49 
and Helminthostachys zeylanica. 
important to determine as far as possible the relatively 
primitive and the highly modified forms of prothallus in each 
group, and to confine the comparison mainly to the former. 
The heterosporous forms will therefore be left out of account 
here and only mentioned when they seem to weaken the 
evidence afforded by the homosporous forms. 
The Lycopodiales may be taken first, since in this group 
prothalli, which are practically autophytic, and subterranean 
holosaprophytic forms occur: this makes it possible to as- 
certain the characters which remain constant in spite of this 
profound physiological change. In a previous paper 1 reasons 
have been given for regarding the Lycopodium cernuum type 
of prothallus as the relatively primitive one in this group. 
The recently discovered prothallus of Phylloglossum 2 gives 
further support to this view. In these prothalli an out- 
standing feature of the general plan of construction is the 
intercalary position of the meristem : from this additions are 
made to the vegetative region below, while the sexual organs 
originate above the meristematic zone, the youngest being 
found nearest to it. This special type of radial symmetry is 
retained in many of the subterranean forms, so that it is not 
necessarily obscured or lost in relation to such a change in the 
economy. Now the radial symmetry of the Ophioglossaceous 
prothalli, and the apical position of the meristem in them, 
stand in marked contrast to this and afford no suggestion of 
a common origin. Comparison is however at first sight more 
justifiable between the Ophioglossaceous prothalli and those 
of Lycopodium Phlegmaria. The fine cylindrical branches of 
the latter have apical growth, though, when sexual organs are 
formed, they originate on the upper side above the meristem. 
In the paper cited above reasons have been given for regarding 
this type of Lycopodium prothallus as derived from the type 
with an intercalary meristematic zone by limitation of growth 
1 Lang, Prothallus of Lycopodium clavatum. Annals of Botany, vol. xiii, 1899, 
p. 279. 
2 Thomas, Preliminary Account of the Prothallium of Phylloglossum. Proc, 
Roy. Soc., 1902. 
E 
