Occurrence of Medullary Bundles in the Apple Shoot

Abstract

MEDULLARY bundles are known to occur in some thirty-eight families of flowering plants; but hitherto they have not been reported in the Rosacese. Investigations are proceeding on the apple (var. Worcester Permain on Mailing II stock), from material collected in the orchards at Wye College, to try to determine not only their development and extent, but also to see whether their presence can in any way be connected with some physiological phenomenon, which will perhaps provide an indication as to their possible function. In this connexion, Alexandrov and Alexandrova¹ reported the presence of medullary bundles in the first two to three internodes of the inflorescence axis of Ricinus communis, appearing only after flowering and being completely absent from decapitated plants. From this they concluded that the medullary bundles were developed to meet the demands of a congested type of inflorescence. Westmaier (quoted in Haberlandt²) was of a similar opinion regarding their function as a result of a study of medullary bundles in the Begoniaceæ. He observed that in this family their occurrence is restricted to the stems of species that perennate by means of tubers or rhizomes, where the quantity of translocated materials passing through the stem is greater than in woody forms; and concluded that medullary bundles are formed to meet this need. Worsdell³, in a paper on the Cucurbitaceæ, concludes that the internal phloem, which he considers a vestigial structure, being medullary bundles which have lost their xylem, is a remnant of a monocotyledonous condition. I myself regard this latter theory as improbable, owing to the occurrence of medullary bundles in families of plants which bear no systematic relation to each other.