408 Dr. G. Ogilvie on the Forms and Structure of Fein-stems. 



probably always depend on the shrinking of a lax cellular tissue 

 originally filling the space. Hollow stems are not described as 

 occurring in any of our indigenous Ferns, but it may generally 

 be observed that in the decayed portions of a rhizome the central 

 cellular tissue is the first to disappear. 



The most elegant and instructive preparation of a fern-stem 

 is made by dissecting off the outer cortical layer so as to expose 

 the reticulated structure of the fibro-vascular cylinder. Two sets 

 of fasciculi will then be seen to be connected with its exterior, 

 — the one derived, as already noticed, from the bases of the 

 petioles, the other continuous with the rootlets. 



The rootlets of Ferns are emitted in succession from below 

 upwards, in proportion as they are required for the nutriment 

 and mechanical support of the plant. The latter is probably the 

 main use of the so-called adventitious roots, which are sent 

 forth in such numbers from the lower part of the stem in some 

 Tree-ferns as to cause a remarkable increase in the diameter of 

 the base. The nutriment introduced by the roots must be 

 carried up by imbibition through the cellular matter of the 

 stem, or through the fibro-vascular network derived from the 

 bases of petioles long since decayed. In any case, it is evident 

 that, whatever may be alleged in favour of regarding a Dicoty- 

 ledonous tree as a mere aggregation of slender stem- bound phy- 

 tons, whose leaf-bearing shoots form its present foliage, while 

 their downward extremities are continuous with the active spon- 

 gioles of the roots, such a view cannot be held tenable in the 

 case of Ferns. In this group at least, and probably also in 

 Monocotyledons, we must admit two fundamentally distinct foci 

 of vital action — the leaf-bud and the rootlet, — the former ori- 

 ginating from the upper extremity of the stem, the latter con- 

 necting itself with its lower portions. In neither of these forms 

 of vegetation can the rootlets be regarded (as some would regard 

 them in Dicotyledons) merely as the onward continuation of 

 fasciculi sent down from the leaves, for the bundles of the leaf- 

 stalks descend, in Monocotyledonous plants, but a small way 

 along the stem, and in Ferns not at all ; while, in arborescent 

 species, the radicles are emitted only from the inferior parts, far 

 below the point reached by the former. 



There is every reason to believe that the emission of the root- 

 lets is a purely local action, and Prof. Henfrey's description of 

 it in Monocotyledons seems to apply also in Ferns*. According 

 to this author, the first rudiment of the rootlet is a funnel- 

 shaped circle of fasciculi in the cortical region at the base of the 

 stem, which, on the one hand, implant themselves upon the 

 fibrous layer within, and, on the other, converge to form the 

 • Ann. Nat. Hist. i. 187. 



