284 
Lang . — T he Prothallus 
outset that the formation of new tissue is not confined to one 
end, but takes place all round the edge of the prothallus. 
Leaving aside for the moment the structure of the two ends, 
the results obtained from the study of sections across the 
lateral margin may be first considered. The merismatic 
region to which growth can be referred is situated at the 
margin of the prothallus toward the upper surface ; its posi- 
tion is_ indicated by a cross in Fig. 5. Toward the lower 
surface the tissues of the vegetative half of the prothallus 
become differentiated, while to the other side the cushion- 
layer is formed. When the latter, in cases where the sexual 
organs are numerous and near to the margin, attains a con- 
siderable thickness, a marginal groove is the result, which can 
be recognized in Figs. 5, 8, and 9. In these cases the initial 
region occupies the upper side of the lower lip of the groove. 
Periclinal divisions sometimes occur in the superficial cells of 
the merismatic region, but it is very doubtful whether these 
cells can be considered to be initials. The comparison of 
a number of sections rather suggested that the meristem is 
imperfectly stratified. The superficial layer can be traced on 
the one hand into the limiting layer of the under surface, the 
cells of which undergo anticlinal divisions only. On the other 
side this layer gives rise at a greater or less distance from the 
margin to the small-celled generative layer with the sexual 
organs. Below the superficial layer in the merismatic region 
are a few layers of fairly large cells, which can be traced on 
the one hand into the band of longitudinally extended cells 
(£), and on the other into the cushion-layer (e). Still deeper 
in is a small-celled meristem, from which the storage-layer ( d ) 
and the layer of palisade-cells (c) are differentiated. This 
differentiation is complete at a very short distance from the 
meristem, the whole appearance suggesting that the addition 
of new cells to the permanent tissue is very slow. This 
explains the difficulty that was experienced in following the 
course of divisions in the meristem. 
The vertical sections through the ends of the prothalli 
presented considerable differences in the various specimens 
