364 



BOTANY OF THE LIVING PLANT 



archegonium (Fig. 307). As it develops further the lower part remains thin, 

 forming the seta, which maybe traversed by a conducting strand (Fig. 305). 

 But the distal part enlarges to form the capsule. 

 A layer of cells is there cut off from the periphery 

 of the endothecium, and acquires dense contents. 

 This is the archesporium, which is shaped like a 

 barrel without ends (arch. Fig. 305). Within it is 

 the large-celled water-storage tissue of thecolumella. 

 The amphithecium, limited now bya superficial epi- 

 dermis with stomata, forms a lacunar photo-syn- 

 thetic tissue, with a large and continuous air-space 

 outside the archesporium. This tissue is specially 

 active in that region called the apophysis, where the 

 stomata are most frequent. In some Mosses it is 

 enlarged as a very effective organ of nutrition. 



As the development proceeds the cells of the 

 archesporium divide repeatedly, forming a thick 

 cylinder of sporogenous cells surrounding the 

 columella, and limited externally by a double 

 layer of cells of the amphithecium. This con- 

 stitutes the spore-sac. The cells then separate, 

 and rounding off in a fluid that fills the sac, each 

 undergoes tetrad-division, and finally produces four 

 spores. Reduction takes place as usual, common 

 numbers of chromosomes for Mosses being 12-6. 

 The numbers are low for Bryophytes generally. 

 The mature spore is very minute, and almost 

 spherical, and it contains globules of oil. 



Meanwhile above the fertile region certain inner 

 cells of the amphithecium undergo changes of 

 induration of the cell-walls to form the peristome, 

 which is closely related to the liberation of the 



spores (Fig. 305). As its structure differs in detail in various Mosses, it 



provides facts valuable in theirclassification. The 



case of Fontinalis serves as a good example of a 



complicated peristome, as it is seen after the 



operculum falls away. It is double (Fig. 308). 



The inner peristome forms a sort of connected 



lattice-work which will allow the spores singly 



through its pores, but prevents them all falling out 



at once. The outer consists of 16 teeth, which are 



really strips of thickened cell-wall, separated from 



one another by the breaking down of the thinner 



lateral connections. They show movements with 



changes of moisture in the air, and catching one 



on another by their rough edges, they give flicking 



movements on release, which throw the spores to 



a distance. The spores thus shed germinate to 



form protonema, as seen above (Fig. 298). 



FIG. 307- 



Young sporogonium of Physco- 

 iintrella patens, shortly before 

 the rupture of the archegonial 

 wall. (After Hy.) 



ap 



FIG 



Fontinalis, apex of capsule (A') 

 after shedding the operculum. 

 a= outer peristome. ip = inner 

 peristome. (After Schimper.) 

 ( 50.) 



