ii6 



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 t rue mosses this earliest filamentous stage persi sts. This fact is 

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

 characteristic of mosses. W ith green tissue di splayed-h-y-£rect branches 

 the__display of green tissue by the jthaUus 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 tq 

 the sex organs. 



The leafy branch 

 Figs. 251-254. — True moss: 251, leafy branch arising , . , 



from protonema and putting out rhizoids; a well-developed QCVeiOpS Dy means 

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

 organs; 253, rosette containing archegonia; 254, a sporo- ^j^Jj three Cutting 

 phyte arising from a cluster of archegonia. /• rm 



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 manyjnosses 



