270 PRIMA RY ARRANGEMENT OF TISSUES. 



A Structure deviating from that described occurs in the bundles of the small 

 submerged stems of the floating Utricularias, the Lemnaceae, the Podostemaceae, 

 Vallisneria spiralis, and in the rhizomes of Epipogon and Corallorhiza, which inhabit 

 humus, all of these likewise remaining rudimentary. 



In Utricularia vulgaris ^ an axial, approximately cylindrical bundle is present, 

 and gives off a branch into each of the so-called leaves. Its smaller half, which with 

 reference to the horizontally floating stem is the upper, consists of elongated prismatic 

 cells, usually with flat ends and thick collenchymatous walls ; the lower half consists 

 chiefly of thinner-walled elements, namely, of wide sieve-tubes and numerous narrower 

 prismatic cells. Near the boundary between the upper and lower half, but within the 

 latter, next the centre, lies a single row of very long, wide tracheides, with their 

 pointed ends applied to one another, showing alternately annular and spiral thicken- 

 ing. In the small and younger specimens no other tracheides occur. In very thick 

 stems, on the other hand, I found a second single or double row of annular tracheides, 

 lying at the side of those mentioned, near the periphery of the bundle. These are 

 similar to the former in structure, but only about half as wide. Their development 

 seems to take place very late. All the tracheides are persistent, no intercellular 

 spaces whatever are present in the bundle. 



The bundles in the frond of Lemna ' consist in the native species of a thin row 

 of annul3,r tracheides, surrounded by one or a few layers of elongated cells. 

 Spirodela polyrrhiza has several parallel rows of tracheides instead of one ; in Lemna 

 valdiviana, on the other hand, the development of tracheides is suppressed. The 

 absence of any vascular bundles in the Wolffias has already been mentioned above. 



The bundles of the Podostemacese, which are still much in want of accurate 

 investigation, consist, according to Trdcul's short statement ', of a bundle of fibres and 

 some small annular vessels, the latter being often absent in old stems and replaced 

 by a cavity ' Vallisneria also requires more exact investigation ". 



The axial bundle in the rhizome of Corallorhiza contains in the middle two 

 multiseriate strands of reticulated tracheides with narrow transverse meshes, and 

 from these the simple bundles branch off for the distichous leaf- rudiments. It 

 further consists of elongated, usually thin-walled, elements, which still require more ' 

 exact investigation. In the rhizome of Epipogon the bundle consists, so far as 

 investigations extend, only of uniform, moderately elongated cells, with oblique ends 

 and thin walls. 



In the roots of those aquatic plants, which in the stem possess incomplete or small 

 bundles containing passages, the bundles, which here, it is true, are always feebly de- 

 veloped, may be complete, and provided with a persistent xylem constructed according 

 to the radial type, e. g. Potamogeton lucens °- As a rule, however, they here also either 

 become incomplete by disappearance of the vessels, intercellular passages appearing in 

 their place, or they remain rudinnentary. 



' Compare van Tieghem, Ann. Sci. Nat. 5 ser. torn. X. p. 54. 



' Hegelmaier, Lemnaceen, p. 48. ' Archives du Museum d'Hist. Nat. torn. VI. p. 4 



* [Compare Warming on Podostemacese, Mem. Acad. Roy. Copenhagen, 1881, &c. ; also Cario, 

 Tristicha hypnoides. Bet. Ztg. 1881, p. 25.] 



° [Compare Franz Mtiller, die Entwickelung von Vallisneria spiralis, Hanstein's Abhandl. 

 Bd. III. Heft. 4, 1878.] 



" Van Tieghem, Ann. Sci. Nat. 5 s^r. torn. XIII. p. 164, pi. VI. 



