[Chap. IX ENVIRONMENT AND LEAF DEVELOPMENT 77 



longer than the apical portion. Hence these leaves continue to elongate 

 from the base after the other portions are mature. When growing plants 

 of grasses and aloe are placed in a dark box the leaves continue elongat- 

 ing; their white or yellow basal portions indicate the growing regions. 

 In early spring one may often see the same thing in bluegrass lawns 

 before sunrise. When bluegrass is clipped the clipped leaves continue 

 their elongation. Cutting the tip of a fern leaf removes the growing 

 region, and growth of the leaf stops. Appreciation of these three types 

 of leaf development (Fig. 37) may aid in the interpretation of many 

 leaf phenomena. 



During development leaves may be subjected to drought, to intense 

 light, shade, or darkness, to high and low temperatures, and to numer- 



Fig. 38. Cross sections of leaves of sugar maple from different environments: 

 A, section of a very young leaf with the usual three layers of mesophyll cells which 

 do not increase in number in some environments; B, section of a leaf from the 

 base of a tree in a forest; C, section of a leaf from the middle of the crown of an 

 isolated tree; D, section of a leaf from the side of the crown exposed to the sun. 

 Drawings redrawn from G. H. Smith (A) and from H. C. Hanson (B-D), American 

 Journal of Botany. 



ous other external conditions. The effects of these environmental factors 

 on the growth processes within the cells may bring about an increase or 

 decrease in the division, enlargement, and differentiation of cells. Conse- 

 quently any change in the intensity of these factors during the growth 

 period of a leaf may result in distinct differences in form and structure. 

 Light and leaf size. Leaves that have developed in full sunlight are 

 usually smaller and thicker than the corresponding leaves that have 

 developed in partial shade. The lobes of leaves, such as those of oak 



