CHAPTER XX 



THE GREAT GROUPS OP BRYOPHYTES 



HhpaticI'; ( Lii'crirtirfs) 



202. General character. — Liverworts live in a variety of 

 conditions, some floating on the water, many in damp 

 places, and many on tlie Ijark of trees. In general they are 

 moisture-loving })hints (hydrophytes), though some can en- 

 dure great dryness. The gametophyte body is prostrate, 

 though there may lie erect aiul leafless gametophores. 



This prostrate ha1)it develops a i/orsircii/nd body — that 

 is, one whose two surfaces {ilarsal and venlral) are exposed 

 to dilferent conditions and liecomo unlike in structure. In 

 Liverworts the ventral surface is against tlie siilistratum, 

 and puts out numerous hair-like processes (rh i:niils) for ab- 

 sorption and anchorage. Tlio doi'sal region is exposed to 

 the light and its cells develop chlorophyll. If thethallus 

 is thin, chlorophyll is developed i]i all the cells ; if it Ije so 

 thick that the light is cut off from the ventral cells, the 

 thallus is differentiated into a green dorsal region doing the 

 chloroiihyll work, and a colorless ventral region producing 

 absorbing rhizoids. This latter represents a simple differ- 

 entiation of the nutiltivo body into working regions, the 

 ventral region absorbing material a,nd conducting it to the 

 green dorsal cells which use it in making food. 



There seems to have Ijeen at least three main lines of 

 dev(!lopment a.inong Liverworts, each beginning ill forms 

 with a very sim])lo thallus, and developing in dilTcrent di- 

 rections. They are briefly indicated as follows : 

 30.S 



