THE GAMETOPHYTE GENERATION OF THE PSILOTAOE^I. 
101 
of the latter. Some specimens appeared to be lighter coloured, more of a light 
yellow-brown than the darker colour of Tmesipteris. As in the latter, there is no 
trace of chlorophyll in any of the prothallial cells. It branches irregularly, and 
each branch terminates in a merismatic apex. It is clothed with numerous long 
rhizoids, which extend out from all sides and become intimately associated with 
particles of soil. A fair sample is that shown in fig. 18. This represents an entire 
prothallus. It shows the more or less cylindrical form, and the irregular manner 
of branching. Numerous rhizoids are to be seen growing out in all directions 
from the superficial cells. It should be noted that many rhizoids were broken 
away in freeing the prothallus from the particles of soil ; so that in nature 
they are much more numerous than indicated in this figure. It will be observed 
that there is a merismatic region at the apex of each branchy and in this parti- 
cular specimen a branch has apparently just bifurcated, and consequently there 
are two terminal meristems close together. On the surface of this specimen we 
may see numerous archegonia and antheridia in various stages of development. The 
endophytic fungus inhabiting the cells of the prothallus is quite observable from the 
surface view. 
The size of the prothalli obtained varied considerably. For instance, in fig. 19 
we have a superficial view of an exceptionally large prothallus. The portion here 
represented is only half the length of the specimen. The entire prothallus from 
which this figure was drawn measured just under three-quarters of an inch in length, 
and it was greater in diameter than any other specimen found. The immense number 
of antheridia and archegonia projecting from its surface was very striking. From this 
view alone there are forty antheridia and twenty archegonia visible, and quite as 
many on the opposite side. Several such specimens # as this were found, and they 
afforded excellent material for microtome sectioning, and following out the various 
stages in the development of the reproductive organs. The average size and form 
of the prothallus found is that represented in fig. 24. This is probably a younger 
state, but sufficiently mature to bear numerous antheridia and archegonia. The 
curious and rather indefinite sort of branching is indicated, as well as the merismatic 
apices of the branches. In such specimens as those represented in figs. 19 and 24 
one may very easily observe the spherical form of the antheridia, and the straight 
tube-like neck of the archegonia. 
An examination of the prothalli shown in figs. 18 and 24 will reveal a meristem 
at the apex of each branch, just exactly as it is in Tmesipteris. A longitudinal 
section through such a merismatic apex is represented in fig. 21. A study of these 
sections taken in series shows quite clearly that the apex consists of a mass of small 
cells, with dense finely granular cytoplasm and deeply staining nuclei. One of 
these cells (marked Ap.) I interpret to be the apical cell. An apical cell has been 
* Comparing such specimens as these with the figures and descriptions given by Lang (1904) of the prothallus 
which he provisionally referred to Psilotum, it will be seen that there is not the slightest resemblance. 
TRANS. ROY. SOC. EDIN., YOL. LII, PART I (NO. 4). 
16 
