340 PRIMARY ARRANGEMENT OF TISSUES. 



surrounded successively by the wider vessels, the phloem, and the endodermis. It 

 is manifest that this structure may come about owing to the frequent junctions of the 

 smaller collateral bundles. 



The above-mentioned isolated occurrence of concentric arrangement in the 

 Cycadese was found by me in some of the small bundles in the petiole of Dion ; 

 they had a round xylem, surrounded by a phloem with its elements in radial rows. 



Lastly, the axial bundle (described at p. 277) in the internodes of several Dicoty- 

 ledonous water-plants must be placed here : namely, Hippuris, Trapa (?), Callitriche, 

 Bulliarda, Elatine ', Hottonia, and Myriophyllum. It consists in general of a central 

 xylem, completely surrounded by a phloem, both parts usually having abundant 

 delicate parenchyma between the essential elements. In the cases of Hippuris, 

 Trapa, Hottonia, and Elatine Alsinastrum, the persistent vessels are arranged in an 

 interrupted ring, which surrounds a relatively thick cylinder of parenchyma {' pith '). 

 The phloem is bounded on the outside by an endodermis. In the leaves of these 

 plants, with the exception perhaps of Callitriche, the bundles are collateral, with 

 normal orientation. The axial bundle of Verhuellia (p. 278) also seems to belong 

 to this series, though Schmitz could only detect that it consists 'of a strand of 

 prosenchymatous cells, in the middle of which runs a single spiral-vessel.' All the 

 cases here cited, with the exception of the three elucidated by Sanio, Vochting, and 

 Hegelmaier, require still more accurate investigation. 



In Bulliarda aquatica, according to Caspary's description, the middle of the stem is 

 occupied by a thin cylindrical strand consisting chiefly of elongated cells, in which, about 

 midway between periphery and centre, lie two indistinctly separated groups of annular 

 and spiral vessels, which run out to the leaves. In Elatine Alsinastrum the axial cylindrical 

 strand consists permanently, as regards its main bulk, of much elongated cells ; a few cell- 

 layers inside its periphery, one vessel for each leaf of the whorl next above it first appears, 

 and in the node this vessel bends out into the leaf at right angles ; or, to express the fact 

 differently, the vascular elements running into the leaf here abut on the cauline vessels. 

 The vessel itself appears to be the continuation of the one which has passed out at the 

 next lower node. Later on other isolated wider vessels are formed side by side with 

 the original ones; in cross-section all are arranged in an irregular ring. Sieve-tubes 

 lie in the zone outside the vessels. Axial vessels are not present at the beginning ; after 

 those of the leaf-trace however 1-2 vessels appear and are permanent. Hottonia appears 

 (judging from very incomplete investigation) to behave similarly, apart from the obvious 

 differences due to difference of Phyllotaxis, and with the further distinction that axial 

 vessels do not occur. 



The often investigated axial bundle in the stem oi Hippuris '' shows in its early stages, as 

 first accurately described by Sanio, annular and spiral tracheae at its centre, which are 

 scattered among thin-walled prismatic cells, and are cauline, with acropetal growth. At 

 a later stage vessels are formed at the periphery of the cylinder, and from these the 

 bundles branch off, which pass transversely through the cortex into the leaves. They 

 are connected with one another in the node, and in the cross-section of the internode 

 they represent an irregular, many-layered and often-interrupted ring, in which the 

 vessels increase in width in the centrifugal direction, according to their order of origin. 

 Outside the vascular ring lies a many-layered ring of prismatic cells, and between the 



' [F. Miiller, Struktur einiger Arten von Elatine ; Flora, 1877.] 



* Von Mohl, Verm. Schr.; Palm. Structura, Tab. g, fig. 2.— Niigeli, Beitr. I.e. p. 56.— Sanio, 

 Eotan. Zeitg. 1865, p. igi. ^ 



