IV THE JUNGERMANNIACE^ 75 



number of growing points formed by the rapid dichotomy of 

 the original apex, and separated only by a few rows of cells. 

 From the lower side of the thallus grow numerous root-hairs 

 of the thin-walled form. The whole upper surface is covered 

 with the sexual organs, each of which is surrounded by its own 

 very completely developed envelope. 



A vertical section passing through one of the growing 

 points (Fig. 28, C) shows a structure closely like a similar 

 section of Riccia. The apical cell (a-) produces dorsal and 

 ventral segments, and from the outer cells of the former the 

 sexual organs arise exactly as in Riccia. On the ventral 

 surface the characteristic scales of Riccia are absent, and are 

 replaced by the glandular hairs found in most of the anacro- 

 gynous Jungermanniaceae. 



The development of the archegonium shows one or two 

 peculiarities in which it differs from other Hepaticae. The 

 mother cell is much elongated, and the first division wall, by 

 which the archegonium itself is separated from the stalk, is 

 some distance above the level of the adjacent cells of the 

 thallus, so that the upper cell is very much smaller than the 

 lower one. The upper cell has much denser contents than the 

 lower one, which instead of remaining undivided as in Riccia, 

 divides into two nearly equal superimposed cells, this division 

 taking place about the same time as the first division in the 

 archegonial cell (Fig. 29, B). The divisions in the latter are 

 the same as in Riccia, and the general structure of the arche- 

 gonium offers no noteworthy peculiarities. The number of 

 neck canal cells is small, probably never exceeding four, and in 

 this respect recalls again Riccia. The central cell is relatively 

 large, and the ventral canal cell often nearly as large as the 

 ^gg- -^^ the archegonium develops, its growth is stronger on 

 the posterior side, and it thus curves forward. At first the 

 young archegonium projects free above the surface, but 

 presently an envelope is formed about it exactly as in Riccia, 

 but arising at a later stage. After this has begun to form, its 

 growth is very rapid, and it soon overtakes the archegonium 

 and grows beyond it, and finally forms a vesicular body, 

 plainly visible to the naked eye, at the bottom of which the 

 archegonium lies. The formation of this involucre is quite 

 independent of the fertilisation of the archegonium, and as 

 these peculiar vesicles cover completely the whole dorsal 



