738 BIOLOGICAL EFFECTS OF RADIATION 



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ultra-violet region. Light of wave-lengths greater than 5700 A evidently 

 has very little action in inhibiting growth. 



Triimpf (114) illuminated leaves and petioles of the bean plant 

 w^hile the epicotyl remained in darkness. The leaves developed like those 

 of fully illuminated plants, while the epicotyl developed like that of a 

 completely etiolated plant. Similarly, illuminating the epicotyl had no 

 effect on the leaves. When one portion of a leaf or epicotyl was illumi- 

 nated while another portion remained in darkness, the portion in darkness 

 developed somewhat similarly to that in the light. The light effect, 

 therefore, is local in its action and cannot be transferred from an irradi- 

 ated organ to one kept in darkness. From his experiments it is possible 

 to conclude that the action of light on one part of an organ may affect 

 other parts of the same organ, but whether or not this actually occurs 

 cannot be detected without more refined methods of examination than 

 he used. 



Coupin (25) reported that lentils germinated in darkness in a hori- 

 zontal position continued to develop horizontally, i.e., they showed no 

 response to gravity. Since this plant produces runners which grow 

 horizontally, he assumed that the stem in darkness takes on the form 

 of a runner. Whether this actually occurs is open to considerable 

 question since weakness of the stem, injurious effects of illuminating 

 gas, or other factors may have influenced his results. 



THEORIES AS TO THE CAUSE OF ETIOLATION 



Numerous theories have been advanced by early workers to explain 

 etiolation, many of which are discussed by MacDougal (65). Sachs 

 considered etiolation to be pathological, although etiolated plants become 

 normal in light. Kraus explained the failure of leaves to grow in darkness 

 by postulating that they could use for growth only substances manu- 

 factured locally, Godlewski assumed etiolation to be a response of the 

 plant to overcome temporary obstructions. Palladin thought that 

 etiolation might be caused by the lack of sufficient transpiration in dark- 

 ness. MacDougal attributed to light a morphogenetic influence which 

 causes the differentiation and development of the various plant organs. 

 Without light, development is arrested. He thought of light as having 

 a stimulative action which does not need to act directly upon a partic- 

 ular tissue but could be transmitted from the illuminated parts to those 

 held in darkness. 



Coupin (24) added to the cultural solution in which etiolated plants 

 were growing a sterilized and filtered extract from green plants. Such 

 an extract inhibited stem elongation of the etiolated plants more than 

 a similar extract prepared from etiolated plants. He concluded, there- 

 fore, that plants develop a substance in light which determines their 



