Lang —On a Prothallus provisionally referred to P silo turn. 575 
the primary tubercle. The state of things is much the same as in the 
prothallus of Lycopodium clavatum 1 . 
The zone intervening between the vegetative and sexual regions must 
be regarded as the meristem of the prothallus on account of the succession 
of antheridia in the tissues above and the gradual differentiation of the 
mycorhizal tissues below. But little or nothing in the character of the 
cells of this zone indicated their meristematic nature. Probably, as is the 
case also in the subterranean prothalli of Lycopodium , this may be placed 
in relation to the slowness of growth. Whether this is the true explanation 
or not, it was impossible to arrive at any conclusion as to the succession of 
divisions in the meristematic cells from the single specimen available for 
study. 
Comparison with other prothalli, such as those of the Lycopodiaceae and 
Ophioglossaceae, justifies us in regarding the endophytic Fungus, the strictly 
limited distribution of which has been described above, as mycorhizal. The 
general differentiation of the tissues of the prothallus, the absence of 
chlorophyll, and the position in which the prothallus grew, all support the 
conclusion that it was a total saprophyte dependent in its nutrition upon 
the co-operation of the endophytic Fungus. 
It now remains to inquire to what extent we are justified in ascribing 
this prothallus to P silo turn . Our knowledge of the characteristics of the 
gametophyte in the great groups of Vascular Cryptogams is sufficient to 
enable us at once to limit the inquiry as to the systematic position of this 
prothallus to the homosporous Lycopodiaceae and the Psilotaceae. The 
gametophyte is known in a considerable number of species of Lycopodium 
and in Pkylloglossum, and I have previously discussed the several types 
occurring in the former genus 2 . The subsequent discovery of the prothallus 
of Pkylloglossum 3 , which appears to resemble most closely that of 
Lycopodium cernuum % though without the assimilatory lobes of the latter, 
confirms the view that this is the primitive type of prothallus in the group. 
In the paper cited I suggested several lines of adaptation to explain the 
various types of wholly saprophytic prothallus in the genus. A special 
type characteristic of the epiphytic forms (L. Phlegmaria, &c.) can be 
distinguished from the massive subterranean prothalli of a number of 
species normally growing in soil rich in humus. This latter type, as 
Bruch mann’s 4 exhaustive work shows, presents various grades of specializa- 
tion of the tissue containing the endophytic Fungus. 
The prothallus under discussion, with its radial symmetry and its growth 
referable to a meristematic zone between the vegetative and sexual regions, 
is obviously constructed on the general plan traceable throughout the 
known prothalli of Lycopodium, I11 size and general appearance it 
1 Bruchmann, loc. cit. 2 Annals of Botany, vol. xiii, 1899, p. 279. 
3 Thomas, Proc. Roy. Soc., vol. Ixix, 1901, p. 285. 1 Loc. cit. 
