LIVERWORTS AND MOSSES. 



expanded in a horizontal plane, though sometimes much 

 crisped. The simpler ones consist of several layers of uniform 

 cells* (£, fig. 57). 



57. The dorsiventral thallus. — In other forms there is a 

 more decided difference between the upper and under sides 

 of the thallus. The upper cells contain chloroplasts, 

 while the under ones have none or very few. In the Mar- 

 chantia family there are large air-chambers in the upper part 

 of the thallus, from the floor of which arise filaments or 

 cactus-like rows of chlorophyll-bearing cells (fig. 58). On 

 the under side, also, are frequently found scale-like out- 

 growths (superficial aggregates), as in fig. 58, i. 



A part which shows constant 

 differences between an upper (dor- 

 sal) and an under (ventral) side is 

 said to be dorsiventral, and the 

 state of being thus different is 

 termed dorsiventrality. 



58. Branching. — The branching 

 of the thallus is always by forking, 

 in a single plane or direction, as in 

 Fucus, but the branches do not 

 always develop equally. Some- 

 times special branches, instead of 

 remaining horizontal, grow upright 

 and develop into peculiar forms 

 adapted to producing the sexual 

 reproductive organs (fig. 59). 



59. The growing point of the thallus is usually in a notch 

 at the apex (fig. 60). There is a single apical cell of wedge 

 shape (rarely tetrahedral), from whose inner faces segments 

 are cut off (fig. 61). These, by division and growth, 



oung, one mature), f< >r 

 g sex-organs. Nal ui tl 

 \ti,r Bischoff. 



*Ccenocytes rarely appear in the vegetative bodies of this or any 

 higher group. 



