MORPHOLOGY 



101 



plants, the cell walls of the epidermis 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 off 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. 



Fio. 108. Transverse section of a node of the sugar-cane, Saccharum oflleinaru,m, showing wax- 

 incrustation in the form of small rods, (x 540.) 



The wax coverings may consist of grains, small rods (Fig. 108), or 

 crusts. 



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

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

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

 cerifcra, 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. 

 In many cases, in place of the wax coverings, small grains and scales of a fatty 

 substance, which is soluble even in cold alcohol, are excreted. The dusty cover- 

 ings thus formed appear either mealy white or golden yellow, and are the cause of 

 the striking appearance of the Gold and Silver Ferns, especially in species of 

 Gymnogramme. 



In many cases, slimy or sticky excretions are produced between 

 the thickening layers of the epidermis and the cuticle ; these press up 

 the latter and finally burst it. Such excreting surfaces often occur 

 on bud-scales. Sticky zones are frequently formed on stems, as in 

 the case of Lychnis viscaria and other Sileneae, as a means of protection 

 to the flowers higher on the stem from undesirable visitors. Small 

 creeping insects, which would otherwise rob the flowers of their honey, 



