744 BIOLOGICAL EFFECTS OF RADIATION 



plants are grown in extremely weak light, stem structure is similar to 

 that of etiolated plants, but leaf structure is decidedly different. The 

 leaves completely unfold and develop epidermal layers and show some 

 differentiation among the inner cells. Ordinarily, in plants grown in 

 weak light, the leaves are very thin and have only one layer of palisade 

 cells and a more or less loosely organized parenchyma. When the light 

 is increased in intensity up to moderate values, stem and leaf differentia- 

 tion take place, and the plant has the general appearance of a normal 

 plant. In moderate intensities the leaves reach their maximum size, 

 and stem elongation, if dependent upon food elaborated by the leaves, 

 is greatest. Moderate root development occurs. Under such conditions 

 the leaves will have from one to two layers of palisade tissue and will 

 develop the typical spongy parenchyma. Vegetative development 

 reaches its highest point at moderate light intensities. At extremely 

 low intensities nutrition limits the development of the leaves and height 

 growth. At moderate intensities there is evidently sufficient food 

 available for most vigorous vegetative growth. Flowering and fruit 

 development does not occur in very weak light but at moderate intensities 

 flowers are produced. The intensity for optimum development of 

 flowers and fruit, as well as for maximum production of dry matter, is 

 considerably higher than that required for best vegetative development. 



At extremely high light intensities transpiration is excessive. Under 

 such conditions the plants develop various devices which protect them 

 against excessive heating and drying. This resvilts in short stems, thicker 

 leaves, but less total leaf area, increased water-conducting tissues, a more 

 rapid rate of transpiration and, in general, features typical of xerophytic 

 plants. 



Root development is greatly enhanced by high light intensities. If 

 plants abundantly .supplied with nitrates could be grown in very high 

 light intensities and at the same time in atmospheres containing a large 

 supply of moisture, it is possible that vegetative growth would not 

 be checked. However, there are other effects of high light intensity, 

 aside from that resulting in a moisture deficit within the plant. Pro- 

 longed illumination of leaves at high intensities results first in an accumu- 

 lation of starch and later in the entire disappearance of starch. High 

 light intensities also tend to increase the alkalinity of the cell sap. In 

 extreme cases this may interfere with the iron nutrition of the plant. 

 In such cases the plants become chlorotic through the breakdown of 

 chlorophyll. 



These reactions of the plant to variations in fight intensity are in 

 general of an adaptive nature. At low intensities, the plant requires a 

 very efficient photo.synthetic apparatus. This is pro\dded in large, thin 

 leaves, high in chlorophyll concentration and widely spaced on the 

 stem. At high light intensities the plant with large thin leaves is at a 



