366 MOSSES AND FEKiYS chap. 



the investigations of the latter it seems that here, as in nearly 

 all other Ferns, the stem apex has the usual three-sided initial 

 cell, but only a small part of the segments give rise to leaves, 

 which are arranged in two ranks. As in Gleicheiiia, there is a 

 single vascular bundle in the stem, and this, according to Prantl,^ 

 is collateral in HemipJilebium as in Osmunda. The tracheary 

 tissue lies upon the ventral side of the stem, the phloem on 

 the dorsal side. The pericycle,^ which at points shows clearly 

 its common origin with the endodermis, surrounds the whole 

 bundle. The cortex is composed in part of parenchyma, and 

 partly of sclerenchyma. In the sub-genus Hemiphlebium the 

 latter occupies the periphery of the stem, in the others the 

 position is reversed, and it lies next the vascular bundle. In 

 the other forms also the stem bundle is concentric, and cor- 

 responds closely with that of Gleichenia. 



The Leaf 



The observations on the earliest stages of the leaf are very 

 incomplete, but in some cases at least a two-sided apical cell is 

 present. In those with palmately lobed or entire kidney-shaped 

 leaves, the later growth is marginal, and of the same type found 

 in similar leaves among the Folypodiacese. The venation in 

 these forms is exclusively dichotomous, in those with pinnate 

 leaves, e.g. TricJwinanes radicans, this is only true of the last 

 formed veins. 



With the exception of a very few species, e.g. T. reniforme, 

 H. dilatatum, where the mesophyll of the leaves is three to four 

 cells thick, the whole lamina, with the exception of the veins, is 

 single-layered, and of course stomata are completely absent. 

 The form of that leaf is either pinnate, as in the larger species 

 of Trichoinanes and Hymenophylhivi (Fig. 190), reniform {T. 

 reniforme^, or palmately divided {T. parvulum, Fig. 190, B). 

 The smaller veins, as in other Ferns, have collateral vascular 

 bundles, and in the smallest ones the xylem may be reduced to 

 a single row of tracheids. The latter may be spiral, reticulated, 

 or scalariform. In the phloem Prantl could not distinguish any 

 well-marked sieve-tubes, but it was mainly composed of bast 



1 Prantl (i), p. 26. 



^ Van Tieghem (3) considers the endodermis to be double, and that no true 

 pericycle is present. 



