LEAF SURFACES 



above 140° and some micro-roughness was 

 inferred to be responsible for these high con- 

 tact angles. Two examples of the micro- 

 roughness revealed by this technique are 

 shown in Figs. 2 and 4. Projections above 

 the level of the cuticle of many different 

 forms have been discovered. They vary in 

 height from 0.25 /x to 4 /x and are assumed to 

 be mainly of wax. We believe that it is this 

 fine structure which is responsible for the 

 bloom in many species, for contact angles 

 above 100°, and (most important to the 

 agronomist) for the lack of adhesion of water 

 droplets. All species which are found to have 

 this fine Svax blanket' structure on their 

 surfaces have initial contact angles for those 

 surfaces above 140°. 



A number of problems connected with the 

 retention of liquids on plant surfaces and 

 thought to be associated with properties of 

 the cuticle have been investigated with this 

 technique. Fogg (1947) showed that different 

 parts of the same plant differed in their abili- 

 ties to shed water droplets. The electron 

 microscope shows that the fine structm'e of 

 the surface of a plant may vary with its 

 position on a plant. Figures 3 and 4 show, 

 respectively, the abaxial and adaxial surfaces 

 of the same leaf of a pea plant. 



Differences were thought to occur in the 

 fine structure on the surfaces of plants with 

 variations in the light intensity (Dorschner 

 and Buchholtz, 1956). Figures 4 and 5 show 

 the adaxial surfaces of pea leaves grown at 

 5000 and 1500 ft-candles, respectively; as 

 the light intensity falling on the leaf is pro- 

 gressively reduced, the wax projections de- 

 veloped on those surfaces are smaller, less 

 dense and more angular in appearance. This 

 agrees with the observation that the sur- 

 faces of plants grown under reduced light 

 conditions are more delicate and more sus- 

 ceptible to mechanical damage. 



Differences in susceptibility to mechanical 

 damage are apparent between different spe- 

 cies of plants grown at the same light in- 

 tensity. The leaf surface of Oxalis corniculata 



Fig. 3. Abaxial leaf surface of Pisum sativum 

 var. 'Alaska'. 



Fig. 4. Adaxial leaf surface of Pisum sativum 

 var. 'Alaska'. 



is practically impervious to mechanical dam- 

 age and the leaves usually remain unwettable 

 until senescence; Hyacinthus orientalis, on 

 the other hand, is readily damaged bj'' ordi- 

 nary aerial weathering, and the leaves are 

 wettable soon after they have emerged. Very 

 little is known about the comparative chem- 

 istry of leaf waxes which must contribute to 

 hardness. 



179 



