Boodle . —Anatomy of the Hymenophyllaceae . 481 
2. A solid mass of xylem with internal protoxylems ac¬ 
companied by only scanty parenchyma. 7 . radicans , 7 . 
Prieurii . 
3. The sub-collateral type. 7 ". trichoideum . 
4. The collateral type. T. muscoides . 
5. Only one tracheide or no tracheide, and no phloem 
present. 7 . lahiatum and 7 . Motleyi . 
6 . A solid mass of xylem with scattered indefinite proto- 
xylem. 7 *. spicatum. 
7. A solid mass of xylem with distinct peripheral proto- 
xylem. 7 . scandens . 
Of these types, 4, 5, 6 and 7 do not occur in Hymeno - 
phyllum. A comparison with the latter genus, where transi¬ 
tional forms between types 1 and 3 occur, suggests that in 7 rz- 
chomanes also the sub-collateral structure has probably been 
derived from the two-banded type ( 7 . reniforme) by reduction, 
which has here gone still further and produced collateral 
structure in 7 . muscoides. As to the remaining types, it may 
appear doubtful whether they are to be regarded as more 
primitive than 7 . reniforme or more specialized forms. 
In the petiolar bundle simple collateral structure is found in 
small forms of both genera, in larger forms in both genera 
also, the xylem is an arc with a protoxylem at the two 
ends, and the xylem arc closes to a ring in the base of 
the petiole. Larger bundles with three protoxylems are found 
in Trichomanes only. 
Phylogeny, &C. 
With regard to the phylogenetic questions in Hymeno - 
phyllum and Trichomanes , several views may be suggested, e.g. 
1. That the solid stele (as in 7 . scandens) is primitive, that 
the type of 7 . reniforme was derived from this (by the replac¬ 
ing of many of the central tracheides by parenchyma), and 
that further specialization took place in the direction of 
reduction, ending with collateral structure ; that the genus 
Hymenophyllum is a series of reduction parallel with what is 
found in Trichomanes , and that, as its most complicated 
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