38o MOSSES AND FERNS chap. 



is much greater. According to Boodle (1. c. Fig. 24), there 

 are two or three protoxylems, accompanied by parenchyma 

 cells, surrounded by a massive ring of large tracheids. There 

 is an approach in this species, and still more in T. reniforme, 

 to the form characteristic of Hymenophyllum scabrum and its 

 allies. In the small species, T. muscoides, apparently by reduc- 

 tion, the stele becomes collateral, and this, according to Prantl 

 ( ( I ) , p. 26) , is the rule in the sub-genus H ennphlehium, where 

 the xylem lies on the ventral side of the stem, the phloem on the 

 dorsal side. The pericycle, at certain points, shows clearly its 

 common origin with the endodermis. Van Tieghem (3) con- 

 siders that there is a double endodermis, and that no true peri- 

 cycle is present. In T. lahiatum (T. microphyllum) Giesen- 

 hagen ( i ) found the bundle reduced to a single tracheid sur- 

 rounded by four or five parenchyma cells immediately within 

 the endodermis. The reduction is carried still further in T. 

 Motleyi, where tracheary tissue has entirely disappeared from 

 both stem and sterile leaf. In the sporophylls, however, trach- 

 eary tissue is present (Karsten (2), p. 135). 



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 Polypodiaceae. The venation in 

 these forms is exclusively dichotomous, in those with pinnate 

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

 formed veins. 



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

 H. dilatatiim, 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 the leaf is either pinnate, as in the larger species 

 of Trichomanes and Hymenophyllum (Fig. 219), reniform 

 {T. reniforme) , or palmately divided (T. parvulum, Fig. 219, 

 B). The smaller veins, as in other Ferns, have collateral vas- 

 cular bundles, and in the smallest ones the xylem may be re- 

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

 reticulated, or scalariform. In the phloem Prantl could not 



