[Chap. IX ENVIRONMENT AND LEAF DEVELOPMENT 79 



and more per unit area; likewise their guard cells are smaller, and the 

 pairs of guard cells per unit area are more numerous. When moisture 





B/ 



SHADE FORM 



SUN FORM 



Fig. 39. Relation of growth of leaf tissues of wandering jew (Tradescantia) to 

 light. Leaves (A) on plants shaded by two layers of cheesecloth are broader and 

 thinner, and have shorter pahsade cells, larger epidermal cells, and fewer stomates 

 per square millimeter than leaves (B) on plants exposed to full sunlight. From 

 F. H. Norris. 



conditions are controlled so that evaporation from the leaves is about 

 the same in sun and in shade, the numbers are approximately the same 

 for equal surface areas. The effects of intense light upon leaf develop- 

 ment are similar to those of drought. 



