30] 



CHAPTER II. THE TISSUES. 



107 



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. Thisjwax 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 some- 

 times attains a considerable bulk, as 

 in the fruits of Myrica cerifcra and 

 the trunks of some Palms (Ccratoy- 

 lon andicola and Klopstockia ceri- 

 fcra}. The epidermal cells are some- 

 times sclerotic, as in prickles, thorns, 

 and leaf -spines. Chloroplastids are 

 not usually present in the epidermal 

 cells of land-plants : they are, how- 

 ever, to be found in the epidei-mal 

 cells of most Terns, of Selaginella, 

 and of some Phanerogams, and 

 generally in those of aquatics. 



The form of the epidermal cells, as 

 seen in surface view, presents con- 

 siderable variety. Generally speak- 

 ing, the cells of an elongated member 

 are themselves elongated in the 

 same direction as the member ; 

 whereas, in broad, flattened mem- 

 bers, there is less difference between 

 the diameters of the cells : in either case the side-walls of the 

 cells very frequently have an undulating outline, so that adjoining 

 cells fit closely together forming a continuous membrane, the con- 

 tinuity of which is, however, interrupted in certain cases by well- 

 defined apertures, termed stomata, which permit communication 

 between the intercellular spaces of the internal tissues and the 

 external air. 



T^& Stoinata^YQ confined exclusively to the sporophyte-genera- 

 tion, and make their first appearance in the Moss-sporogonium. 

 Each stoma is an aperture_ bounded by two (sometimes only one, 

 as in the Mosses) specialised epidermal cells, termed guard-cells, 

 which always contain chloroplastids. The aperture of the stoma 



FIG. 87. Part of a transverse sec- 

 tion of the air-root of an Orchid : v 

 many-layered epiblem, or velamen ; 

 c cortex; e exodermis. (Magnified; 

 after Unger.) 



