80 TEXTBOOK OF BOTANY 



Following is a comparison of leaf structures- in a cross section of 

 Rhododendron catawbiense from a spruce-fir forest ( shade ) and from a 

 heath-bald (sun) on Mt. LeConte in the southern Appalachians, eleva- 

 tion 6600 ft. Evaporation rate in the heath-bald w^as about 5 times that 

 in the forest, whereas transpiration was only about 3.5 times as great. 



Shade Sun Ratio 



Shade 



Average leaf dimensions 13.5 X 5.9 cm. 6.9 X 3 cm. 0.25 



Mean thickness of leaves 0.358 mm. 0.525 mm. 1.5 



Mean thickness, upper cuti- 

 cle 6.0 microns 8.9 microns 1.5 



Mean thickness, upper epi- 

 dermis 11.0 " 13.5 " 1.2 



Mean number layers of pal- 

 isade 2.5 " 4.5 " 1.8 



Mean thickness, palisade 



layers 70.0 " 144.0 " 2.1 



Mean thickness, spongy 



parenchyma 253.0 " 321.0 " 1.6 



Mean thickness, lower epi- 

 dermis 10.5 " 16.2 " 1.5 



Mean thickness, lower cuti- 

 cle 2.9 " 6.7 " 2.3 



The development of the finer veins of the leaf is similarly affected, 

 and the meshes ( "vein-islets" ) enclosed by the veins are smaller in sun- 

 light than in shade. The area of vein-islets probably is determined not 

 directly by sunlight, but indirectly by the increased evaporation and 

 the decreased expansion of the leaf cells, as are leaf size, thickness, 

 number of stomates, and epidermal cells per square millimeter. In all 

 these phenomena growth hormones may play an appreciable part. 



Light and leaf position. Plants placed at a window are exposed to only 

 a fourth or a fifth as much light as plants out-of-doors; moreover, most 

 of the light strikes onh' one side of the plant. This one-sided illumina- 

 tion of the young leaves results in unequal growth of the petioles. The 

 cells on the shaded side lengthen more than those on the lighted side. 

 The greater elongation of the shaded side of the petiole is evident in its 

 bending toward the window. Apparently minute quantities of growth 



- Calculated from sections near the midrib. From S. A. Cain and J. D. Oli\er Miller, 

 Amer. Midland Naturalist, 14:69-82, 1933. 



