THE PALM-STEM. 33 



the inner possess but a few thin-walled, wide liber-cells, scarcely to be dis- 

 tinguished from the parenchyma- cells and proper vessels in the transverse 

 section, e. g. in Asparagus officinalis, Lilium bulbiferum, Orchis militant, and 

 Sagittaria sagittifolia. When the inner bundles do possess thick-walled 

 liber-cells, these exist but in small quantity, and form only a narrow crescent. 

 Sometimes their diameter is so great in proportion to their thin walls, that 

 these cells are only to be recognised as the organ so greatly developed in the 

 Palms, by their position and their greater development in the outer bundles ; 

 for example, in Nusa paradisiaca, Hemerocallis flava, Tulipa gesneriana, 

 Fritillaria imperialis, Ruscus Hypophyllum, Iris sibirica, Epidendron elonga- 

 tum, Aloe Commelini, and Scirpus lacustris. 



The proper vessels occur throughout the whole series of Monocotyledons, 

 and lie in the same position between the liber and wood as in the Palms ; 

 they also possess the same structure, and contain the same opaque sap. In 

 many plants they are more developed in reference to number and size than in 

 the Palms ; e. g. in Asparagus officinalis, where single ones, lying toward the 

 inner side of the bundle, exhibit uncommonly large cavities. In Mma 

 paradisiaca, the proper vessels, with their thin investment of liber-cells, form 

 a large bundle, lying separate, in part, from the woody bundle, and not 

 much inferior to it in size. In Dioscorea villosa also, and Tamus, some of 

 the vessels lying toward the interior are developed to an unusual size. In 

 Sagittaria sagittifolia the proper vessels form about half of the entire vas- 

 cular bundle, and are difficult to discriminate from the surrounding cells, 

 since these likewise have very thin walls. 



The woody portion consists, as in the Palms, of more or less elongated 

 parenchymatous cells ; its vessels may also be divided into large and small ; 

 but the large are not so much separated from the small in their position in 

 most Monocotyledons, for all the vessels generally lie together. A resem- 

 blance to the vascular bundle of the Palms does, however, occur here in so 

 far that the large vessels lie in front and at the two sides, and the smaller 

 behind and between the former, so that the total mass of vessels forms a 

 more or less regular crescent, with its concavity outward, as for instance in 

 Asparagus officinalis, Convallaria Polygonatum, and Lilium bulbiferum. In 

 all these cases the large vessels lying at the two extremities of the crescent 

 are reticulated tubes, while the smaller, lying further back, have the form 

 of scalariform ducts, and the smallest, most posterior, that of spiral or an- 

 nular vessels. Ruscus Hypophyllum makes an exception to this arrange- 

 ment, as the largest vessels here lie behind and toward the interior. One 

 consequence of this crescentic position of the vessels is, that the bundle of 

 proper vessels retreats back between the horns of the crescent, and thus is 

 half inclosed by the wood ; e. g. in Asparagus, Convallaria Polygonatum, and 

 Lilium bulbiferum. This is the case in the highest degree in Tamus and 

 Dioscorea, where the proper vessels are retracted so much between the 



3 



