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of Lycopodium clavatum , L. 
surrounds a central strand of elongated cells which broadens 
out above where it is continuous with the cushion of the 
generative half. In this prothallus the endophyte is never 
found between the cells, and the reserve materials are stored 
within the cells occupied by its hyphae. The differences 
presented by the other subterranean prothalli almost entirely 
concern the band of tissue in which the endophyte is found ; 
in all of them there is a central region of more or less 
elongated cells, and the external limiting layer is free from 
the mycelium. In L. complanatum a few layers of oval cells 
containing the Fungus within their cavities are present within 
the latter, while between them and the central strand is found 
a single layer of cells enormously elongated at right angles to 
the surface ; the mycelium extends between these cells, but 
is absent from their cavities which contain reserve materials. 
The differentiation of the corresponding region of the pro- 
thallus of L. clavatum has been described in the earlier part 
of this paper. In this type several layers of oval cells with 
intracellular mycelium are found within the limiting layer, 
then a single layer of palisade-cells also containing the 
endophyte, while next to the central tissue is a broad band 
of smaller cells stored with reserve materials between which 
the hyphae extend. Finally, in the case of the prothalli of 
the L. Phlegmaria type, the endophyte is absent from the 
outermost layer, but is found in the cavities of most of the 
cells within this ; in the thicker branches a central strand of 
elongated elements can be recognized. This simplicity of 
structure, which is associated with the thin cylindrical branches 
of which these prothalli consist, finds its nearest analogue in 
the much thicker dorsiventral branches of the prothallus of 
L. Selago , the tissues of which are, however, more sharply 
differentiated. 
In considering these structural differences it must be borne 
in mind that they concern the vegetative region of the pro- 
thallus, those tissues which there is reason to believe are 
occupied in the assimilation of the absorbed plant-food and 
its storage in the form of starch and oil. Our ignorance of 
