30] 



CHAPTER II. THE TISSUES. 



109 



bundles on the margins or at the apex of the leaf ; when chalk-glands are 

 pivsunt i"p. !)7), water-stomata are developed in connexion with them. 

 ~Tn some plants (e.g. Grasses) which excrete drops of water, the water 



escapes through fissures in the epidermis of the leaf. 



The epidermis of the submerged shoots of water-plants differs 

 from that of land-plants in that it is not cuticularised, in the 

 absence of stomata, and in that its cells frequently contain chloro- 

 plastids. 



The epiblemof_the subterranean root is commonly known as 

 the piliferous layer be- 

 TTause it is the layer from 

 which the root-hairs (see 

 p. 46), when present, .are 

 developed. Its cell-walls 

 are not cuticularised, but 

 are frequently (especially 

 in the root-hairs) more or 

 less mucilaginous. It is 

 generally of but short du- 

 ration, and to be found 

 only on the younger parts 

 of roots which are the 

 regions of active absorp- 

 tion : on its disappearance 

 the exodermis becomes the 

 superficial layer. 



In aerial roots (Orchids, 

 etc.) where the epiblem persists as a velamen (see Fig. 87, p. 107) 

 of one or several layers of cells, the walls are thickened, cuti- 

 cularised (especially the superficial layer), and somewhat lignified. 



The many-layered root-cap (see p. 103), in^its younger, more 

 internal part, consists of parenchymatous cells, with cell-walls cf 

 cellulose, forming a compact tissue without intercellular spaces. 

 As the cells grow older, and come to be situated more externally, 

 they lose their protoplasmic contents. TEe" disintegration of the 

 root-cap is due, iu some cases, to the mucilaginous degeneration of 

 the middle lamella of the cell-walls ; whilst in other cases, where 

 the cell-walls become cuticularised, the superficial layers of the 

 cap are successively split off and exfoliated by the pressure of the 

 internal growing tissues. 



Hairs (see p. -46), are frequently developed on the primary 



Water-stoma from the margin of the 

 leaf of Tropccolum majus, with surrounding epider- 

 mal cells. (After Strasbureer : x 240.) 



