THE PALM-STEM. 31 



growth in thickness by Dupetit-Thouars, we meet with a 

 structure of the vascular bundle which is aberrant in 

 many respects. In a cross section of the stem of these 

 plants, two evidently distinct layers may be detected, the 

 inner being soft and pith-like, and acquiring no increase 

 of size with the age of the plant, while the outer forms a 

 firm mass, and gradually increases in thickness with the 

 age of the plant. Anatomical investigation demonstrates 

 that the inner soft substance has wholly the structure of 

 the Palm-stem, its vascular bundles running in the same 

 way from the periphery to the centre, and from this to 

 the leaf. The most external of these bundles are smaller, 

 poor in vessels, and closely crowded, the inner consist of 

 several rows of dotted liber-tubes, a bundle of proper 

 vessels, and a woody portion, which differs from that of 

 the Palms only in so far that its large vessels are not iso- 

 lated, but united with the smaller vessels into the form of 

 a crescent. Tracing these vascular bundles downwards, 

 we see them enter the outer firm layer of the stem ; 

 instead, however, of running down the stem in the form 

 of fine fibres into a laxer cellular tissue, as in the Palms, 

 they retain a considerable diameter, notwithstanding that 

 they lose their woody portion, since the prosenchymatous 

 cells of which they are composed are very wide. They 

 contain a small bundle of proper vessels in the centre. 

 Under this form they run downward in the stem, as in 

 the Palms, but are not isolated, for they unite together 

 laterally into a network, like the ligneous bundles of the 

 Dicotyledons. The parenchyma in which they are im- 

 bedded consists of tolerably thick-walled, porous cells, 

 elongated in the radial direction. This outer firm layer 

 therefore corresponds to the fibrous layer of the Palms. 

 No trace of it occurs at the apex of the stem, since it is 

 composed of the lower extremities of vascular bundles*: 

 the lower down in the stem we examine, the thicker 

 this layer appears, and thus the stem is conical and not 

 cylindrical. The younger layers of the evascular fibrous 

 bundles are applied upon the outer side of the older, and 



