of light on plant structure has also provided important morpho- 

 genetic information. 



Temperature has been found to be of importance in speeding up or slowing 

 down early stages in growth and thus affecting the character of the develop- 

 mental cycle. The concept of phasic development, of changing physiological 

 states during growth, has largely grown from studies of temperature effects. 

 These are closely concerned with the effects of light and of growth substances. 



Mechanical factors, such as tension, compression, and swaying during 

 growth, have been found to affect plant structure in various ways. They 

 seem to be particularly effective on cambial growth. A tree that is freely 

 swayed by the wind, for example, will grow considerably thicker than one 

 which is prevented from swaying. Sometimes the influence of mechanical fac- 

 tors is direct, as in the determination of the plane of cell division, but oftener 

 it is apparently indirect and simply influences the distribution or operation 

 of other factors. 



The field of bioelectricity is an extraordinarily difficult one, but enough 

 substantial results have been obtained to make sure that it is importantly 

 concerned in many developmental processes. This is what we should expect 

 from our knowledge of animal physiology, but the advantage of similar work 

 with plants is that their effects are simpler and not complicated by the pres- 

 ence of a nervous system. 



One of the active and fruitful fields of research in plant morphogenesis is 

 a study of the effects of chemical substances. The role of various individual 

 elements in plant development has long been examined, especially in con- 

 nection with agricultural practices. More important is the ratio between 

 some of them, notably between carbohydrate, on the one hand, and water 

 and available nitrogen, on the other. This ratio affects the relative develop- 

 ment both of vegetative to reproductive structures and of root to shoot. 



Still more significant are the various growth substances. Auxin is known to 

 be involved in almost all the developmental processes of plants — cell division, 

 cell enlargement, cambial activity, polarity, differentiation, and dominance. 

 Other normally occurring hormone-like substances are concerned with wound 

 healing, the induction of flowering, reproductive processes in algae and fungi, 

 and other activities which have important implications for morphogenesis. 

 Of only a few are their precise chemical nature and action known. A wide 

 variety of synthetic substances provide valuable means for experimental work. 

 A study of the role of growth substances is now the most active one in 

 morphogenetic research. 



To the environmental factors which have been mentioned the plant reacts 



