460 



OPHIOGLOSSALES 



that all the three genera show either a solid xylem-core or a slightly 

 medullated stele in the young axis. 



Proceeding to the upper part of the shoot the medulla increases, while 

 the vascular tissue forms a more or less interrupted cylinder surrounding 

 it : the interruptions are the leaf-meshes, for above the exit of each leaf- 

 trace there is a gap in the cylinder. Jn Ophioglossum the meshes are large, 

 and as the leaves are arranged in a compact spiral, the whole system assumes 

 a form clearly shown in Rostowzew's drawings (Fig. 236, Nos. 4, 5). In 

 Botrychium a similar arrangement is found ; but as the proportion of the 

 leaf-meshes to the whole surface of the cylinder is less, it approximates 

 more nearly to a continuous tube. This is still more clearly seen in 



Helminthostachys, where the shoot is 

 dorsiventral ; for there the foliar gaps 

 are disposed obliquely upon the 

 upper side only of the cylinder, while 

 the lower side of it is uninterrupted 

 (Fig. 257). It would thus appear 

 that the vascular system of the axis 

 is essentially similar in them all, and 

 is referable in origin to the amplifi- 

 cation of a primitive stele, with a 

 distending pith, and perforation of 

 the vascular cylinder by foliar gaps. 

 As regards the tissues themselves, 

 the most important of them for 

 comparison are the xylem and the 

 endodermis. The latter shows 

 curious irregularity of occurrence in 

 this family. In Botrychium there 

 is a well-marked outer endodermis 

 throughout the length of the stock : 

 there is also an inner endodermis in the pith, but it is found only at 

 the base of the stock (Poirault). In most species of Ophioglossum 

 there is no endodermis in the stock at all; but in O. Bergianum, 

 capense, and ellipticum, all small species, an outer and inner endodermis 

 are 'both present, though at the base of the stalk only : passing upwards 

 they fade gradually away, the inner disappearing first. 1 In Helmintho- 

 stachys, curiously enough, the converse is seen : here there is throughout 

 the stock a well-marked outer endodermis, as in Botrychium, but the inner 

 appears only in the older stems, the young plants being quite destitute of 

 it. 2 It is difficult to draw any definite conclusions from such discordant 

 facts : it must suffice for the moment to remark that, on the one hand, 

 there is want of constancy of the endodermis also in the Psilotaceae, and 

 on the other, that in the Marattiaceae the endodermis is present in 





FIG. 257. 



Helminthostachys zeylanica^ Hook. The upper 

 figure represents the vascular skeleton, dissected out. 

 L = leaf- trace ; R root-strand ; Fg foliar gap. The 

 lower figure shows the rhizome-stele giving off a leaf- 

 trace, L.T., which breaks up above into separate 

 petiolar bundles. /?= root-trace. (After Farmer 

 and Freeman.) 



1 Poirault, I.e. , p. 169. 



Farmer and Hill, Ann. of Bot. xvi., p. 401. 



