The Developmental Anatomy of the Annonaceae. II. Well-Developed Seedling Structure

Abstract

In well-developed seedlings the axis attains the maximum diameter in the region where tissues are undergoing transition rather rapidly. Only primary tissues are involved in all the phases of the transition phenomenon. In most of the examined species of Annonaceae the transition from a radial to a collateral arrangement of vascular bundles, and from exarch to endarch xylem, is completed in the upper half of the hypocotyl. A remarkable uniformity is found in this respect among all 13 species observed. The primary xylem of the root and that of the hypocotyl are in direct continuity and diverge entirely into the two cotyledons. The epicotyledonary xylem does not show any connection with the primary xylem of the root-hypocotyl-cotyledon system. However, the epicotyledonary phloem strands are in connection with the phloem of the future cotyledonary bundles in the upper part of the hypocotyl. These two vascular tissues demonstrate their individuality with respect to the pattern of differentiation. The axis attai...