244 PHYLUM VIII. BRYOPHYTA 



the top and surrounding the row of canal cells, but later 

 open with a continuous passage to the egg (owing to the 

 dissolution of the canal cells). In fertilization which 

 takes place in water, the sperms pass down the tubular 

 neck to the egg below. 



425. Mossworts are of small size, rarely exceeding 10 or 

 15 centimeters in height. They generally prefer moist 

 situations upon the ground, or on the sides of trees or 

 rocks. All told there are somewhat more than 16,000 

 species. Two classes may be distinguished, as follows: 



Mostly bilateral, often thalloid, creeping gametophytes, 

 usually with splitting sporophytes, and mostly having 

 elaters Class HEPATICAE. 



Multilateral, leafy-stemmed, mostly erect gametophytes, usu- 

 ally with circularly dehiscing sporophytes, and without 

 elaters . . . . .Class Musci. 



CLASS HEPATICAE. LIVERWORTS 



426. In the lower Liverworts the gametophyte is a flat, 

 expanded thallus of parenchymatous tissue, and this 

 gradually differentiates into a leafy stem as we pass to 

 the higher forms, but in all cases the plant body has two 

 distinct and well-marked surfaces, an upper and an under 

 one, the latter bearing the root-hairs (rhizoids) by which 

 the plant is fixed to the ground. About 4000 species are 

 known. 



427. Among the simplest of the Liverworts are the 

 little round, flat Riccias (Riccia) which grow on wet earth 

 or even float on the water. In the upper surface of the 

 loose green tissue are the sunken antherids which pro- 

 duce biciliated spiral sperms. In a similar manner the 

 archegones are sunken in the upper surface. After fer- 

 tilization the egg develops into a globose cellular body 



