LIVERWORTS AND MOSSES. 5 I 



expanded in a horizontal plane, though sometimes much 

 crisped. The simpler ones consist of several layers of uniform 

 cells* (B, 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 „ 



m # Fig. 59. — Lunularia cruczata, 



and develop intO peculiar forms showing horizontal thallus and 



rhizoids with two erect branches 



adapted tO producing the Sexual (one young, one mature), for 



1 carrying sex-organs. Natural 



reproductive organs (fig. 59). size.— After Bischoff. 



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, 



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

 higher group. 



