3°8 
Lang . — The Prothallus 
the physiological processes taking place in such organisms as 
these renders any explanation of the differences in the arrange- 
ments of tissue in the several species at present impossible ; 
but as regards their morphological value, they may fairly be 
considered as comparable to variations in the details of 
arrangement of the assimilating tissue of leaves in closely 
related plants. The furthest degree of separation of the types 
that they justify us in assuming is that these types represent 
independent modifications of less differentiated prothalli, some- 
thing like those of Z. cernuum , which have enabled them to 
become wholly saprophytic. Such close similarity in the 
arrangement of the various layers as is seen in Z. clavatum 
and annotinum on the one hand, and in Z. complanatum and 
alpimtm on the other, probably indicates comparatively recent 
origin of these pairs of species from ancestors the prothalli of 
which were already adapted to saprophytic life. The close 
relationship here indicated by the gametophyte is borne out 
by the similarity of the sporophytes of the species named. 
With regard to the Z. Phlegmaria type of structure, the most 
probable view would appear to be that, along with the reduc- 
tion in thickness of the branch, the structure had remained 
or become greatly simplified. The dorsiventral cylindrical 
branches of the prothallus of Z. Selago afford a suggestion of 
how such a state of things may have come about, whether the 
Z. Selago type be regarded as in any way related to that of 
Z. Phlegmaria or not. It will be evident that if the complexity 
of structure of some of the saprophytic forms of Lycopodium 
has arisen as an adaptation to the mode of life, it cannot be 
regarded as evidence of the descent of the Lycopodium game- 
tophyte from highly differentiated ancestors as Bruchmann 
suggests 1 . The similarity of the regions into which the tissues 
harbouring the mycelium are divided in the Lycopodium pro- 
thalli to those which Janse 2 has been able to distinguish in 
the roots of many plants containing a mycorhizal Fungus, 
affords considerable support to the point of view advocated 
above. 
1 Loc. cit., 1898, p. hi. 
2 Ann. Jard. Bot. Buit., xiv. p. 53. 
