u6 



MORPHOLOGY 



and the liverworts. In these groups the thallus in its development 

 passes through two or three stages, the first one being filamentous ; in 

 the true mosses this earliest filamentous stage persists. This fact is 

 associated with the development of the erect leafy branch (gametophore) 

 characteristic of mosses. With green tissue displayed by erect branches 

 the display of green tissue by the thallus body declines, and the 

 thallus finally remains in an embryonic stage. Although the leafy 



branch is the con- 

 spicuous part of 

 mosses, it should 

 not be thought of 

 as the gametophyte, 

 but as a branch of 

 the gametophyte 

 (fig. 251). In cer- 

 tain mosses, known 

 as reduced forms, 

 this branch is not 

 so prominent in its 

 display of green 

 tissue, only a few 

 leaves appearing ; 

 in fact it may bear 

 only a single scale 

 leaf in addition to 

 the sex organs. 

 The leafy branch 



FIGS. 251-254. True moss: 251, leafy branch arising . . , 



from protonema and putting out rhizoids; a well-developed develops by I 



"resting bud" is shown; 252, terminal rosette containing sex of an apical Cell 



organs; 253, rosette containing archegonia; 254, a sporo- w j t jj t j iree cu tting 

 phyte arising from a cluster of archegonia. 



faces. The segments 



are cut into outer and inner cells, the former, for the most part, 

 developing the leaves ; the latter the axis. The leaves usually consist 

 of a single plate of green cells, often thickened in the middle so as to 

 resemble a midrib. 



Vegetative multiplication. The power of vegetative multiplication 

 is remarkably developed. The leafy branch bears the sex organs 

 above the moist substratum, so that the conditions are not favorable 

 for swimming sperms. As a consequence, fertilization in many mosses 



254 



