The Anatomy of Rumex with Special reference to the morphology of the internal bundles and the origin of the internal phloem in the Polygonaceae

Abstract

The structure, development, and course of the vascular bundles in two species of Rumex, one possessing internal bundles (R. orientalis) and the other lacking internal bundles (R. dentatus), have been studied. It has been found that in both these species about 75 per cent of the bundles of the normal ring are first formed as a group of phloem cells alone. Later, on the side of the phloem facing the pith, a cambium is differentiated that forms all the xylem. Thus there is no xylem of primary origin. This condition is comparable with the development of the internal bundles in R. orientalis and R. crispus. An interfascicular cambium is absent in the internodes of Rumex dentatus. The development of internal bundles in R. orientalis resembles the development of similar bundles in R. crispus as described by Maheshwari. But the former species differs from the latter in the presence of internal bundles: (a) even in very young plants; (b) up to the base of the first internode; and (c) in the hypocotyl, where the internal bundles are found in a perfect medullary position and consist of phloem and a surrounding cambial ring only. Freely scattered medullary bundles are also rarely found in the internodes of Rumex orientalis. In Rumex orientalis, the internal bundles of one internode are seen to be formed from the internal bundles of the upper internode, from some adaxial bundles of the axillary branch, and from some of the vascular bundles of the normal ring of the upper internode. In the node, the internal bundles run a truly medullary course. From consideration of the above facts and from comparison with related plants, it is concluded: (a) The specialized internal bundles found in the genus Rumex have been derived from true medullary bundles such as are found in the genus Rheum. (b) The perennial species of Rumex without internal bundles are the oldest forms of the genus. From these have arisen the perennial forms with internal bundles and the annual forms without internal bundles. In some cases, however, the annual forms may have been derived from perennial forms with internal bundles by loss of these structures. (c) The internal phloem in the family Polygonaceae has been derived by reduction from a preexisting system of medullary bundles in the manner suggested by Worsdell for Cucurbitaceae and Compositae.