MARCHANTIA POLYMORPHA 191 



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 are 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 : the walls are thin, 

 and marked with shallow pits : the protoplasmic 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. 



d. Attached at the lower surface of the thallus may be seen 

 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. 



ii. The amphigastria, which may now be seen to be plates 

 of tissue one layer of cells in thickness : their cell-walls are 

 often coloured violet or brown. 



Returning now to the "stomata/ ' note under a high power 

 their structure as seen in a good transverse section : each will 

 appear as consisting of tiers of small cells (four or more in 

 depth), which surround a large central cavity. 



IV. Cut tangential sections so as to strip off the so-called 

 " epidermis " : mount with the outer surface uppermost in 

 weak glycerine : observe under a low power the diamond- 

 shaped areas above described, and a single large " stoma " in 

 the middle of each. Under a high power note 



1. That the cells of the "epidermis" contain chlorophyll. 



2. That each " stoma " is bounded by four or five of the tiers 

 of cells above described. 



3. That these cells contain but little chlorophyll. 



4. That on focusing downwards it becomes apparent that the 



