WAX 



107 



epidermis as an instrument for restricting evaporation. From obser- 

 vations extending over a month Unger deduced a much smaller value 4 ' J 

 for the ratio between evaporation from a freely exposed water- 

 surface and the foliar transpiration of Digitalis purpurea : hut the 

 discrepancy is explained by the fact that Unger's method does not 

 eliminate stomatic transpiration. 



The effectiveness of the control exercised over transpiration by the 

 epidermis is often enhanced by the addition of a coating of wax. 00 

 The characteristic "bloom" on a grape or a plum, and the glaucous 

 appearance of many leaves [and 

 stems] are caused by the presence of 

 such waxy coatings. There are three 

 principal types of waxy covering. 

 In the great majority of cases the 

 wax is deposited as a layer of minute, 

 closely-crowded granules, with an 

 average diameter of "001 mm.; this 

 condition is exemplified by the leaves 

 and stems of many Gramineae, 

 Liliaceae and Iridaceae. Less 

 frequently the covering takes the 

 form of a layer of rods, which stand 

 up vertically above the cuticle ; 

 these rods are often much taller than the epidermal cells, and may be 

 hooked or rolled up at their distal ends. This type of coating is found 

 in a number of Gramineae and Scitamineae (e.g. Musa, Strelitzia, 

 Carina). Unusually tall rods reaching a length of "1 to '15 mm. 

 occur on the nodes of the Sugar-cane (Saccharum officinarum, Fig. 24). 

 A wax-crust or -stratum, finally, such as covers the leaves of Thuja and 

 Sempervivum, consists of a brittle, translucent amorphous glaze, which is 

 generally intersected by numerous cracks and flaws. As a rule the 

 depth of such a crust scarcely exceeds 1/j. ; in special cases, however, 

 it may be much greater. Exceptionally thick crusts occur, for instance, 

 on the leaves and stems of Panicum turgidwn ('03 mm.) and on the 

 young leaves of Copernicia cerifera ( , 015- , 019 mm.). In the Wax Palms 

 (spp. of Ceroxylon and Klopstockia) the waxy incrustation may even 

 reach a thickness of 5 mm. As regards the mode of formation of waxy 

 coverings, De Bary has shown that the wax does not, as might be sup- 

 posed, arise by chemical modification of the cuticle or of other layers 

 of the outer wall, but that it is on the contrary a product of secretion. 



F. Haberlandt was the first to investigate the effect of waxy 

 coverings upon transpiration experimentally ; he examined the leaves 

 of the Swedish Turnip, which have a well-developed granular covering. 



Pio. 24. 



Coating of wax-rods on a node of Saccharum 

 officinarum. X142. After De Bary (from De 

 Bary, Comp. Anat.). 



