THE PALM-STEM.

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very similar to that of the Grasses; but in Sagittaria, in which exist manyvessels of tolerably equal size, these accommodate their position to the air-canal, and form a crescent, convex to the outer side, enveloped by a largebundle of proper vessels. This is the case also in Alisma Plantago ; only aportion of the vessels are also scattered irregularly in the woody portion.I did not find proper vessels in the vascular bundles of this plant, but theliber-tubes immediately adjoined the wood.

As I have already observed, the large vessels of the Monocotyledons, asa general rule, exhibit the form of reticulated vessels; but that this doesnot hold without any exception, is shown by the masterly researches ofMoldenhawer, on the vascular structure of the Grasses. In these, however,the vessels are still reticulated in the greater portion of their course; inothers, they exhibit the form of spiral vessels. Thus, in the petiole of-Musa paradisiaca , a very large vessel occurs in the middle of the woodybundle, in place of which, in the middle vascular bundles of the stem, threeor four vessels occur. Now this vessel, as a general rule, exhibits the formof a spiral vessel, with many parallel fibres; and only in rare cases—for in-stance in the lowest parts of the vagina of the leaf and in the rhizome—didI find the fibres of this vessel blended together, and this frequently merelyat particular places. In the same way I found, as a rule, only spiral vesselsin the stems of Tgpha angustifolia, Sparganium ramosum, and in the petioleof Calla AEthiopica.

I shall subjoin but a few words on the cellular tissue of the Monocotyle-donous stem. It consists of large cells, mostly with thin walls, yet fre-quently having pores; they have intercellular passages between them, andexhibit a transition of form between the polyhedral and cylindrical. Thesecells become of smaller diameter towards the surface of the stem, withwhich is combined an increased thickness of the walls. This thickeningoccurs to such an extent in many Monocotyledons, at the place where theouter vascular bundles lie, that a firm ring is formed around the stem, forinstance, in Arundo Donax, Ruscus Hgpophyllum, Asparagus officinalis, Con-vallaria Polygonatum, Lilium bulbiferum, Iris sibirica, Dioscorea villosa,Tamus Elephantipes, Sparganium ramosum, Triglochin palustre, AlismaPlantago, &c. From their thickened walls, their smaller diameter, andelongated form, these cells resemble the liber-cells, but it would be a greatmistake to compare this ring with the liber of the Dicotyledons , since1. There are many plants, e. g. Pritillaria imperialis and Tulipa gesneriana,in which these cells are wide and less thickened in the walls, forming adistinct transition into the parenchymatous cells. 2. This ring is notsharply defined at its inner side, but passes gradually into the parenchymaof the stem. 3. The relation to the vascular bundles and the leaves isaltogether different from that of the liber-bundles of the Dicotyledons tothese. 4. Many herbaceous Dicotyledons exhibit this ring and liber-