SECT. I 



MORPHOLOGY 



111 



is the effect of a muddy colourless medium. The opacity in the cellulose 

 thickening layers of the epidormal cells may possibly be due to granules of 

 cutin in the wall. A similar intluence gives rise to striated or furrowed thicken- 

 ings which affect the whole thickness of the walls. A blue iridescence of the 

 epidermis occurs in many shade plants though usually in a slight degree ("•'). 



By the thickening and the cutinisation of their outer walls the 

 mechanical rigidity of the epidermal cells is increased, and the loss 

 of water by transpiration is lessened. Plants in dry climates, or so 

 situated that, for any reason, transpiration from their outer surfaces 

 must be diminished, are characterised by the greatly thickened and 

 cuticularised walls of their epidermal cells. In some of the Gramineae, 

 Equisetaceae, and many other plants, the cell walls of the epidermis 



Fig. lis. — Transverse section of a node of tlie sugar-cane, Sacchamm offlcinarum, showing 

 wax incrustation in tlie form of small rods. (x540.) 



are silicified. In the Equisetaceae the impregnation with silica is so 

 considerable that these plants are used for polishing. Heating, even 

 to redness, does not destroy the structure of such silicified epidermal 

 cells. 



Deposits of wax are also present in the cutinised layers of the 

 epidermis, and consequently water will flow ofi' the epidermis 

 without wetting it. The wax is sometimes spread over the surface 

 of the cuticle as a wax covering. This is the case in most fruits, 

 where, as is so noticeable on plums, it forms the so-called bloom. 

 The wax coverings may consist of grains, small rods (Fig. 118), or 

 crusts. 



The wax deposits attain their greatest thickness on the leaves of some of the 

 Palms ; on the Peruvian Wax Palm, Ceroxylon andicola, the wax covering is more 

 than 5 mm. thick. This wax as well as that obtained from the fruit of Myrica 

 eerifera, is known as vegetable wax, and possesses an economic value. The wax 

 incrustations may be melted by heat ; they are soluble in ether and in hot alcohol. 



