22 PRACTICAL BOTANY. 



weak glycerine, others in Schulze's solution, or in 

 iodine solution : examine under a high power, and, 

 starting from the upper surface, observe 



a. The superficial layer, or so-called " epidermis," 

 consisting of a continuous layer of cells of small size 

 which contain chlorophyll, as may be seen in prepara- 

 tions from fresh material : the continuity of the layer 

 is broken here and there by the so-called " stomata " : 

 these however differ from the true stomata of the higher 

 plants in the mode of their development. This layer 

 may be seen to be attached to the lower lying tissues at 

 points between the " stomata," and the lines of attach- 

 ment thus cut through correspond to the limits of the 

 diamond-shaped areas above noted. 



b. 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 



