12

THE PALM-STEM.

interior. The thickening of their walls is usually indeed,although always noticeable, not very striking, but in cer-tain Palms , on the contrary, e. g. Diplothemium caudescens,Cocos botryophora, they attain such thickness as we areonly accustomed to see in wood- and liber-cells. In conse-quence of this thickening of the walls, the dots (which ingeneral occur in all Palm-stems) are converted into dis-tinct canals, which correspond in contiguous cells. Inthese two conditions, the thickening of the walls and thepunctation, these cells approach no less than in theirform, the cells of the medullary rays of Dicotyledonous trees, since these also are constantly thick-walled andpunctated.

The cellular tissue of the central part of the stem ex-hibits in like manner many variations, which in greatpart are connected with the position of the vascularbundles. In all Palms it agrees in these characters—thecells are thin-walled, in most cases arranged in perpendi-cular rows; the cells lying upon the vascular bundles aremostly somewhat elongated, and depend for the directionof their transverse diameters on the position of the vas-cular bundles.

In the interior of the Cocos-like stems, the cellulartissue exhibits a regular parenchyma, the cells are thin-walled, finely punctated, and only in the investment ofthe vascular bundles, or where two bundles lie neartogether, do they form transitions to the muriform cellulartissue, without, however, thereby acquiring thicker walls;in Cocos botryophora and Diplothemium caudescens, thecellular tissue even becomes thinner-walled the nearer itlies to the centre.

In most Palm-stems, on the other hand, in which thevascular bundles stand much further apart in the middlethan at the circumference, the cellular tissue of the centreexhibits considerable differences from that of the outerlayers, becoming very lax, and this in two ways.

In some cases the cellular tissue in the middle of thestem has very large cells, and thus forms a very soft,