20

THE PALM-STEM.

the stem are also in a less crowded condition than in thetwo preceding forms. Partly by this, and partly by thesmaller amount of decrease of the liber in the middle ofthe stem, is explained the more uniform hardness of thedifferent layers of the latter.

The vascular bundles of Calamus exhibit a very pecu-liar structure. The liber is here also strongly developedin the outer layer of the stem, but the woody portiondisplays this peculiarity, that, disregarding rare excep-tions, instead of several large vessels, it contains onlyone of unusual dimensions, occupying the centre of thebundle. Behind this large vessel (except in the outer-most vascular bundles) lie small spiral vessels. The cellsof the woody portion have thick walls, and thus mayreadily be confounded with the liber-cells in a cross sec-tion. The proper vessels are distributed in two groups,which, with the spiral vessels, form as it were the pointsof a triangle inclosing the large vessel.

Of the Structure of the particular Anatomical Systemsof the Vascular Bundle .

The cells of that part which I denominate liber alwayshave a diagonal septum. In a young condition, they,like all other thick-walled cells, are composed of delicate,colourless membrane. When with increased age theyhave become thicker, they afford clear evidence that themembrane of the vegetable cell grows in thickness by thedeposition of layers. In transverse sections of the wallsof the liber-cells of all Palms, delicate concentric linesmay be seen, and that these lines form the boundariesof the different layers composing the cell-membrane, ismanifest from the fact that sometimes, when the sectionhas been made with a razor which is not very sharp, theselayers separate from each other, and the slice of cell-membrane appears in the form of distinct concentricrings. Frequently the colour of these cells is not uniformthroughout the whole thickness of the membrane, and