of Lycopodium cluvatum , L. 285 
examined, and also when the two ends of any one prothallus 
were compared. In the prothallus represented in Fig. 4, no 
difference could be made out between sections of either end 
and those across the lateral margin ; in all these regions the 
appearance of the tissues and the succession of archegonia 
showed that growth was proceeding. But in the other 
prothalli the sections of the ends differed from each other 
and from those across the lateral margin. The nature of 
these differences will be gathered from Fig. 7 a and b, which 
represent the sections through the ends of a prothallus which 
resembled the one in Fig. 3 ; the crosses indicate the points 
in relation to which the tissues seemed to be arranged. The 
insufficiency of material will not permit of any more detailed 
statement being made, but the existence of such differences 
is of interest with regard to the appearance of symmetry 
presented by these old prothalli. 
Although the details of the merismatic regions will require 
to be investigated in younger prothalli, the facts of which an 
account has just been given suggest the explanation of the 
main changes of form exhibited by these old prothalli. 
Starting from a flat plate of tissue (Fig. 2), the growth of 
which is proceeding all round the edge, irregularities of 
growth would cause the lateral margins to assume the folds 
seen in Figs. 1, 3, and 4, while the main curvatures of the 
prothallus may find their explanation in differences in rapidity 
of growth between the upper and the lower sides of the 
marginal meristem. A much larger number of prothalli would, 
however, be needed to enable this to be traced in detail. 
Antheridia and Archegonia. 
From what is known of the distribution of the sexual 
organs on the prothalli of other species of Lycopodium , it 
might have been anticipated that archegonia and antheridia 
would have occurred in numbers upon the same prothallus. 
It is of course impossible to draw any general conclusion as 
to the distribution of the sexes in L . clavatum from the few 
U 
