598 FACTORS AFFECTING GROWTH 



very low intensity is sufficient to prevent the development of any pronounced 

 earmarks of etiolation. When seedlings grow in very weak light the leaves 

 expand and synthesize chlorophyll and the internodes do not elongate as much 

 as on similar plants growing in the dark, although the plant will usually show 

 a more attenuated development than in stronger light. Even a short tempo- 

 rary exposure to light often results in the development of a much more nearly 

 normal configuration in plants which are subsequently returned to a dark 

 room (Priestley, 1925, 1926). This suggests that substances of the general 

 nature of hormones are synthesized in plants under the influence of light, and 

 that it is the absence of such substances which permits the development of 

 the characteristic symptoms of etiolation. This supposition is supported by 

 the fact that the elongation rate of seedlings, including those parts which 

 remain underground, is immediately checked upon emergence of the hypocotyl 

 or plumule into the light. 



The attenuated growth of the leaves (monocots) and stems (dicots) of 

 etiolated plants appear to be due chiefly to an increase in the length of their 

 component cells as compared with cells of similar tissues developed in the 

 light. Some of the increased length of most etiolated plant organs is also due, 

 however, to more frequent cell divisions in organs growing in the absence 

 of light than in those which are illuminated. 



The phenomenon of etiolation indicates convincingly that light exerts a 

 retarding effect upon the enlargement phase and probably also on the cell 

 division phase of growth. Maturation, on the other hand, appears to be 

 favored by exposure to light. 



The retarding effect of relatively high light intensities upon growth, how- 

 ever, is usually at least in part a temperature and desiccation effect. High 

 light intensities favor rapid transpiration rates which in turn almost invariably 

 i result in a reduction in the water content of the plant. Reduced hydration of 

 plant cells, if at all marked, always causes a retardation of cessation of the 

 division and enlargement phases of growth. 



For a more comprehensive picture of the effects of different light intensi- 

 ties upon growth we must turn to investigations such as that of Popp (1926) 

 on soy beans. He grew plants of this species under six different light inten- 

 sities averaging 4285, 1536, 560, 390, 250, and 26 foot candles respectively. 

 While during the initial period of growth the rate of stem elongation varied 

 inversely with the light intensity, when growth was considered for a 7 weeks 

 period a somewhat different relation was found to hold. For such periods the 

 greatest height was attained at intermediate light intensities, in this particular 

 experiment at 560 foot candles. The thickness of the stems was found to vary 



