BRYOPHYTA 161 



sterile cells. It gives rise to numerous small biciliate sperms, two of 

 which arise from each sperm mother cell. The presence of swimming 

 sperms, universal among bryophytes and pteridophytes, represents the 

 retention of a primitive algal character. 



The archegonium is a very characteristic organ of bryophytes and 

 pteridophytes. Although corresponding to the oogonium of the algae, 

 it is much more highly developed. The archegonium is usually stalked 

 and flask-shaped. It is composed of an axial row of cells surrounded 

 by a sterile jacket. The axial row consists of an egg — the basal and 

 largest cell of the series — and a variable number of canal cells, which 

 disorganize and become mucilaginous prior to fertilization. The fertilized 

 egg gives rise to an embryo that develops within the archegonium, the 

 basal portion of which enlarges to form a protective covering, the calyptra. 



In all bryophytes the sporophyte is without differentiation into stem 



and leaves and is w^ithout a direct connection with the soil. In nearly 



all bryophj^tes the sporophyte consists of a ba.sal absorbing organ {foot), 



a stalk (seta), and a terminal spore-producing portion (capsule). The 



capsule is a sporangium. All bryophytes are homosporous, the spores of a 



given species being alike in size and form. On germination, the spore 



produces either the main gametophyte directly or, more commonly, a 



filamentous protonema from which the main gametophyte sooner or later 



arises. 



1. HEPATICAE 



The liverworts are primitive land plants, most of them growing in the 

 presence of abundant moisture on soil, rocks, and tree trunks. With 

 very few exceptions, the gametophyte is dorsiventral. It may be thalloid, 

 but more commonly is leafy, the leaves being nearly always without a 

 midrib. Unicellular unbranched rhizoids maintain a connection with the 

 substratum. The Hepaticae are widely distributed but are more numer- 

 ous in the tropics than elsewhere. A few fossil forms are known from the 

 Upper Carboniferous of England. There are about 6,000 species of 

 liverworts, nearly all being included in four principal orders, the r^Iarchan- 

 tiales, Sphaerocarpales, Jungermanniales, and Anthocerotales. 



1. Marchantiales 



The Marchantiales are a well-defined order of about 30 genera and 

 400 species. They range from arctic to tropical regions and are well 

 represented in the Temperate Zones. In the tropics they occur chiefly 

 between altitudes of 900 and 1,500 m. Nearly all of them are terrestrial, 

 growing mainly on damp soil or rocks. Some common genera of Mar- 

 chantiales, all of widespread distribution, are Riccia, RehouUa, Asterella, 

 Conocephalum, and Marchantia. The largest genus is Riccia, wth over 

 100 species. 



