20G PLANT MORPHOLOGY 



than the mosses. Both groups may have had a common origin, but it is 

 more Hkely that the mosses were derived directly from the liverworts. 

 Each has subseciuently pursued its own course of evolution, the mosses 

 having advanced considerably beyond the liverworts. 



The Land Habit. The establishment of the land habit was one of the 

 most important events in the history of plant life, for it made possible all 

 subsequent progress. Its advantage lies in the greater opportunity for 

 photosynthesis in the presence of more light. Its disadvantage lies in the 

 danger of excessive transpiration. The first land plants may have arisen 

 from some green alga consisting of a simple plate of cells, perhaps from a 

 form somewhat like Coleochaete. Adjustment to the land environment 

 must have involved structural changes facilitating the absorption of water 

 from the soil and the retention of water by parts exposed to the air. This 

 adjustment on the part of bryophytes is manifested chiefly by the devel- 

 opment of compact bodies, absorptive filaments (rhizoids), jacketed sex 

 organs, heavy-walled aerial spores, and, in some cases, by an epidermis 

 with air pores, in others, by primitive conducting cells. 



Because the simplest liverworts are thalloid, it is reasonable to suppose 

 that they have given rise to the leafy forms. An alternative view is that 

 the leafy body is primitive, the thalloid type having been derived from it 

 by reduction. Since an erect leafy body permits the exposure of a greater 

 photosynthetic surface to the light than is afforded by a flat thallus, it 

 would seem to represent a more advanced state of adaptation to the land 

 habit. 



The Gametophyte. The bryophytes are a group in which the gameto- 

 phyte is the dominant generation. It is always a green, independent 

 plant body. In its simplest form it is a flat thallus, one to several layers 

 of cells thick, and without any internal differentiation of tissues. Such a 

 gametophyte is seen in Sphaerocarpus, P cilia, Notothylas, and a number of 

 other Hepaticae. It may be regarded as having given rise to two diver- 

 gent lines of descent: (1) a line in which the gametophyte, remaining 

 thalloid, has undergone differentiation in structure; (2) a line in which the 

 gametophyte has remained simple structurally but has become differen- 

 tiated in form, finally becoming a leafy shoot. The first line of evolution 

 has been followed by the Marchantiales, the second by the Jungerman- 

 niales. In the Musci the gametophyte reaches its highest development, 

 the erect leafy shoot of the higher mosses, with its radial symmetry, show- 

 ing differentiation in both form and structure. 



The Sporophyte. The simplest sporophyte among the bryophytes is 

 that of Riccia, where, except for the single layer of wall cells, all the cells 

 are sporogenous. If this sporophyte be regarded as primitive rather than 

 reduced, all subsequent progress has resulted from sterilization of poten- 

 tially sporogenous tissue and its diversion to other functions. This is 



