LIVERWORTS AND MOSSES. 



43 



thallus, from the floor of which arise green filaments (fig. 38). 

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

 growths as in fig. 38, i. 



A part which shows constant differences between an upper 

 (dorsal) and an under (ventral) side is said to be dorsiveniral. 



Fig. 38 Fig. 39. 



Fig. 38. — Portion of a vertical section of the thallus of Lunnlaria cruciata. a, dor- 

 sal, ^, ventral epidermis ; c, an air-pore ; e, air-chamber, from whose floor rise green 

 filaments, d\ f. partition between adjoining air chambers ; g, colorless cells contain- 

 ing starch, some showing net-like thickenings of the walls, others with oil-bodies, h ; 

 i, a ventral scale ; /, a rhizoid. Ma^ified 110 diam. — After Nestler. 



Fig. 39. — Lunulayia cruciata, showing horizontal thallus and rhizoids with two erect 

 branches (one young, one mature), for carrying sex-organs. Natural size. - After 

 Bischoff. 



These differences are usually called forth by the action of 

 light (see f 325). 



53. 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. Sometimes 

 special branches, instead of remaining horizontal, grow up- 

 right and develop into peculiar forms adapted to producing 

 the sexual reproductive organs (fig. 39). 



