PORELLA 415 
into other kinds of work is spoken of as sterilization of sporog- 
enous tissue. One can now see that a sporophyte could become 
as complex as a Corn plant by becoming more and more multi- 
cellular while at the same time most of the cells were used in 
forming structures, such as roots, stems, and leaves. In this 
way sporophytes became more and more com- 
plex until the highest plant forms were pro- 
duced. 
Porella. — This Liverwort belongs to the 
Jungermaniales, which order contains the 
largest number of Liverworts. 
The Jungermaniales vary widely in their 
moisture requirements, some being able to 
live in dry situations. They are especially 
abundant in the tropics where they grow on 
the trunks of trees, on leaves of other plants, 
and on the ground. Some have thallose game- 
tophytes like the Marchantiales, while others, 
known as foliose forms, have gametophytes 
that are differentiated into leaf- and stem- Fic. 370. — A 
like structures and resemble the Mosses. branch oF seotelas 
: a foliose Liverwort 
Porella is one of the foliose forms of the of the Jungermani- 
Jungermaniales and is common on the trunks ales. x 3. 
of trees and fallen logs in the north temperate 
regions. The character of the gametophyte is shown in Figure 370. 
It has a slender, creeping, branched, stem-like axis bearing two hor- 
izontal rows of larger leaves on the dorsal surface and one horizontal 
row of smaller leaves on the ventral surface. Although much more 
differentiated as to form, the gametophyte of Porella is much less 
differentiated as to tissues than the gametophyte of the Marchantias. 
The two kinds of sex organs may occur on the same plant or on 
different plants. The Archegonia occur in groups on the ends 
of short lateral branches. The antheridia occur in the axils of the 
leaves of certain branches which can be identified by the closely 
imbricated leaves. 
The sporophyte has a long stalk and the sporangium splits 
into four valves which spread out and allow the spores to escape. 
There is more sterilization of sporogenous tissue and a more 
definite provision for the shedding of spores than in the sporo- 
phyte of the Marchantias. 
