THALLUS OF MARCHANTIA. 



235 



and poorer in chlorophyll than on the upper side. The rhizoids 

 which spring from the ventral surface show an alternative 

 structure. They are either more slender, and provided with peg- 

 like projections into the interior, or thicker and without such 

 projections. Those with the peg-like projections arise out of the 

 frond, as far as the median or laminar scales, or only the former, 

 extend. They lie close to the frond, and follow the mid-rib in 

 bundles, covered by the scales. They probably serve to stiffen 

 the thallus. The ordinary rhizoids proceed chiefly from the 

 midrib, and turn at an acute angle towards the substratum, to 

 which they fix the thallus. All the ventral scales are unilamellar, 

 the median consist of still living cells, the laminar and marginal 

 scales of cells which are already dead. 



FIG. 93. Marchantia polyrnorpha. A , an air-pore from above. B, in cross-section 

 ( x 240). 



A cross-section through the thallus shows us on the dorsal 

 surface first a zone of chlorophyll-containing tissue. The interior 

 of the thallus is composed of broader cells, almost devoid of 

 chlorophyll. In the walls of these cells here and there broad 

 elliptic pits are to be seen. At the ventral surface the two last 

 layers of cells are again narrower, flatter, rich in chlorophyll, and 

 form the so-called ventral cortical layer. Oil-bodies are scattered 

 through the entire tissue. Certain other individual cells are 

 noticeable from their size and highly refractive contents ; these are 

 the mucilage cellSi A closer study of the outer layers, rich in 

 chlorophyll, of the dorsal surface, completes the conception which 

 we had obtained from the surface view. Outermost we see a 

 single layer of flat cells, which extend free over the air-chamber, 



