574 Lang. — On a Prothallus provisionally referred to Psilotum. 
which ran concentrically round the cell and almost completely obliterated 
the cell-cavity. So far as could be seen the hyphae were non-septate : they 
were very fine, and in well-stained specimens could be seen to contain 
numerous small nuclei. In a small proportion of the cells of this zone 
vesicular structures occurred in addition to the hyphae. They were 
relatively small and thin-walled, but otherwise resembled the vesicles, which 
are such a striking feature of the layer next to be described (c). This 
consisted of a single layer of long narrow cells standing at right angles to 
the surface. Fungal hyphae could in favourable preparations be demon- 
strated running between the cells in the thickened cell-wall, but the 
prominent feature of this layer was the presence of numerous oval vesicles, 
the position and general appearance of which are sufficiently shown in 
Fig. 7. The vesicles, which were intercellular, bulged out the septa in 
which they lay, and frequently obliterated the lumen of the cells on one or 
both sides. In suitable specimens it could be seen that they were borne on 
hyphae, but whether they were always terminal, as they appeared in all 
the cases observed, could not be determined. Close to the meristematic 
region they were thin-walled (Fig. 9, a), but in all the older regions the 
walls were thick and stained intensely with some dyes, e. g. safranin when 
used in combination with haematoxylin. The vesicles were filled with 
cytoplasm, no central vacuole being as a rule present ; in most cases 
numerous small nuclei were evenly distributed through the cytoplasm 
(Fig. 9). In other cases the nuclei appeared to be aggregated in one or 
several groups, the rest of the cytoplasm being practically free from them. 
No signs of any further development was observed in the vesicles within 
the tissues of the prothallus. Internal to the layer of elongated cells is 
the parenchymatous tissue ( d ), which composes the central portion of the 
vegetative region of the prothallus. This tissue was entirely free from 
the endophyte, and its cells presented no characters which call for special 
note. 
The arrangement of the various tissues in the conical base of the 
prothallus must be referred to in order to complete the description of the 
vegetative region. The diagram in Fig. 8, which is founded on a section 
passing through the middle of the projection, will make the matter clear. 
The region occupied by the Fungus is shaded, and the several layers are 
indicated by the same letters as in Fig. 7. It will be evident that in the 
upper portion of the conical projection the arrangement is exactly the same as 
has been described above. It is only at the extreme tip of the projection, 
the region which was first developed on germination, that a difference is 
found. There, as is the rule in the similar prothalli of Lycopodium , the 
endophytic Fungus inhabits the superficial cells as well as the layers 
beneath ; the inner layers are also less regular here. Strictly speaking it 
is this tip only, and not the whole of the projection, that is comparable to 
