102 



THE OPHIOGLOSSALES 



through the mesophyll do not project at the surface as veins. In the section Phyllo- 

 trichtum, where the sterile lamina is horizontal, stomata are developed only upon the 

 lower surface. In these forms the mesophyll is more compact upon the upper side, 

 but does not develop a distinct palisade layer. The epidermal cells may be simply 

 elongated, e. g., B. lanuginosum, or they may be undulate, e. g., B. virgtntanum. 

 The development of the stomata was examined in B. lanuginosum and the stoma 

 was found to arise directly from an unmodified epidermal cell. The section Eubo- 

 trychium shows a further approach to Ophtoglossum in the character of its stipular 

 sheath, which, as we have seen, forms a hood-like appendage at the base of the leaf, 

 while in B. virginianum the stipular sheath is open in front, so that there are really 

 two stipules. Moreover, the leaf in the latter is sharply bent over in the bud in a 

 way to suggest somewhat the circinate vernation of the Marattiaceae. For the most 

 part the surface of the plant is quite smooth, as in Ophtoglossum, but the sheaths 



Fig. 75. 



A. Stem apex of young sporophyte of 5o/r)'c//iMm u/rj;m/anum. .v, apical cell; /, youngest leaf. X200. 



B. Cross-section of stem apex. 



C. Part of vascular cylinder of stem, pi, phloem; cam, cambium; xy, xylem; m, medullary ray. 



D. Some of the tracheids, more enlarged, showing bordered pits. 



enveloping the young leaves, as well as the very young leaves themselves, are often 

 hairy, and in a very few species, like B. lanuginosum, long and slender unicellular 

 hairs are sparsely scattered over the surface of the adult leaf. The stem, as we have 

 seen, is always an upright rhizome and usually unbranched, but adventitious buds 

 may be developed (see Bruchmann 2). These adventitious buds may develop into 

 lateral branches. In one specimen of B. lanuginosum examined by me there were 

 two equal branches, presenting the appearance of a dichotomy, but a careful exam- 

 ination indicated that the original apex had been destroyed and that these two 

 branches were lateral shoots, which had probably arisen as adventitious buds. In 

 Eubotrychium the leaves are arranged spirally upon the rhizome, as they are in 

 Euophioglossum. This is also true of certain members of the section Phyllotrichium, 

 but others, e. g., B. ternatum, are arranged in two rows, thus suggesting the dorsi- 

 ventral arrangement found in Helminthostachys. 



The structure of the adult stem does not differ, except in size, from that of the 

 younger plants already described. In Eubotrychium the cambium is absent and 

 apparently there is no secondary increase in diameter, but all of the larger species 

 of Phyllotrichium show a greater or less development of the cambium, which we have 



