OBSERVATIONS ON THE VASCULAR SUPPLY TO AXILLARY BRANCHES

Abstract

Surprisingly little information is available concerning the manner in which the vascular system of a lateral branch is connected with that of its parent stem. It is clear, however, that in the majority of dicotyledons the vascular supply to an axillary branch is derived entirely from the margins of the gap left by the passing out of the median trace of the axillant leaf. For many years it has been known that exceptions to this rule occur in certain families (de Bary, I884, pp. 309-I0). In particular, a condition has been described for Aralia japonica and for several species of Umbelliflorae in which the axillary shoot receives vascular strands also from the margins of lateral leaf gaps. Recent investigations have brought to light some further examples of this type of structure, and by means of a new dissection technique it has been possible to elucidate the vascular connexions with a completeness and accuracy not previously attained. In Fig. i is shown a piece of the rhizome of Petasites vulgaris which has had all the tissues external to the xylem removed, this operation being made possible by previous maceration with hydrochloric acid. The stele lies horizontally with its apex to the right of the figure. The portion represented bears the xylem strands of several adventitious roots, three of which are visible (R). These strands have no effect on the anatomy of the node and will not be further described. Eight foliar traces are visible out of a total of about fifteen (L). These traces are spaced out fairly uniformly round the whole circumference of the rhizome. As each leaf trace leaves the stele it is accompanied or followed by a number of other bundles, and the strands arising from any particular gap, including the foliar trace, form themselves into a kind of miniature stele (M), which soon becomes completely free from the main stele of the rhizome and runs parallel to it in the rhizome cortex. On reaching the level of the leaf insertion these miniature steles unite into a vascular girdle (G) which completely encircles the node and which is continuous with the base of the axillary branch (B). The bundles of the girdle are numerous, and undergo complex and irregular anastomosis and bifurcation. For the sake of simplicity only a few of these bundles are shown in the figure. It should be noted that at the level of the girdle the foliar traces separate from the miniature steles and continue directly into the base of the leaf, taking no part in the anastomoses of the girdle. At the level of the node and for some distance below, the entire system of miniature steles, girdle, foliar traces and branch base is quite free from the stele of the rhizome, being separated from it, in life, by masses of cortical parenchyma. Dissections cannot reveal the course of the phloem strands, which are removed with the cortical tissues, but serial sections of intact rhizomes show that the phloem everywhere closely follows the course of the xylem. A nodal structure essentially the same as that just described has also been found in