STRUCTURE OF CONCENTRIC BUNDLES. 



341 



latter are small bundles of sieve-tube, each consisting of one, or rarely two sieve-tubes' 

 surrounded by a layer of cambiform-cells. — Both vessels and sieve-tubes run vertically 

 and separately in the internode. In the node the equivalent elements anastomose with 

 each other, and the two kinds of elements unite to form the vascular bundle, and enter 

 the leaf. The peripheral elements of the axial strand which we have described are 

 persistent. The cauline axial tracheal elements begin to disappear on the first appear- 

 ance of the peripheral vessels, and become so crushed by the surrounding prismatic cells 

 that a mature bundle encloses a dense parenchymatous ' pith ' within the peripheral 

 vascular ring. 



One bundle runs out into each of the leaves, which form multifoliate whorls, and 

 usually each has a separate course from the rest ; yet it not uncommonly happens that 

 a common trunk arises from the vascular cylinder of the stem, and then, towards the 

 periphery of the stem, divides into two or even three leaf-strands. 



In the stem of CalJitriche'' the thin axial bundle, consisting chiefly of delicate elongated 

 prismatic cells, contains at its apex an axial annular or spiral vessel, which grows acro- 

 petally, and projects far above the last node which contains vessels ; close by this a 

 second (and a third) soon appear. The first two primordial vessels have a position in 

 the internode corresponding to the two opposite leaves of the adjoining node ; in the 

 node a small bundle branches off from them for each leaf. As the internodes become 

 elongated 3-12 wider annular or reticulated vessels appear, by the side of and somewhat 

 external to the primordial elements ; they are arranged in two iiTegular groups, and are 

 persistent, while the primordial vessels soon disappear in the internode, and are replaced 

 by an axial intercellular passage, to the wall of which their remnants adhere. This pas- 

 sage may subsequently be filled up again by the luxuriant growth of the neighbouring 

 cells. In the node the axial passage between the vascular elements, which are here 

 densely crowded, is absent. The vascular group is surrounded by a small zone of phloem, 

 consisting of a few rows of narrow elements, and bounded on the outside by the 

 endodermis. 



In Trapa the wide central portion of the axial strand consists, in the fully elongated 

 internode, of loose, large-celled parenchyma, traversed by numerous longitudinal air- 

 passages ; a relatively narrow, peripheral, annular zone consists of thin-walled prismatic 

 cells. Among the latter are large vessels arranged in a circle at wide intervals. Their 

 primitive elements appear to run out into the leaves, but to be distorted and indistinct, 

 each being in many cases replaced by an air-passage when the internode has attained its 

 definitive elongation. The large persistent annular vessels are apparently of later origin. 

 Outside and inside the vascular circle Sanio' found a circle of scattered sieve-tube- 

 bundles, each of these consisting of one sieve-tube with horizontal cross-walls, surrounded 

 by a layer of cambiform tissue. 



Myrittphyllum spicatum * has in the young internode, when elongation begins, in the 

 middle of the axial cylinder a central group consisting first of one and then of 2-4 spiral 

 vessels, which are in close contact with each other. This group is cauline and grows 

 acropetally, and in the node branches grow out from it centrifugally into the leaves, 

 which are ranged in alternating, usually quadrifoliate whorls. In the leaf the vessels are 

 united with a small phloem to form a collateral bundle. From the node the four phloem- 

 bundles of the whorl — which require further histological investigation — run down, as a 



' The sieve-tube nature of these elements is disputed by Russow, who however includes them in 

 his protophloem. 



" Nageli, I.e. — Hegelmaier, Monogr. d. Gattg. Callitriche. — Idem in Martins, Flora Brasiliensis, 

 fasc. 67. 



* Botan. Zeitg. 1865, p. 193. 



* Vochting, Zur Histologie und Entwickelungsgesch. v. Myriophyllum, Nova Acta Leop. 

 XXXVI. 1872. 



