28 Lang . — Prothalli of Ophioglossum pendulum 
hair. Numerous colourless plastids are present in the cyto- 
plasm around the nucleus. The outer cell-walls, whether of 
the hairs or of other superficial cells, possess a moderately 
thick cuticle. 
To complete the description of the vegetative portion of 
the prothallus, the distribution and structure of the endophytic 
Fungus must be referred to. It has been seen that the Fungus 
in the young prothallus occupies the superficial cells at the 
base and extends into the central tissue of the lower half of 
the prothallus. This distribution is also recognizable in the 
old branched prothallus when the section passes through the 
central region, which corresponds to the base of the young 
prothallus. In the further growth of the prothallus, however, 
one or two superficial layers of cells always remain free from 
the endophyte, save for hyphae making their way into the 
deeper tissue. The diagram (Fig. 15) will make this clear. 
In the older region of a branch the Fungus occupies all the 
cells save the superficial zone of one or two layers. On 
passing towards the apex of the branch the Fungus may dis- 
appear from the central cells also, persisting however for 
a longer or shorter distance in an intermediate zone of tissue. 
In other cases the endophyte stops at the same level in both 
the axial and the more external cells. 
Since the first stages of germination of the spore have not 
been followed information as. to the first infection of the pro- 
thallus with the symbiotic Fungus is wanting. There is how- 
ever a process of repeated infection, to which the extension of 
the Fungus towards the apex of the branch must in great 
part be ascribed. It will be convenient to trace the entrance 
of the Fungus at one of these points of infection and to 
consider the several structures in the order of their appearance. 
When a branch is examined from the surface, coarse brown 
hyphae can be seen more or less closely applied to its surface. 
The filaments which penetrate the prothallus are in con? 
nexion with these, and as a rule enter by one of the hairs. 
That they are branches of the external mycelium entering at 
these points, and not branches of the endophytic mycelium 
