322 



PART IV. CLASSIFICATION. 



this sort are found in the receptacles of other Marchantiese, the vegetative 

 parts of which have simple pores. 



In many forms, the cells containing chloroplastids (assimilatory tissue) 

 are simply those which form the walls of the air-chambers ; in Marchantia 

 (Fig. 199) the cells forming the floor of the air-chamber, or the sides, or 

 even the roof, grow out into branched or unbranched filaments which fill 

 most of the air-cavity, thus largely increasing the assimilatory tissue. 



Beneath the air-chamber-layer is a compact tissue, consisting of several 

 layers of cells elongated in the direction of the long axis of the branch, 

 which is without intercellular spaces, and contains but few chloroplastids. 

 In the Marchantieae the walls of these cells are generally thickened and 

 pitted ; some of the cells contain mucilage, and in Fegatella the mucilage- 

 cells form continuous rows ; other cells contain a dark-coloured oil-drop, 

 though such cells also occur in the air-chamber layer. 



The ventral surface is formed by a layer of cells which, in the simpler 



FIG. 199. Marchantia polymorplia. A A pore seen in surface view. B Section of a portion 

 of the dorsal region of the thallus, showing the air-chamber containing assimilatory tissue, 

 and the compound pore. ( x 210 : after Strasburger.) 



forms, is not specially differentiated, but in some the cells of this layer 

 are remarkable for their small size; in Marchantia and Preissia there 

 are several layers of these small cells, forming a sort of ventral cortex. 



The ventral scales consist of a single layer of cells, the walls of which 

 generally assume a violet colour ; each scale is developed from a single 

 superficial cell, or, as generally in the Bicciese, from a transverse row of 

 cells. In Marchantia polymorpha, in addition to the scales which arise 

 from the midrib, there are others which spring from the surface of the 

 lamina. 



Unicellular root-hairs are produced in all Marchantiacese ; the com- 

 monest form has thin walls ; in the Marchantieae a second form occurs, in 

 which peg-like thickenings of the wall project into the cavity of the cell : 

 the simple root-hairs are developed mainly on the midrib, the thickened 

 hairs mainly on the lamina. 



