288 PROTONEMA 



In fact, the latter readily grow out into protonema, if exposed 

 to the light, and can thus bring about the development of new 

 plants. 



Vegetative multiplication of the protonema, by the de- 

 tachment of branches or of small terminal groups of cells 

 (Fig. 157, C, g.), is often prolific, and may set in at a comparatively 

 early stage. Sooner or later, however, some of the short lateral 

 branches become pear-shaped and undergo segmentation by three 

 oblique walls, which cut out the characteristic tetrahedral apical 

 cell of the stem of the adult Moss (Fig. 157, A, b.). By its 

 continued divisions a small mass of cells is formed from which 

 leaves soon arise, and it is not long before the new Moss attains 

 recognisable dimensions. This formation of Moss-plants takes 

 place at many points on the protonema, and, as the latter usually 

 dies away soon after, the individuals become independent. A 

 protonema readily develops from most parts of the adult Moss, 

 in fact, in all cases of vegetative propagation from fragments 

 of the thallus (cf. p. 277), a protonema is first formed, and from 

 this the new plants only arise secondarily. 



The protonema of the foliose Liverworts is similar in appear- 

 ance to that of Mosses, but in most of the thalloid species the 

 early stages are not sharply marked off from the adult. 



The life-history of the Bryophyta thus includes two distinct 

 phases : the one, concerned in the production of the sexual 

 organs, is relatively highly organised and self-supporting, whilst 

 the other, concerned solely with the production of asexual spores, 

 is always dependent, and relies for its sustenance, either entirely 

 (most Liverworts) or partly (Mosses), upon food obtained from 

 its host. These two phases normally alternate with one another 

 and arise from one another, and a similar alternation is encoun- 

 tered in all the higher groups of plants. For convenience of 

 designation the spore-producing phase is spoken of as the sporo- 

 phyte, and the sexual phase as the gametophyte. 



