432 Farmer and Freeman. — On the Structure and 
in a zone occupying a position about midway between the 
interior and the periphery of the xylem, although they are 
not very regular in their exact locality. The xylem then 
belongs to the Mesarch category. This is of especial interest 
considering the weight which has been attached to this 
peculiarity in certain groups of plants in which it character- 
istically occurs. But it is clear that, as with almost any other 
single character, it is possible to overstrain its importance, 
and as a matter of fact the relative position of the protoxylem 
is seen to vary in some cases within quite narrow limits. 
Thus in the Ophioglosseae so far as at present known, 
mesarch strands are confined to Helminthostachys , the rest of 
the species investigated possessing strictly endarch xylem. 
We have ourselves studied Ophioglossum vulgatum , not only 
in material prepared by ourselves but also in other excellent 
serial sections kindly lent us by Mr. Boodle, and we find that 
the protoxylem is invariably endarch in development. The 
same arrangement also occurs, apparently exclusively, in 
Botrychium Lunar ia 1 as well as in the other species of which 
the anatomy is known. 
Again, in the Lycopodineae, the bundles are typically 
exarch, the protoxylem being placed peripherally, but 
Harvey Gibson 2 has shown that in Selaginella spinosa 
the protoxylem is centrally situated. And again, as Bower 3 
has pointed out, the xylem in the peduncle of P hylloglossum 
is mesarch, just as we find it to be in Helminthostachys. The 
case of Phylloglosswn is interesting, since it closely resembles 
in the general outline of its xylem that of the Fern now under 
discussion, and at the same time a similarly formed xylem- 
strand is met with in some specimens of Ophioglossum Ber- 
gianum 4 ; in the last named plant, however, the xylem is 
endarch as in the rest of the genus. Our own observations 
confirm this. And, to quote Selaginella once more, the species, 
.S', laevigata , var. Lyallii , which bears superficially a close re- 
semblance to Helminthostachys in the structure of its vascular 
1 See Russow, Vergl. Unters. d. Leitbiindel-Kryptogamen. 
2 Loc. cit. 3 Bower, loc. cit. 4 Loc. cit. 
