138 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 



of leaves. Intense light results in elongated palisade cells and often 

 the production of two or more layers of them. Conversely, weak 

 illumination favors the production of sponge cells. A leaf that, 

 with average illumination, has a single layer of palisade and several 



FlG. 69. Seedling of turkey oak (Quercus catesbaei), a sandhill species, 

 whose leaves have already assumed the vertical position they maintain 

 throughout life. 



layers of sponge cells might have had, in intense light, two or three 

 layers of palisade and a proportionate reduction in sponge tissue. 

 In reduced light the sponge tissue is increased at the expense of 

 the palisade. In extreme cases there may be no palisade or no 

 sponge tissue. The thickness of cutin and the amount of support- 

 ing tissue in the veins are likewise greater or less depending upon 

 light intensity. These characters affect the relative toughness of 

 the leaf. 



What forces cause a developing cell to elongate at right angles 

 to the leaf surface to form palisade or parallel to the surface to 

 form sponge tissues, cannot be stated with any certainty. The 

 causes may not be entirely controlled by light, for unfavorable 

 moisture conditions favor palisade production as does poor aera- 

 tion. Sucker sprouts from stumps often produce leaves of the shade 



