of Lycopodium clavatum, Z. 283 
axes at right angles to the surface, are filled with the mycelium 
of the Fungus, which is here entirely intracellular. In con- 
sequence of this they appear darker than the other regions of 
the section. Above the layer of palisade- cells comes a region 
(d, Figs. 5, 6) about eight cells deep, from the cell-cavities of 
which the Fungus is absent. These cells are smaller than 
those of the preceding layer, but are not infrequently elongated 
in the same direction. Their walls appear thicker than those 
of the rest of the prothallus, especially at the angles. 
Throughout the whole of this region the mycelium of the 
Fungus can be traced within these thickened walls, but never 
penetrating into the cavities of the cells. The latter contain 
a large quantity of starch, which is also present, though less 
abundantly, in the tissues above, but is practically absent from 
those in which the mycelium is intracellular. Whether, as in 
L . annotinum , oil was also stored in this region was not 
determined. The appearance of the four layers of tissue 
described above is shown in greater detail in Fig. 6. The 
region of the prothallus above the layer last described usually 
forms more than half its thickness ( e , Fig. 5). The cells 
composing it are thin-walled and possess a scanty protoplasmic 
lining. Those situated more internally are of large size, but 
those surrounding the antheridia or archegonia are much 
smaller. The latter form a fairly definite layer (/, Fig. 5) on 
the upper surface, the origin of which, from the superficial 
layer of cells at some distance from the growing region, can 
be followed in Fig. 5. This uppermost layer may be distin- 
guished as the generative layer, while the large-celled tissue 
beneath maybe compared to the cushion of a Fern-prothallus. 
The prothallus is thus seen to consist structurally of six more 
or less clearly defined regions, the four lowest of which are, as 
their structure indicates, concerned with the nutrition of the 
organism, while the uppermost layer contains the sexual 
organs. 
Great difficulty was experienced in tracing the origin at the 
growing regions of the different tissues, the mature condition 
of which has now been described. It may be stated at the 
