78 TEXTBOOK OF BOTANY 



and maple, are also thicker as well as narrower and longer when the leaf 

 grows in bright light. The thickness of leaves in full sunlight may be 

 as much as 2 or 3 times that of similar leaves that developed in shade. 

 Studies of leaf development have shown that in moderate shade the 

 number of cell la vers of mesophvll is the same as in the embryonic leaf, 

 while in full sunlight the number of these layers may be increased ( Fig. 

 38). Moreover, there is increased elongation of cells at right angles to 

 the leaf surface; that is, more of the mesophyll cells become palisade 

 cells. 



In dense shade the leaf blades of many plants mature at an early stage 

 of development and remain smaller and thinner. In total darkness such 

 blades develop little or not at all. Corn and other parallel-veined leaves 

 are exceptions in that the blades of seedlings may enlarge in darkness 

 even more than in light. Petioles generally lengthen more in shade and 

 darkness than in light. Internally, the vascular tissues of the petiole 

 become differentiated more in full sunlight than in darkness. 



Light and epidermal cells. What evidence there is seems to indicate 

 that in anv one species the larger size of shade leaves is due more to the 

 greater enlargement of the cells than to the increase in number of cells 

 (Fig. 39). Likewise, the total number of stomates in these leaves may 

 not increase, since the proportion of epidermal cells from which guard 

 cells develop is about the same in both sun and shade. 



The mean number of stomates per 100 original epideiTnal cells is as 

 follows : ^ 



European beech Shade leaves 10.7 Sun leaves 10.4 



Black elderberry Shade leaves 15.5 Sun leaves 16.1 



Myrtle blueberry Shade leaves 12.2 Sun leaves 14,5 



There are more epidermal cells and more stomates per square milli- 

 meter of surface in sun leaves than in shade leaves when the plants grow 

 out-of-doors. Under these conditions the leaves are heated more than in 

 shade, and evaporation of water from them reduces the water content 

 of their cells. Since, in growth,* enlargement of cells depends in part on 

 water pressures inside them, sun leaves have smaller epidermal cells, 



iData from E. J. Salisbury, Philos. Trans. Roy. Soc. London, 216:1-65, 1927. The 

 mean nmnber of epidermal cells and pairs of guard cells, respectively, per square mm. 

 in sun and in shade are given as follows: 



European beech Shade leaves 1157 and 145 Sun leaves 1905 and HI 



Black elderberry Shade leaves 268 and 47 Sun leaves 581 and 112 



Blueberry Shade leaves 545 and 90 Sun leaves 1049 and 132 



