400 A TEXTBOOK OF THEORETICAL BOTANY 



thallus was composed of a more or less uniform parenchyma ; in Marchantia 

 we can distinguish a number of distinct layers (Fig. 382). Starting from the 

 upper surface there is first of all a single layer of epidermal cells. Embedded 

 in this are special ventilating pores which allow the passage of gases through 

 the epidermis. These pores are surrounded by four rows of four cells, the 

 rows forming rings one above the other, leaving an aperture in the centre. 



Ventilating r 

 pore 



Assimilating 

 chamber 



Thallus 



Rhizoid 



Fig. 382. — Marchantia polymorpha. Vertical section 

 showing structure of thallas. 



Thus the pore wall lies half above and half below the epidermis. Below the 

 epidermis there are a number of chambers, within which are short, sometimes 

 branched, filaments of cells, which develop from the flat base of the chamber, 

 which is composed of cells similar in shape to those of the epidermis. The 

 chambers, as seen from above, are diamond-shaped areas and are bounded 

 by walls of three or four cells in height, which unite the epidermis above 

 with the parenchyma below. All these cells, and particularly the filaments 

 within the chambers, are dark green in colour, due to the presence of numerous 

 ovoid chloropiasts. Each chamber lies directly under one of the ventilating 

 pores, and there is no doubt that the green filaments are the principal centres 

 of photosynthesis in the thallus. 



The lower layers of the thallus are made up of relatively large parenchyma 

 cells similar to those in the thallus of Pellia, among which are a number of 

 large mucilage cells. The lower surface is covered with an epidermis 

 composed of cells similar to those of the upper epidermis, and from these 

 cells two types of outgrowths arise. Firstly, there are the rhizoids, which 

 are unicellular and contain very little protoplasm, and, secondly, a number 

 of multicellular scales or amphigastria, which form two overlapping rows, 

 one on each side of the midrib. The amphigastria no doubt serve to retain 

 moisture below the thallus which the rhizoids can absorb. These rhizoids, 

 like those in other members of the Marchantiales, are of two kinds ; one set 



