of Lycopodium clcivatum , L. 281 
allow of the development of the prothallus being followed, 
the specimens differed from one another in size and shape, and 
probably illustrate changes of form associated with the later 
stages of growth ; all conclusions drawn from the comparison 
of a small number of prothalli must, however, be tentative. 
The smallest of the prothalli, which is represented as seen 
from above in Fig. 2, measured 4 mm. in length by 3 mm. in 
breadth. It was an almost flat plate of tissue about 0*5 mm. 
thick, of a dirty white colour. Numerous rhizoids of con- 
siderable length projected from the under surface, especially 
around the margin ; they were absent from the upper surface, 
which was lighter in tint than the rest of the prothallus. 
From external examination an apical 1 region could not be 
distinguished, though in this and all the other prothalli a 
median plane dividing them into equal and similar halves 
could be recognized. In this longitudinal direction the 
prothallus was slightly curved, the upper surface being convex. 
Owing to the lateral margins being slightly bent up, this 
surface was also concave from side to side. It will be seen 
that these curvatures increase in larger and presumably older 
specimens, but it is of interest to note that in a prothallus of 
such relatively considerable size they were so slight. In the 
prothallus represented in Fig. 3 both these curvatures were 
much more marked. Owing to the increase of the longitudinal 
curvature, the ends were approximated to one another; while the 
transverse curvature gives rise to the trough-like form which 
will be apparent from an examination of the figure. Each of 
the lateral margins exhibited a secondary fold rather nearer 
to one end. As in the younger prothallus, the rhizoids, which 
were here much shorter, were more numerous near to the 
margin. Another prothallus, which resembled this in shape, 
save in the absence of the secondary folds, was found. Two 
1 This difficulty is explained, as Bruchmann has shown, by the flattened structure 
being preceded by a vertical, radially symmetrical stage. The longer diameter of 
these old prothalli has thus no relation to the true longitudinal axis. On re- 
examination of serial sections of my prothalli, the conical projection from the 
under surface, which is all that remains of the earlier stage, was to be found, 
though small and inconspicuous. It is not visible in any of the figures. 
