LEAVES 



571 



under leaf surface, where the stomata are the more abundant. The 

 leaves of many desert xerophytes (as in the creosote bush) are coated 

 with resin, and often have a varnished aspect, shining in the sun- 

 light. Many tropical forest leaves also are shiny. The factors influ- 

 encing the formation of resin coats are unknown. Like wax coats, they 

 may retard transpiration, and it has been suggested that they reflect 

 light, an excess of which may injure the chlorophyll. It has been 

 shown that shiny leaves become more slowly heated in the sunlight 



FIG. 811. A surface view of a 

 part of the under epidermis of a leaf of 

 Coletis, showing the wavy lateral walls 

 characteristic of the under epidermis of 

 mesophytic dicotyl leaves ; the shaded 

 cells contain anthocyan; note the sto- 

 mata with their crescentic guard cells, 

 chloroplasts, and central slit; highly 

 magnified. 



FIG. 812. A surface view of a part 

 of the upper epidermis of a leaf of Coleus, 

 showing the straight lateral walls character- 

 istic of the upper epidermis of mesophytic 

 dicotyl leaves; note the absence of an- 

 thocyan and of stomata; magnification as 

 in fig. Si i. 



than do similar leaves that are not shiny, so that evaporation probably 

 is reduced thereby. 



Floating leaves (as in the water lilies) often have waxy or resinous sur- 

 faces, which are highly advantageous in that they prevent the wetting of 

 the stoma-bearing surface, thus facilitating gas exchanges. Many meso- 

 phytic leaves also (as in the meadow rue) are not readily wetted, their 

 silvery aspect when thrust into the water being due to an air film next to 

 the waxy surface. Probably few stomatal surfaces are readily wetted, 

 contrasting thus with the surfaces of submersed leaves and with the 

 stoma-free upper surface of many tropical leaves. 



The lateral walls. In mesophytic dicotyls the lateral epidermal walls commonly 

 are straight on the upper leaf surface and irregularly wavy on the lower or stoma- 



