Thomas. — Notes on Cephaleuros . 787 
possible to decide from the material available, whether this structure of the 
cell-wall is due to some purely physiological condition, or is due in some 
way to the proximity of the Fungus. 
fS. Dark-celled Form. 
This occurs also on the upper surface of the leaf under the same 
conditions and mixed up with the large-celled discs (Plate LIX, Fig. 7). 
At first it was thought that these might be alternative forms in the life- 
history of one individual, but the evidence seems to point to a distinct 
organism. Although the majority of plants are in a fairly mature con- 
dition, young stages have been observed which are undeniably early stages 
of the older dark-celled thalli, such as occur in profusion over the leaf 
surface. The young stage is circular in outline, with indentations of the 
cell-wall similar to those already described in form a. The cell-walls are 
dark brown, and within the limiting walls no structure can be observed, but 
a uniform brown colour prevails. The mature plant also has dark brown 
cell-walls, unaffected by any dyes and strongly resistant even to concentrated 
acids. 
The method of growth appears to be identical with that of the large- 
celled type. The marginal cells, as they grow, are divided radially by 
a cell-plate into two. This membrane grows from the periphery inwards 
until it is equal in length to about half the total length of the undivided cell ; 
transverse walls are then formed, and the same curious free posterior end of 
the radial walls may be seen after the formation of the transverse septa 
(Fig. 8). 
In section, the dark cell-walls are seen to be thickened, especially 
on the upper surface. The cells are much smaller in all their dimensions 
than those of the previously described a form, each cell only measuring 
8 /x x 3 /x in surface view as compared with an average of 1 8 pt x 8 pt in 
the form a. 
This organism is simpler in structure than the a form with which 
it grows. Not only are the cells smaller, but there are no Fungal hyphae 
connected with it, and the cell-walls do not show the lamellose structure of 
those of the a type. 
This investigation suggests that the two forms belong to perfectly dis- 
tinct species, but, on the other hand, one must not neglect the possibility that 
the a form may turn out to be a luxuriant variety of the /3 organism. It is 
well known that association with a Fungus frequently causes hypertrophy ; 
perhaps one has here another instance of this, in which case the a form 
probably owes its larger cells and other peculiarities of structure to its early 
connexion with the Fungal hyphae. 
From an examination of Dr. Cunningham’s slides, a few ^points of 
comparison were made ; although, as one would naturally expect in 
