26 Lang. — Prothalli of Ophioglossum pendulum 
and the old prothalli (Fig. 6) thus come to consist of short, 
stout branches radiating in all directions into the humus. 
All the branches are similar in external form. The end of 
one is represented as seen from the side in Fig. 7. As this 
figure shows, the apical region is conical and its surface 
smooth. A short distance from the growing-point, however, 
the branch has attained its full thickness, and its surface has 
become covered with short wide hairs. Rhizoids appear to 
be entirely wanting. Both kinds of sexual organs occur on 
the same branch, but only the antheridia are conspicuous on 
external examination. 
The general structure of a young prothallus is shown in 
longitudinal section in Fig. 8. The details of apical growth 
and of branching have not been followed in the few specimens 
of this age, but this omission is the less important, since there 
is no reason to expect any essential differences in this respect 
between the young prothallus and the branches of older ones. 
The tissue of the young prothallus is throughout parenchy- 
matous: the cells of the lower half contain an endophytic 
Fungus, while those of the upper portion are free from it. At 
the extreme base, where presumably the spore membrane was 
attached, the surface is smooth and a few superficial cells 
contain the endophyte. A little above this the superficial 
layers become free from the Fungus, and throughout the 
further growth of the prothallus the endophyte is confined to 
internal tissue, the outer cells being only traversed by infecting 
hyphae. Many of the cells of the outermost layer give rise 
to short wide hairs, but these are absent at the extreme base 
and over the meristematic region. The breadth of the apical 
region in this specimen probably indicates that it was 
destined to give rise almost at once to several branches. 
Other specimens, while agreeing in general structure, have 
this region less flattened, and were probably about to con- 
tinue their growth without for the time branching (Fig. 9). 
A more detailed account must be given of the structure of 
a branch of the prothallus such as that described above (Fig. 7). 
The apical growth of such a branch can be referred to a single 
