COMPARISON OF SPOROGONIA 285 



cells of the columella. 1 Such facts again indicate a probability that 

 the whole product of the endothecium was fertile in more primitive 

 forms. 



A general comparison of the sporogonia of Mosses (excluding the 

 Sphagnaceae) thus leads to the conclusion that two distinct tissue-tracts 

 are consistently produced in them by early segmentation, the endothecium 

 and the amphithecium. As these are differentiated early, and with great 

 constancy, while they differ also in their products, they are to be accepted 

 as morphologically distinct. The amphithecium is always sterile, and to its 

 modifications the chief mechanical and assimilatory tissues owe their 

 origin ; the modifications may involve expansion of tissues, but no initia- 

 tion of new parts. The endothecium, theoretically fertile in the first 

 instance throughout its length and breadth, underwent progressive sterili- 

 sation, parts of it being diverted to other uses : a central tract became 

 the sterile columella, while the fertile region became abbreviated both 

 at its upper and lower limits ; and thus the actual archesporium in 

 typical Bryales is a mere truncated residuum, with its barrel-like form 

 open at both ends : the structural indication that its origin was as thus 

 suggested is seen in its apparently arbitrary limitations at either end 

 (compare Figs. 135, 136, 139). 



This is well illustrated in Funaria and Phascum, where there is a 

 continued growth with an initial cell at the apex of the sporogonium ; 

 the archesporium appears in longitudinal sections of young sporogonia as 

 a definite row of cells on either side of the columella ; but it is impossible 

 at first to tell in those rows of cells where the exact limit of spore- 

 development will be. Below the lower limit the cells of the row develop 

 sterile, above it fertile ; but in either case the segmentations which define 

 the cell-row are the same. Passing to the apex, the archesporial row is 

 continued beyond the limit of fertility : passing downwards, the cell 

 row may also be traced into the seta : structurally the possibility of 

 further spore-production seems to be there, but arrested. In different 

 types of Mosses the fertile zone thus limited is not always located at 

 the same level in the sporogonium as a whole : it is sometimes preceded 

 by a shorter, sometimes by a longer, seta. By comparison of these 

 different types, an idea is acquired of a residual and limited fertile zone, which 

 has been liable to be shifted in the course of descent ; and such shifting 

 is made possible by the continued apical growth seen in the developing 

 sporogonium. It is important to have a clear conception of the fertile 

 zone as a residuum thus movable in the course of descent ; the vari- 

 able balance thus established between sterile and fertile tissues is not 

 only interesting in its bearing on the study of sporogonia, but it will 

 come into comparison later with similar features seen in certain strobiloid 



1 Lantzius Beninga, ficiti-. ;. K,nuln. d. Mooskapscls, 1847, Tab. 58, Figs. 9', 9". 

 Also Kienitz-GerlofT, fiot. Zeit., 1878, p. 47, Taf. 2, Fig. 52. 



