EPIDERMIS OF ALOE. 



87 



whole thickness of the leaf need not be taken, but a piece of the 

 tissue, say about ^ inch thick, is cut off from one surface of the 

 leaf. As the stomata run parallel to the long axis of the leaf, we 

 arrange the piece of leaf so that it shall be cut at right angles to 

 this axis. We cut the sections from the inner towards the outer, 

 i.e., from the soft towards the harder part of. the tissue. The 

 strong thickening of the epidermal cells is at once noticeable in 



these sections (see 



Fig. 31); this thick- ^ 



ening affects only 

 the outer half of the 

 cell; corresponding 

 to it, the cavity of 

 the cell narrows 

 outwards. The 

 thickened parts of 

 the cell -wall ' are 

 white, strongly re- 

 fractive, and are 

 covered externally 

 by a cuticle more 

 strongly refractive still, but not sharply delimited. The lateral 

 boundaries of the cells are only indicated by delicate lines in 

 the thickened mass, and externally by a slight ridge. The 

 interior of the strongly refractive thickening layer is covered 

 by a comparatively thin, feebly refractive layer (i). This sur- 

 rounds especially the keel-shaped narrowed part of the cell- 

 cavity ; while gradually thinning off, it ends on the side walls 

 simultaneously w r ith the refractive thickening layer. This thick- 

 ened part of the epidermis, viewed in the aggregate in the section, 

 appears like a curtain cut into regular scollops. At the places 

 where a pit leading up to a stoma is found, is first to be noticed 

 the projection which encloses the stomatic pit as with a rim ; 

 next, that the scollop, formed by the thickening layers, is here 

 only half as wide, and has also only half its usual depth. The 

 guard-cells show, both above and below, on the stomatic side, 

 projecting ridges (of " entrance " and of " exit "), which in cross- 

 section appear beak-like. Above the guard-cells are found the 

 thin parts of the wall which serve as epidermal hinges. The 

 ventilating chamber is narrow and deep. Commonly a parallel, 

 more or less oblique, striation will be observed on the thickened 



FIG. 31. Cross-section through the epidermis and stonm 

 of Alw niyricaas. i, inner thickening layer ( x 240). 



