of Vascular Strands in Angiopteris evecta. 401 
foliar gaps. It is only subsequently that the inner and outer 
phloem and endodermis become united by way of the leaf 
gaps. In Angiopteris , as has already been pointed out, the 
internal phloem clearly arises in relation to, and in consequence 
of, the occurrence of the leaf gaps. Again, in Platyzoma 
microphyllum 1 there is no internal phloem, but a well-marked 
endodermis, whilst in Helm in tkos tacky s there is only an 
irregular internal endodermis in the older stems, the young 
plants being quite destitute of any tissue, apart from pith- 
parenchyma within the xylem. The latter point is singular, 
since, according to Poirault 2 , precisely the reverse obtains in 
Botrychium. But it is of interest to note that, in spite of 
these very diverse modes of development and of completeness, 
the siphonosteles all exhibit, as seen in transverse sections, 
the ( polystelic ’ or ‘ dialystelic ’ condition in consequence 
of the enlargement and frequency of the foliar gaps. 
The examples thus quoted might be greatly extended and 
amplified ; they may serve to show how readily the vascular 
system of the Ferns may be treated from a comparative 
standpoint, if the salient and more permanent features are 
clearly divested of variable details of subordinate importance ; 
and in this connexion the principal characters and mode of 
fractionation of the vascular strands and the primary relation 
of the process of the leaf gaps is clearly of first-rate import- 
ance. Cases of apparent exceptions, e. g. Kaulfussia , in 
respect of the ventral gaps in the siphonostele, are readily 
brought into line with the rest. But the degree of import- 
ance of this or that tissue in the composite strands themselves 
is shown by comparative anatomy to be a feature of minor 
significance, inasmuch as considerable variations may exist 
not only in related forms, but also in the body of one and the 
same individual. 
1 Boodle, On the Anatomy of the Gleicheniaceae. Annals of Botany, xv, p. 735. 
2 Poirault, Rech. anat. sur les Cryptogames vasculaires. Ann. Sci. Nat. (Bot.), 
s&r. 7, t. xviii. 
