Lecture XVI. 123 



The minute structure may be best made out from a transverse 

 section of the thallus. Under the low power this section shows 

 that the mid-rib of the plant is much thicker than the edges. The 

 upper region of the section is green owing to the presence of 

 tissues containing much chlorophyll, the lower two-thirds to seven- 

 eighths is colourless or at least not coloured green. From the 

 lower surface spring numbers of tubular structures, the rhizoids, 

 and what in the section look like chains of cells, which are the 

 transverse sections of the scales. 



With a high power further details may be made out. The cells 

 of the upper superficial layer are small and almost square in cross- 

 section. They have thin walls and their protoplasmic lining con- 



FIG. 23. Marchantia polymorpha, thallus transverse section, x 100. 

 c, cuticle; e, ventilating pore leading into air-chamber; p, partition; 

 r, rhizoids; s (on upper surface), superficial layer; s (on under side), 

 scales ; t, layers of large cells. 



tains small ovoid chloroplasts embedded in it. They are uni- 

 form in size, and fitting closely together, they form a continuous 

 membrane covering the upper side of the thallus. Here and there 

 the section cuts through a ventilating pore or the cells forming 

 the short tube or collar surrounding it. One can see that the 

 collar consists of four tiers of narrow cells. Two of these tiers 

 are raised above the superficial layer and two are below it. In- 

 spection of the collar in a surface section of the thallus shows that 

 each tier is an oval ring composed of four cells, so that the collar 

 surrounding the pore is built up of sixteen cells in all. The four 

 cells of the lowest tier have each a blunt tongue-like bulge upon 

 them more or less obstructing the aperture of the pore. These 



