104 



CELLULAR TISSUE. 



walls. The other part of these cells however becomes expanded to form a great oval 

 bladder which intrudes deeply into the sub -epidermal parenchyma. As soon as the 

 processes of expansion and division in the neighbouring cells begin, there grows from 

 the middle of the thickened outer wall, perpendicularly into the interior, a peg-shaped 

 process (consisting of cellulose), the blunt end of which swells into a knob {B). 

 When the leaf is fully developed {E) the swelling has the form of an egg-shaped or 

 almost spherical body, which attains a size filling half or more than half of the cell. 

 This is the cystolith, which is thickly covered on its exterior with pointed and blunt 

 radially diverging conical warts, and is suspended in the cavity of the bladder-like 

 cell by an irregularly cylindrical stalk, which is continuous directly into the original 

 thickened outer wall. The whole body is impregnated with calcium carbonate, and 



FIG. 44.— Ficus elastica ; leaf, transverse section, e—e in each case the thickness of the epidermis ; ^ (600) upper 

 side ; W, (390) under side of the same very young leaf; in yl^ an already complete stoma, which remains superficial, 

 and a hair (transient) ; in ^ two cystolith-cells, recognisable by their thickened outer wall, epidermal cells as yet 

 undivided.— .S (600) upper, .fi, (390) under side of a rather older leaf, epidermal cells undergoing division. In J?, x is 

 a younger and jr, an older stage of a cystolith, already showing the peg-like excrescence"of the wall.— C (390) older 

 leaf, under side ; division of the now three-layered epidermis is complete, stoma depressed, but the definitive size 

 and form of the parts is not yet attained.— £■ upper side of a mature leaf, four-layered epidermis, cystolith-cell (37S). 



the stalk also with silica: it has a homogeneous glassy appearance, and in the pointed 

 warts there is often stratification and granulation. Acids dissolve the calcium salt, 

 bubbles of carbonic acid being produced. After solution the stalked body remains 

 behind with its original attachment ; the stalk is but little altered, the swelling re- 

 maining as a delicate cellulose skeleton, the outline having become more irregular : 

 the interior shows well-marked stratification and delicate radial striation, the layers 

 from the end of the stalk onwards being nearly concentric with the surface. 



