MORPHOLOGY 



91 



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, as De Baey has shown, 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. The wax coverings may consist of grains, small rods, 

 or crusts. 



On the nodes of many Gramineae the rod-shaped wax bodies have a consider- 

 able length (Kg. 100). The wax deposits attain their greatest thickness on the 



X3QQQQ 



Fig. 100. — Transverse section of a node of the sugar-cane, Saccharum ojfichwnm, showing wax 

 incrustation in the form of small rods. ( X 540.) 



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 cerifera, 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 fat-like substance, which is soluble even in cold alcohol, are excreted 

 from the hairy surface of the epidermis. The dusty coverings 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 Ferns groups of slightly thickened, epidermal cells are 

 distributed over the leaves. These cells are richly supplied with 

 contents, and exude drops of watery fluid. Cells of this nature, which 

 thus serve the purpose of exuding or, at other times, of absorbing 

 water, have been termed hydathodes by Haberlandt. In many 

 other cases, slimy or sticky excretions are produced between the 



