286 Beer. — On the Development of the Spores of Riccia glauca. 
which exhibit considerable metabolic activity, and we may probably infer 
that some relation exists between the spirem- arrangement and the cell 
activity, A similar relation between the peculiar nuclear structure and 
cell-metabolism no doubt also occurs in the developing spores of Riccia 
glauca. The actual material which is used by the protoplast in forming 
and adding to the spore-walls must be derived from without the spore. 
The reserve material and cytaplasm of the spore suffer very little diminu- 
tion during development, and if these are drawn upon to furnish material 
for the growth of the membranes, this loss is at once fully compensated by 
the arrival of new material from without. The starch-contents of a spore 
at about the middle period of its development is shown in Fig. 34, and 
almost precisely the same appearance is presented by the spore-protoplast 
in the preceding and succeeding stages. 
Both the sterile parietal layers of the sporangium and the inner layer 
of the calyptra 1 degenerate and yield some material which is no doubt 
employed in the growth of the spore- walls. Neither of these layers is, 
however, rich in substance (see Figs. 4, 29), and I scarcely think it is 
possible that their degeneration can furnish all the material required for 
the very considerable growth undergone by the membranes. Most probably 
this source is supplemented by material which is assimilated by the 
vegetative cells of the thallus, and which diffuses into the sporophyte 
in a state of solution. After the first spore-wall has been formed, and 
during all the earlier periods of the growth of the second spore-wall, 
a mucilage is constantly present in the sporangium between the spore- 
tetrads (Fig. 33). The origin and significance of this mucilage are, 
however, somewhat obscure. It is certainly not the material secreted by 
the surrounding cells of the thallus, since not a trace of mucilage can be 
detected in any of these. The callose special-mother-cell walls, which for 
some time continue to surround the spore-tetrads, gradually disappear, but 
there is no evidence to show that their substance makes any contribution 
to the sporangial mucilage which, moreover, gives none of the reactions of 
callose. Two sources remain, both or either of which may be responsible 
for the sporangial mucilage. In the first place the degeneration of the 
parietal cells of the sporangium and of the inner archegonial layer may 
contribute to the formation of the mucilage. In that case it still remains 
to be explained how it is that the parietal cells have degenerated some 
time before the mucilage can be seen, whilst some remains of the inner 
archegonial cells can frequently still be detected after the mucilage has 
again become absorbed. Secondly, the degeneration of the primary 
mother-cell walls and of their thickening layers, which are both lost sight 
of about this time, may give rise to part or all of the sporangial mucilage. 
A difficulty in the way of at once accepting this view of the origin of the 
1 The outer layer of the calyptra persists to a very late stage. 
