THE PALM-STEM. 35 



very similar to that of the Grasses ; but in Sagittaria, in which exist many 

 vessels of tolerably equal size, these accommodate their position to the air- 

 canal, and form a crescent, convex to the outer side, enveloped by a large 

 bundle of proper vessels. This is the case also in Alisma Plantago ; only a 

 portion 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 the 

 liber-tubes immediately adjoined the wood. 



As I have already observed, the large vessels of the Monocotyledons, as 

 a general rule, exhibit the form of reticulated vessels ; but that this does 

 not hold without any exception, is shown by the masterly researches of 

 Moldenhawer, on the vascular structure of the Grasses. In these, however, 

 the vessels are still reticulated in the greater portion of their course ; in 

 others, they exhibit the form of spiral vessels. Thus, in the petiole of 

 Musa paradisiaca, a very large vessel occurs in the middle of the woody 

 bundle, in place of which, in the middle vascular bundles of the stem, three 

 or four vessels occur. Now this vessel, as a general rule, exhibits the form 

 of 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 did 

 I find the fibres of this vessel blended together, and this frequently merely 

 at particular places. In the same way I found, as a rule, only spiral vessels 

 in the stems of Ti/pha angustifolia, Sparganium ramosum, and in the petiole 

 of Calla JSthiopica. 



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, and 

 exhibit a transition of form between the polyhedral and cylindrical. These 

 cells become of smaller diameter towards the surface of the stem, with 

 which is combined an increased thickness of the walls. This thickening 

 occurs to such an extent in many Monocotyledons, at the place where the 

 outer vascular bundles lie, that a firm ring is formed around the stem, for 

 instance, in Arundo Donax, Ruscus Hypophyllum, Asparagus qfficinalis, Con- 

 vallaria Polygonatum, Lilium bulbiferum, Iris sibirica, Dioscorea villosa, 

 Tamils Elephantipes, Sparganium ramosum, Triglochin palustre, Alisma 

 Plantago, &c. From their thickened walls, their smaller diameter, and 

 elongated form, these cells resemble the liber-cells, but it would be a great 

 mistake to compare this ring with the liber of the Dicotyledons, since : 

 1. There are many plants, e. g. Fritillaria imperialis and Tulipa gesneriana, 

 in which these cells are wide and less thickened in the walls, forming a 

 distinct transition into the parenchymatous cells. 2. This ring is not 

 sharply defined at its inner side, but passes gradually into the parenchyma 

 of the stem. 3. The relation to the vascular bundles and the leaves is 

 altogether different from that of the liber-bundles of the Dicotyledons to 

 these. 4. Many herbaceous Dicotyledons exhibit this ring and liber- 



