EFFECTS OF LIGHT INTENSITY 755 



this subject tlie reader is referred to Zon and Graves (123) and to 

 Riibel (86). The importance of factors other than Hght in determining 

 the estabhshment and growth of vegetation under natural canopies is 

 stressed by Fricke (35), Toumey (110), Toumey and Kienholz (111), and 

 Fabricius (32, 33). Furthermore, the ecologist and forester are in reality 

 much less concerned about the minimum amount of light required to 

 keep a plant alive than the amount required for satisfactory growth, 

 since the plant which is living under light conditions sufficient only to 

 maintain life may soon succumb to other unfavorable conditions. 



A correlation between light intensity and the occurrence and growth 

 of various species of plants can be established. Recent studies of this 

 type have been conducted by Gast (38), Atkins and Poole (3), Atkins and 

 Stanbury (4), Atkins (2), Holch (50), and Shirley (96). In practically all 

 cases maximum growth is attained in full sunlight. Similar studies have 

 shown correlation between light intensity and the distribution and 

 growth of water plants (70, 87, 14). 



SUMMARY 



Solar radiation is highly variable in intensity and quality, but no 

 artificial source of radiation gives light comparable in intensity and qual- 

 ity with midday sunlight in summer. In studies of the effects of radiation 

 on plants it is important to have as careful control and measure of the 

 other factors affecting plant growth as it is possible to obtain. 



Since green plants are dependent upon light for the synthesis of their 

 food supply, any attempt to cultivate them without light, or in light of 

 too low intensity for maximum photosynthesis, upsets their nutritional 

 economy. In order to have a true understanding of the influence of light 

 and lack of light on plants, it is necessary to distinguish between those 

 effects due to starvation and other effects caused by radiation. 



Plants grow^n without light have attenuated stems, small leaves only 

 partially unfolded, no green pigment, and only weakly differentiated 

 tissue. They give the impression of plants whose development has been 

 arrested. Etiolated stems, particularly the apical regions, are rich in 

 nitrogenous compounds and relatively poor in carbohydrates, even 

 when ample carbohydrates may still be available in the seed or root. 

 The effect of light on etiolated plants depends upon the quantity of light 

 received, and is local in its action. The true nature of etiolation is not 

 fully understood but it is probable that the nutritional disturbance, 

 caused by the impermeability in darkness of the protein and fatty sub- 

 stances comprising the protoplasm of meristematic tissue, plays a domi- 

 nant role in this phenomenon. 



Light has a very pronounced effect upon the rate of elongation of 

 shoots of juvenile plants. When etiolated plants are exposed to light, 



