Lang . — On a Prothallus provisionally referred to P silo turn. 573 
the meristematic region. The symbiotic Fungus will be considered along 
with the vegetative region, in which it occurs. 
The general relation of the sexual region to the rest of the prothallus has 
been described above. It only remains to point out that, as a comparison 
of the sections in Figs. 3 and 4 indicates, the thick margin was not equally 
developed all round. The former cut missed the overhanging edge on both 
sides, while the latter is taken through a place where it was well developed. 
The hypertrophied appearance of this edge was very striking ; it was 
thrown into folds, and at places tears in its interior showed the strain 
which the development of the surface bearing the antheridia had caused. 
The antheridia originated in regular succession, the youngest being next 
the meristematic region, the position of which is marked with a cross in 
Fig. 4. From the few developmental stages observed it was clear that the 
antheridium originates in the same way as that of Lycopodium , the first 
division separating an outer cell, which forms the wall, from an inner one 
giving rise to the mass of spermatocytes. The outer wall of the mature 
antheridium (Figs. 5, 6) is one layer of cells thick, and is nearly level with 
the surface of the prothallus. Sometimes the antheridium was extended 
parallel to the surface as in those figured, while in other cases it was 
elongated at right angles to the surface. There were no hairs (paraphyses) 
growing from the surface among the sexual organs. 
The vegetative region exhibited greater histological differentiation in 
relation to the presence within it of an endophytic Fungus, the definite 
distribution of which indicated its symbiotic nature. The central mass of 
tissue was entirely free from the Fungus, which was confined to a peripheral 
zone indicated by the shading on the cut surface of the prothallus in Fig. 3. 
The mycorhizal zone is continuous over the whole vegetative half of the 
prothallus (Figs. 3, 4, 8). On approaching the meristematic zone it 
narrows (Fig. 4), and in this situation the development of its constituent 
layers could be traced. 
The relative position and appearance of these layers will be evident 
from the diagram in Fig. 8 and the slightly diagrammatic drawing in 
Fig. 7. The outermost layer {a) consisted of cells free from the Fungus, 
save for filaments passing across from the bases of the rhizoids and entering 
the cells of the deeper layers. The rhizoids are simple protrusions of cells 
of this layer, and most of them contained one or more fungal hyphae ; 
whether these are to be regarded as passing inwards or outwards could not 
be determined, but the latter interpretation is more probable. Within the 
peripheral layer comes a zone of three or four layers of cells ( 5 ), the more 
external of which, like the superficial ones, were extended tangentially, while 
the inner cells have their longer axes at right angles to the surface, but do 
not form a distinct layer. These cells, each of which had a single healthy- 
looking nucleus in a central position, were filled with fine fungal hyphae, 
R r 2 
