Medullary Bundle in Relation to Primary Vascular System in Chenopodiaceae and Amaranthaceae

Abstract

1. An anatomical study has been made of the primary vascular system of a number of species of Chenopodiaceae and Amaranthaceae. 2. A series of diagrams of the longitudinal course of the primary bundles in these species is presented. They set forth in an ascending series a probable method of evolution of the medullary arrangement of bundles as found in Amaranthus, and possibly as found in other dicotyledonous families possessing medullary bundles. 3. It is concluded that the steps in this progression are possibly as follows: (a) the cauline bundles on either side of an entering leaf trace pass into the pith a short distance (probably due to the influence of the node), and return quickly to their normal position just under the extrafascicular cambium; (b) this condition becomes exaggerated: the cauline bundles abandon their temporary course in the pith, and become medullary throughout their course in the stem; (c) the bundles of the trace behave in a similar manner. In the earlier stages of such a development, they are also members of the peripheral ring of bundles; later their course in the stem becomes more and more extended, until they are finally completely medullary. 4. Attention is called to the belief, based chiefly upon the wide distribution of medullary bundles among various dicotyledonous families, that this bundle arrangement has probably had an independent evolution in the various groups in which it is found. 5. Based upon the nature of the primary vascular system, the two families dealt with here are considered to be closely related. This conclusion, based chiefly upon flower morphology, has already been reached by other workers.