PORELLA 



415 



into other kinds of work is spoken of as sterilization of sporog- 

 enous tissue. One can now see that a sporophytc 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- 

 Hke structures and resemble the Mosses. 



Porella is one of the foliose forms of the 

 Jungermaniales and is common on the trunks 

 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 theMarchantias. 



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 bj^ 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. 



Fig. 370. — A 

 branch of Porella, 

 a foliose Liverwort 

 of the Jungermani- 

 ales. X 3. 



