364 PRACTICAL BOTANY 



ment thus cut through correspond to the limits of the 

 diamond-shaped areas above noted. 



I. Beneath the " stomata " are large areas, the air- 

 cavities, in which are seen numerous round or oval 

 cells, grouped in simple or branched series, and attached 

 to the lower surface of the cavity : their cell-walls are 

 thin, and consist of cellulose : these cells contain 

 chlorophyll, and constitute the chief assimilating tissue 

 of the plant. 



c. Below this is a massive tissue, which constitutes 

 the great bulk of the section : it consists of oval cells, 

 with few intercellular spaces, if any at all : the walls 

 are thin, and marked with shallow pits : the proto- 

 plasmic contents are scanty : in the cells nearer the 

 upper surface there are often numerous starch-grains. 

 Individual cells here and there in this tissue have 

 peculiar mucilaginous, or highly refractive, yellowish or 

 brown, oily contents. 



Cut transverse sections from a fresh thallus, mount in water, 

 and having noted cells containing the highly refractive bodies 

 above mentioned, treat with potash solution, and warm gently 

 over a spirit lamp : the bodies are not dissolved, but partially 

 lose their highly refractive quality, without swelling. Treat 

 other sections first with alcohol, and then with ether : wash 

 from the ether with alcohol, and mount : the oil bodies will 

 have been dissolved. 



d. At the lower surface of the thallus may be seen 

 attached organs of two kinds 



i. Hairs, or rhizoids, which are long and unicellular, 

 and are inserted deeply in the tissues of the thallus : they 

 often show dotted or peg-like ingrowths of the cell-wall 

 of various form, 



