110 



PLANT STEtlCTDRES 



G9. Marehantia forms. — In this line the simple thallus 

 gradually becomes changed into a very complex one. The 



thallus retains its simple 

 outlines, but becomes thick 

 and differentiated in tissues 

 (groups of similar cells). 

 The line may be distin- 

 guished, therefore, as one 

 in which the difierentia- 

 tion of the tissues of the 

 gametophyte is emphasized 

 (Figs. 91-93). In 3Iar- 

 chantia proper the thallus 

 becomes very complex, and 

 it may be taken as an illus- 

 tration. 



The thallus is so thick 



that there are Tery distinct 



green dorsal and colorless 



Tentral regions (Fig. 94). The latter puts out numerous 



rhizoids and scales from the single layer of epidermal cells. 



Above the ventral epidermis are several layers of colorless 



Fig. 91. A very small species of Biccia, 

 one of the Marehantia forms ; A, a 

 group of thallus bodies slightly en- 

 larged ; B, section of a thallus, show- 

 ing rhizoids and two sporogonia im- 

 bedded and communicating with the 

 outside by tubular passages in the 

 thallus. — After Strasburger. 



Fig. 92. 7/irrinriirpiis. a Marehantia form, showing numerous rhizoids from ventral 

 surface, the dichotomous brau<]iiug, aud the jtosition of the sporogonia on the 

 dorsal surface aloug the " midribs."— Caldwell. 



