72 



ELEMENTARY STUDIES IN BOTANY 



oxygen is given. The waterproofing epidermis interferes 

 with this exchange, and therefore it must be interrupted 

 enough to let the air into the green cells. In the simpler 

 Liverworts these openings for the air are secured by clefts, 

 while in complex Liverworts there are elaborate pores in 



the epidermis, that open 

 into internal air chambers 

 (Fig. 52). In this latter 

 case there is an internal 

 atmosphere that bathes 

 the green cells, which 

 communicates with the 

 external atmosphere 

 through the pores. In 

 this way the gas exchange 

 is provided for ; but the 

 pores also open up a way 

 for the escape of water 

 vapor from the inner cells. 

 This loss of water is the 

 price the plant must pay 

 for the opportunity to 

 manufacture food. 



The bodies of Liver- 

 worts have advanced in 

 two general directions. 

 Some Liverworts have 

 retained the primitive 



form of the body, a continuous sheet of cells which branches 

 by forking, and have advanced in making the structure of 

 the body more and more complex (Fig. 51). Other Liver- 

 worts, and these are far more numerous, have retained the 

 simple structure of the body, and have advanced in changing 

 the form of the body, so that the flat sheet becomes dif- 

 ferentiated into a central axis (stem) bearing many lobes of 



FIG. 55. Porella, a leafy liverwort : A, a plant 

 showing the two rows of overlapping dorsal 

 leaves, and also three special branches that 

 bear antheridia ; B, a plant with the special 

 branches that bear archegonia (two of these 

 branches are bearing the spore-cases produced 

 by the fertilized egg) ; C, ventral view of a 

 plant, showing the ventral leaves much 

 modified by being in contact with the 

 substratum. 



