TRANSACTIONS OF THE SECTIONS. 139 



fically adapted to elaborate, under the appropriate stimulus of light, its own proper 

 tint or succession of tints, and no other. 



11th. The subordinate but important office of light is to influence the development 

 of whatever colour the tissues are prepared for, which it effects in two ways. As 

 to degree, according to the presence, diminution, or absence of light, the colour may 

 be either brilliantly or feebly developed, or totally absent. As to change of colour, 

 light facilitates, heightens, or even wholly brings on, whatever modifications each 

 particular portion of plant-tissue is prepared for, whether in growth, maturity, or 

 decay ; but it cannot originate changes to which there is no inherent tendency. 



On the Vegetative Axis of Ferns. By George Ogilvie, 31. J). 



This communication embraced two principal points — the general form of the 

 rhizome of ferns, and its internal structure. The stems of our British species at 

 least may be reduced to three forms— the creeping rhizome, and the caudex, simple 

 or branched. These differences depend on the proportionate development of the 

 vegetative axis itself and the two kinds of appendages with which it is furnished, 

 the black wiry rootlets, and the persistent bases of the leaf-stalks. In the general 

 arrangement of the vegetable organization there is a distinct local separation between 

 the leaves and the rootlets, the latter being confined to the lower or underground 

 extremity of the stem. Yet this is not always the case even in arborescent forms ; 

 for in some species (as of Ficus) they arise also from the leaf-bearing portions of the 

 trunk. Further, in all classes of plants we meet with what are termed rhizomes, 

 t. e. prostrate or underground stems, from the whole extent of which leaves and 

 rootlets are emitted side by side : the stems of all our indigenous ferns are of this 

 kind. 



In the variety above referred to as the creeping rhizome, the stem is much drawn 

 out, so as to form a cord which branches frequently iti its course along or just under 

 the surface of the ground, and emits at intervals fronds from its upper, and rootlets 

 from its lower aspect. In the common Polypody the petioles break off from the 

 stem, leaving scar-like marks at the points of articulation ; but in the other species 

 and in the Braken, the lower extremities of the leaf-stalks remain adherent after the 

 upper portions and the fronds have decayed away, and by their comparative state of 

 preservation mark the limits of the growth of the rhizome in successive years. 



In the form of the stem termed a caudex, the axis is less drawn out, and more closely 

 set with rootlets and petioles,[and the latter are arranged in a spiral manner. In the 

 genus Asplenium, in Osmunda regalis, Blechnum bureale, Allosurua crispus, Lastrea 

 Oreopteris, and perhaps some other species, the caudex branches in a dichotomous 

 manner by the repeated duplication of the terminal bud, and the so-called root 

 acquires a shrubby character when the axis is exposed by the removal of the rootlets 

 and petioles. 



In some ferns, however, the caudex never branches at its extremity ; and when off- 

 shoots occur, they arise from lateral buds. This is the case in the massive imbricated 

 root-stock of the Male Fern and some other species of Aspidium. This form presents 

 many points of resemblance to the stem of a Tree-fern, though its small development 

 and horizontal line of growth prevent its forming any conspicuous trunk above the 

 surface of the ground. The resemblance becomes more apparent when the persistent 

 bases of the decayed fronds are cut off, and only the central axis left, marked with 

 spiral rows of cicatrices like the scars which occur on the stem of the Tree-fein. 



The chief peculiarity of the internal structure is the reduction of the fibro-vascular 

 system to a netted cylinder, imbedded in the general cellular tissue of the stem, and 

 giving off fasciculi both to the petioles and the rootlets. The annual increments of 

 the stem — which in Exogens form a series of conical envelopes of continually in- 

 creasing dimensions, each entirely enwrapping its predecessors, and which in En- 

 dogens have been compared to a series of envelopes of similar form, but of uniform 

 size, piled up into a column by successive superposition — are in the fern-stem repre- 

 sented simply by annular additions to the upper extremity of the netted cylinder. 

 The fibro-vascular bundles of the petioles, immediately on entering the stem, branch 

 but to form an upward extension of this cylinder in the growing extremity of the 

 corm ; but they have no downward prolongation, the isolated fasciculi of the interior 



