312 A TEXTBOOK OF GENERAL BOTANY 
to four spores, and in the 
first of these two divi- 
sions the number of chro- 
mosomes is reduced. Thus 
each spore has a single 
number of chromosomes. 
These give rise to gameto- 
phytes with a single num- 
ber of chromosomes, and 
the double number is again 
restored when the egg is 
Fig. 403. Portion of a thallus of Marchan- fertilized by a spermato- 
tia bearing two gemma cups within which 
are many gemme 

zoid. The gametophyte is 
thus characterized by a 
M J 1] - . 
Marchantia reproduces asexually by the for single number of chreaia 
mation of lenticular gemme in gemma cups. 
These gemme produce new plants (Fig. 404), Somes and the sporophyte 
The lines dividing the thallus into polygonal by a double number. The 
areas mark the boundaries of air chambers, alternation ot generations 
while the black dots represent the stomata- 
like openings into the chambers. (x 6) is therefore not only an 
alternation of a gameto- 
phytic and a sporophytic generation but also an alternation of 
a generation with a single and a generation with a double 
number of chromosomes. 
Orders of the Hepaticae. The Hepat- 
wcae, or liverworts, contain four orders: 
the Riecciales, the Marchantiales, the Jun- 
germanniales, and the Anthocerotales. 
ORDER RICCIALES 
The genus Riccia may be taken as 
an example of this order. The game- 
tophyte is a dichotomously branching 
thallus (Fig.397). Dichotomousbranch-  Py¢. 404. Gemma of Maz 
ing is a system of branching in which  chantia after the rupture of 
the main axis forks repeatedly into stalk cell. (x 150) 

