BOTANY AND MORPHOGENESIS 12 5 



A wide field for morphogenetic research is open through experimental ma- 

 nipulation of the environment. Plants provide particularly favorable material 

 for studies of this sort, for a plant, as a result of its fixed habit, is much more 

 sensitive to environmental factors than are most animals, since the latter can 

 in some degree choose or modify their surroundings. We must recognize that 

 there is not only an external environment but an internal one, and that 

 changes in the latter, as in water content, acidity, electrical potential, and 

 chemical substances, may have important effects on development. These inner 

 factors are part of a highly organized system that exists in the midst of a 

 much less organized environment. 



The effect of a given factor on an organism is a much more complex process 

 than it is on a lifeless object. In the latter the factor produces a direct and 

 predictable result, as when water moves a wheel or light modifies a photo- 

 graphic plate. In a plant or animal, however, the factor acts as a trigger, or 

 stimulus, which sets off a response. This response is the result of regulatory 

 reaction by an organized living system, and since the nature of this system 

 is not fully understood, the response is sometimes difficult to forecast. It is 

 the presence of this organized, self-regulating system that makes experiments 

 with living things so much more complex than with lifeless material. 



The morphogenetic effects of various internal and external factors are often 

 hardly to be distinguished from their physiological ones, and much of the 

 knowledge that physiology obtains about biochemical and biophysical proc- 

 esses and protoplasmic activity has made important contributions to our 

 knowledge of developmental processes. This has been particularly evident in 

 the physiology of light effects, water relations, and growth substances. 



For convenience one may classify environmental factors into physical ones, 

 chemical ones, and a third group — genetic or biological — which have not been 

 analyzed into simpler terms. 



Among the first factors to be examined was water. The effects of a shortage 

 of this essential substance in the production of xeromorphic structures was 

 early recognized. These were at first thought to be adaptive in character. This 

 is doubtless true for such as are inherited and have proved their value in 

 selection. There is evidence, however, that the xeromorphy which results from 

 shortage of water during development, either because of its lack in the en- 

 vironment or the inability of a given plant to obtain it, is due to a direct 

 effect on plant structure and is without much adaptive significance. This effect 

 is shown chiefly in reduced cell size and increased deposition of cellulose. 

 Studies in water relations are not as common as they used to be in plant 

 physiology, but their contribution to developmental problems is far from 

 exhausted. 



Light, a factor of peculiar importance in plants because of its role in 

 photosynthesis, has implications for development that were unexpected in 

 earlier years. The influence of the photoperiod on flowering has made possible 



