ANATOMY OF LIVERWORTS 



273 



(Fig. 152, A). An equally simple structure is displayed by 

 most of the foliose forms, the leaves being invariably only one 

 cell thick, and usually devoid of a midrib. In Marchantia and 

 some of its allies, however, the upper part of the thallus, which 

 is always the principal assimilating region, shows considerable 

 complexity (Fig. 149, A). It is subdivided into a large number 

 of shallow polygonal chambers, each of which is roofed over by 

 an epidermis, and communicates with the exterior through a 



D 



FIG. 149. Structure of Marchantia. A, Small part of a transverse sec- 

 tion through the thallus. B, Part of a peg-rhizoid, much enlarged, 

 showing the thickenings (pe.). C, Surface-view, showing a single 

 pore. D, Gemma, seen from the surface, a., assimilatory filaments ; 

 m., mucilage -cell ; p., pores ; r., rhizoid ; s., scales. 



central barrel-shaped pore (p.). From the floor of each chamber 

 arise numerous short filaments of green cells (.), which form 

 the assimilatory system of the thallus. The whole of the lower 

 portion of the latter consists of large colourless cells, serving 

 in the main for the storage of food-reserves, and in part showing 

 reticulate thickenings. On the surface of the thallus the assimi- 

 latory chambers appear to the naked eye as a number of small 

 diamond-shaped areas, each with a minute central dot corre- 

 sponding to the pore. 

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