306 Lang . — The Frothallus 
found in L. Selago give the clue to the more specialized 
saprophytic types which, in the deeper-growing subterranean 
species, retain the radial symmetry while becoming modified 
in shape. On the other hand, the type of prothallus growing 
in rotting wood has lost the radial symmetry, and consists of 
cylindrical but more or less clearly dorsiventral branches. 
The variability of several characters, such as the presence or 
absence of leafy lobes, the distinctness of the primary tubercle, 
and the passage from radial to dorsiventral symmetry within 
the limits of the L. cernuum type, when taken in conjunction 
with the varieties of form of the prothallus of L. Selago , 
appears to justify such a view as that suggested above. The 
differences between the archegonia in the saprophytic forms 
are all such as suggest later modifications rather than deep 
morphological distinctions. 
The importance to be attached to the differences in anato- 
mical differentiation in the vegetative regions of these prothalli 
has now to be considered. The generative half of the pro- 
thallus consists in every case of a parenchymatous tissue into 
which the endophytic Fungus does not penetrate. In L. 
cernuum , &c., the lower half has been seen to be almost 
equally undifferentiated save by the distribution of the 
endophyte, which occupies the cavities of the outer layer of 
cells of the primary tubercle and penetrates between the cells 
of the internal tissue. In the case of all the prothalli in 
which the oldest region adjoining the spore is known, the 
endophyte is present in the limiting layer of cells of this 
region only ; as suggested above this may be regarded as the 
representative in these prothalli of the distinct primary 
tubercle of L. cernuum and salakense. In the vegetative 
portion of the prothallus of L. Selago , which has been seen 
to approach those of the L. cernuum type most closely in 
external form, the structure is correspondingly simple. The 
cells of the limiting layer, with the exception of those pro- 
longed as rhizoids, are free from Fungus. This is found 
within the cells of a broad band of tissue internal to this, the 
cells of which show no alteration of their form. This tissue 
