CHAPTEE XX 



THE GREAT GBOUPS OF BRYOPHYTES 



HepatiojE (Liverworts) 



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

 conditions, some floating on the water, many in damp 

 places, and many on the bark of trees. In general they are 

 moisture-loving plants (hydrophytes), though some can en- 

 dure great dryness. The gametophyte body is prostrate, 

 though there may be erect and leafless gametophores. 



This prostrate habit develops a dorsiventral body — that 

 is, one whose two surfaces {dorsal and ventral) are exposed 

 to different conditions and become unlike in structure. In 

 Liverworts the ventral surface is against the substratum, 

 and puts out hair-like processes (rhizoids) for anchorage 

 and possibly absorption. The dorsal region is exposed to 

 the light and its cells develop chlorophyll. If the thallus 

 is thin, chlorophyll is developed in all the cells ; if it be so 

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

 thallus is differentiated into a green dorsal region doing 

 the chlorophyll work, and a colorless ventral region pro- 

 ducing anchoring rhizoids. This latter represents a simple 

 differentiation of the nutritive body into working regions, 

 the ventral region absorbing material and conducting it to 

 the green dorsal cells which use it in making food. 



There seem to have been at least three main lines of 

 development among Liverworts, each beginning in forms 

 with a very simple thallus, and developing in different 

 directions. They are briefly indicated as follows : 

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