Anatomical and morphological studies in the Chenopodiaceae. II. Vascularization of the seedling

Abstract

An account is presented of the development of the vascular system of seedlings of Atriplex L., Bassia All., Kochia Roth, Malacocera R. H. Anderson, Salicornia (Tourn.) L., and Arthrocnenzum Moq. and the anatomy of the cotyledons is described. The procambial strands of the cotyledons are continuous with those of the hypocotyl-radicle axis but the maturation of the first protoxylem elements occurs independently in each system and eventually they come into contact side by side. There is a certain amount of intrusion by each trace into the tissues of the other. The extent of the transition region between the endarch protoxylem of the cotyledon and the exarch condition of the root as well as the extent of the pith within the hypocotyl and the position of initiation of maturation of the protoxylem elements seems to be determined by the manner in which the hypocotyl elongates. Almost invariably, seedlings of Atriplex, Bassiu, Kochia, and Malacocera which show pronounced elongation in the upper hypocotyl have a long pith and the transition region begins low down and is extensive. Seedlings of Salicornza and Arthrocnemum, on the other hand, have a short pith and the transition region and the initiation of the first protoxylem elements occur at a high level within the hypocotyl. The seedlings of this latter group have a very pronounced elongation in the lower part of the hypocotyl. There is no intermediate condition between -the vascular system of the epicotyl which has endarch protoxylem and the root with the exarch condition, but connexion between the two is established simply by their coming into contact side by side; this occurs in the cotyledon-radicle transition zone. It seems, in the light of this developmental study, that initially there are three separate systems of vascular tissues within the seedling, those of the epicotyl, the cotyledon, and the radicle, but further evidence is needed before a definite conclusion can be made and this could be obtained by an embryological study of the group.