106 
PROFESSOR A. ANSTRUTHER LAWSON ON 
the breaking away of one or more tiers of neck cells. The venter, with its conspicu- 
ous egg-nucleus below the surface, is quite evident in all these cases where the 
section is a median one. These archegonia are. apparently mature, and ready for 
fertilisation. 
Compared with the Tmesipteris archegonium there are one or two minor points 
of difference to note. In the first place, the archegonia of Psilotum are smaller. 
They are a little more than half the size of those of Tmesipteris .- This difference in 
size is quite noticeable in both the longitudinal view and the surface view. The 
surface view represented in fig. 14, compared with fig. 40, shows quite clearly that 
the archegonium of Tmesipteris is just about twice as large as that of Psilotum ; for 
both these figures are drawn at the same magnification. Comparing figs. 15, 16, and 
17 with figs. 41, 42, and 43, the same difference in size is noticeable. Even the egg- 
nucleus of one plant in each case is larger than the other. It should be noted that 
a corresponding difference has been mentioned above in connection with the size of 
the antheridia. 
In Psilotum there are four rows of neck cells to the archegonium, surrounding a 
neck canal. A typical example of this, as seen from the surface view, is represented 
in fig. 40. As shown in figs. 38 and 39, there are evidently six tiers of cells in the 
neck. In fig. 38 is shown a longitudinal section of the archegonium, not quite in the 
median plane. It clearly shows six tiers of neck cells, and the egg cell with its 
nucleus in the venter. Fig. 39 is a similar view, but in a median plane. The neck 
canal is represented ; but I was unable to determine the number of cells it contained. 
The manner of the breaking away of the distal tier of neck cells seems to be 
different from that which 1 have described for Tmesipteris. It was the rule in the 
latter plant that all but the basal tier of neck cells broke away (figs. 15, 16, 17). I 
have observed very few such cases in Psilotum. Fair samples of the mature 
archegonia in the latter are represented in figs. 41, 42, and 43. In the first of these, 
two tiers of neck cells are left ; in fig. 42 one may see four tiers ; and in fig. 43 there is 
but one. From the numerous examples studied it seems evident that the number of 
tiers of neck xells broken away is not as constant as in Tmesipteris. In more than . 
one case, coiled bodies, in every essential resembling spermatozoids, were observed 
associated with the opening of the neck canal. Such a fertilisation stage is repre- 
sented in fig. 43. 
It is evident from this account that the archegonium in Tmesipteris and Psilotum 
is of the same type. The only essential difference is that of size, and this was also 
noted as a conspicuous difference in the antheridium of these two plants. 
Theoretical Considerations. 
In organisms such as the Pteridophyta, where there are two distinct phases in 
the life-history — the gametophyte and the sporophyte, — it becomes a matter of 
great interest and importance to have as complete a knowledge as possible of 
