90 
G. P. DARNELL-SMITH ON 
The young archegonia are almost white and somewhat translucent ; the necks may 
be seen projecting from the broad, obtuse growing point of the prothallus. 
There are indications that the plant resulting from fertilisation leads a sub- 
terranean existence for some time. The formation and development of the embryo 
and the possible relationships of Psilotum will be, however, the subjects of a future 
communication by Prof. Lawson, who has this investigation now well in hand. 
8. Summary. 
The spores of Psilotum require a special environment to induce them to 
germinate. 
They produce a cylindrical, radially symmetrical prothallus. 
The prothallus is a brown, subterranean saprophytic tuberous body without 
chlorophyll, and contains an endophytic fungus. Antheridia and archegonia are borne 
upon the same prothallus. 
The antheridia produce spermatozoids having spirally wound bodies and 
numerous cilia. 
The archegonia are very simple. They consist of a venter containing the 
oosphere sunk in the prothallus, a ventral canal cell, and probably two canal cells. 
These latter are bounded by four tiers of four neck-cells that project at right angles 
to the surface. 
I am indebted to my assistant, Mr. W. A. Birmingham, for much careful work 
in the preparation of material and sections. 
I have to thank Prof. Lawson, in whose laboratory much' of the work was carried 
on, for much kindly advice and criticism during the progress of the investigation. 
EXPLANATION OF FIGURES. 
Fig. 1. A mature spore of Psilotum , showing the cleft from which the first cell of the prothallium emerges 
upon germination. The cleft has a lip on either side throughout its length. 
Fig. 2. A Psilotum spore, seen in optical section, that has lain in a suitable medium for four months, 
showing the condition just prior to germination. The nucleus is distinct and the spore is filled with globules. 
Fig. 3. A Psilotum spore, seen in optical section, that has lain in an unsuitable medium. The protoplasm 
has shrunk around the nucleus ; 'such a spore will not germinate. 
Fig. 4. First stage in the germination of a Psilotum spore. Several chromatophores are seen near the 
apex of the emerging cell, which contains numerous droplets. 
Fig. 5. Another stage in the germination of the Psilotum spore. At the apex are the chromatophores, 
and a small papilla is visible. The droplets show a tendency to congregate into two groups. A slight fold 
is seen near the apex. The exospore is seen as a shell at the base of the figure. 
Figs. 6 and 7. Surface view and side view of a germinating spore, showing a fold in the cell membrane 
produced by the partial invagination of the thin cell-wall. 
Fig. 8. Germinating spore of Psilotum, showing the formation of the first cells of the prothallium by 
