The Anatomy of the Genus Salicornia.

Summary. 1 The succulent “cortex” which covers the internodes of a Salicornian shoot is foliar in origin, and is phylogenetically derived from the basally developed leaf-sheath of the pair of leaves of the node above. This conclusion is based upon the evidence afforded by: (a) The development of the shoot behind the apex. (b) The anatomy, and the course of the vascular bundles. (c) The structure of the flowering shoot. (d) The leaf-fall. (e) The morphology of the seedling. (f) The morphology of species of allied genera. 2 The anatomy of the leaf is similar to that of any other succulent dicotyledon. It has a two-layered palisade parenchyma and well developed aqueous tissue. Numerous vascular bundles lie between the palisade layer and the water-storing tissue. The development of cuticle is slight, except in S. glauca and S. fruticosa. The structure of the free leaf-tips, the tubular leaf-sheath, and internodal sheath is practically identical. 3 Each foliar trace divides into three on passing out from the stem at the node. The median strand branches, and forms the vascular system of the free leaf-tip, the two lateral strands curve outwards and downwards and by their repeated branching form the so-called “cortical” bundles of the internodal sheath. 4 The flowering spike is constructed on the same plan as the vegetative shoot, but the internodal bract-sheath is necessarily modified on account of the two groups of flowers which develop at the node. 5 Leaf-fall occurs in both the annual and perennial species. It is due to the suberisation of the cells of the outer layers of the stem tissues, beginning in the endodermis and extending inwards, this results in the death of the internodal foliar tissue and its subsequent separation at the limit between the stem and leaf. 6 The seedling has two small, fleshy cotyledons and a short thick hypocotyl. The cotydedons fuse and form a cotyledonary sheath to the hypocotyl similar to the leaf-sheath of the vegetative shoot. The hypocotylar sheath has an outer series of bundles derived from the two cotyledonary traces. Transition from stem to root structure takes place high up in the hypocotyl and follows Van Tieghem's Type III. 7 The morphology of Kalidium foliatum and Halocnemum strobilaceam further indicates how the Salicornian condition may have arisen. 8 The discovery of every form of transitional element between spiral cells and stereides leads to the conclusion that the two are homologous structures; the former function chiefly in water storage, the latter in mechanical support. 9 Anomalous secondary growth sets in early in both root and stem. A secondary, pericyclic cambium gives rise on its inner margin to a thick zone of lignified intermediate tissue, in which concentric series of collateral vascular bundles are embedded. 10 At the base of the main stem, the upper part of the main root, and the rhizome of the perennial species, the secondary cambium forms an aerating cortex of greater or less extent. In Salicornia herbacea it develops into a thick coating of aerenchyma, and ribs of this tissue occur in S. ramosissima. 11 Halocnemum strobilaceum has long and short shoots. The long shoots are covered with decussately arranged, fleshy leaf-buds, which latter develop into the short shoots. The long shoots are old “short shoots” from which the foliar sheath has fallen leaving the buds exposed. The jointed segments of the short shoots are similar to those of Salicornia. The anatomy of the free leaves of the bud and the foliar sheath of the segment is essentially similar, and agrees in all Important points with that of Salicornia glauca, but stereides are absent from the palisade parenchyma, and the foliar bundles of both the free leaves and the sheath are associated with numerous transfusion tracheids. May 11th, 1913. —Since the above paper was written a publication* has appeared on the plant-habits and structure of Salicornia australis; as the interpretation of certain anatomical features differs from that given in the present investigation, it seemed desirable to add the following notes. The author states very briefly various considerations which have led to the conclusion that “it is the greatly developed leaf-base which forms the ‘cortex’ referred to by writers on Salicornia. ” Details are given as to the stomata, the aqueous tissue, the palisade layer, the apical growth, and the seedling of the species. The occurrence of spiral cells in the palisade tissue of the leaf and leaf-base is recorded, but their function is ascribed to air storage (cf. p. 333). A full account is given of the mode of secondary growth of the stem and root. A pericyclic extrafascicular cambium ring forms centripetally, xylem consisting of “thick-walled fibrous cells, among which are situated, in irregular rings, the vessels, with very large lumens. ” Scattered among the fibrous cells of the xylem and lying just outside the large vessels are phloem islands. The extrafascicular cambium forms phloem centrifugally, this bast consisted of parenchyma like cells with chlorophyll and starch grains, but no sieve tubes were present; some of the cells of the phloem disintegrate, “those remaining being arranged in radial rows, between which are large intercellular spaces. ” The secondary thickening of the stem and root of the species of Salicornia examined during the present investigation differs essentially from the above account; the extrafascicular cambium is regarded as giving rise centripetally to collateral bundles of wood and bast embedded in fibrous “intermediate tissue,” centrifugally a zone of aerating cortex variable in extent is produced (cf. p. 337 seq.). The paper concludes with a description of the flowers and fruit. The point of greatest interest in this connection lies in the fact that five to ten flowers occur in each leaf axil, while occasionally as many as sixteen may be found, a double row round the stem then being produced.