EFFECTS OF LIGHT INTENSITY 745 



disadvantage in meeting the excessive transpiration losses. In this 

 case, the smaller, thicker, cutinized leaves are an advantage. Also a 

 contraction in the volume occupied by the plant is advantageous. 



REVIEW OF SPECIFIC INVESTIGATIONS 



Beyond the juvenile stage the development of tissue in green plants 

 under different light intensities depends upon the amount of carbo- 

 hydrates produced. The development of dry weight is directly cor- 

 related with photosynthetic activity. The effect of light intensity upon 

 photosynthesis is considered separately by Spoehr and Smith (Paper 

 XXXI) and will not be taken up here. 



Production of dry matter in plants is dependent on many other factors 

 than light, hence experiments on the effect of Ught on the production of 

 dry matter have failed to establish a definite proportionality. 



In a series of experiments on the effects of light intensity on plant 

 growth Lubimenko (62) grew plants in small cages which were pro- 

 vided with shades. Although temperature and humidity control were 

 impossible, the variations in these factors, while important, were evi- 

 dently not large enough to vitiate the results. His light values were 

 given in terms of the amount which would pass through glass 5 mm. in 

 thickness. Intensities varied from the amount of light which would pass 

 through one layer of glass to the amount which would pass through 

 54 layers. The production of dry matter increased with hght intensity 

 up to a maximum and then decreased. He found that the optimum 

 illumination for production of dry matter varied both with temperature 

 and with chlorophyll concentration. The higher the chlorophyll concen- 

 tration in the plant, the lower the light intensity required for maximum 

 production of dry matter. Chlorophyll development was itself depend- 

 ent upon light intensity. The optimum light intensity for chlorophyll 

 development was considerably below that for the optimum production 

 of dry matter. Strong Ught intensities tended to favor root growth 

 more than shoot growth. Leaf areas attained a maximum at moderate 

 light intensities and fell off with further increase or decrease in light. 

 In 1905 Lubimenko (61), with the best methods then available, made a 

 study of the effect of light intensity on chlorophyll concentration and was 

 able to separate the shade-loving plants from the sun-plants on the basis 

 of their ability to increase their chlorophyll concentration at low light 

 intensities. Sun-plants showed approximately the same chlorophyll 

 concentration at all light intensities, but shade-plants showed a con- 

 siderable increase in chlorophyll concentration with decreasing light 

 intensity. 



Combes (22) grew a number of plants under an improved type of cloth 

 shade, ingeniously designed so as to admit free air circulation and still 

 exclude light. Temperature and humidity were maintained at values 



