31.] 



CHAPTER II. THE TISSUES. 



155 



a many-layered epiblema, known as the velamen, consisting of 

 empty tracheidal cells with reticulated and perforated walls 

 (see p. 134). 



The cells of the epidermis of 

 the shoot of land-plants, are cha- 

 racterised by the thickening and 

 cnticularisation of their cell- walls 

 (see p. 132). The external wall is 

 usually much more thickened than 

 the other walls; its outermost 

 layer, termed the cuticle, is always 

 cuticularised, and is clearly defined 

 from the inner layers, which may 

 be also more or less cuticularised. 

 The cuticle may be stripped off as 

 a membrane, over a considerable 

 area ; it frequently forms surface- 

 projections. Particles of wax are 

 included in the cuticle of many 

 plants, and serve to prevent the 

 surface from being wetted by 

 water. This wax often appears 

 on the surface in the form of small 

 granules, rods, or flakes, and this 

 forms the bloom which is easily wiped off : it sometimes attains a 

 considerable bulk, as in the fruits of Myrica cerifera and the trunks 

 of some Palms (Ceratoxylon 

 andicola, and Klopstocltia ceri- 

 fera). The epidermal cells 

 are sometimes sclerotic, as 

 in prickles, thorns, and leaf- 

 spines. Chloroplastids are 



not usually present in the epi- II \\ e lf3ib^/S&??.)/ e 



dermal cells of land-plants ; 

 they are to be found, how- 

 ever, in the cells of most 

 Ferns, of Selaginella, and 

 of some Phanerogams, more 

 especially aquatics. 



The form of the epidermal cells, as seen in surface view, presents 

 considerable variety. Generally speaking, the cells of an elon- 



Fio. 117. Part of a transverse section 

 of the air-root of an Orchid : v many- 

 layered epiblema, or velamen ; c cortex. 

 (Magnified; after Unger.) 



FIG. 118. Epidermis (e) with a stoma(S)from 

 a cross-section of a leaf of Hyadnthus orientalis 

 ( x 800) : p parenchyma of the fundamental 

 tissue ; i an air-cavity. 



