THE PALM-STEM. 21

some of the component layers are often of a darker huethan the rest.

A second remarkable peculiarity of these cells is theirporosity. Both in the transverse and longitudinal sec-tions we see fine striae, which run from the cavity towardthe outer surface of the cell. When a high magnifyingpower is used, there remains no doubt that these striaeare canals perforating the cell-wall. As a general rule,their diameter does not exceed ^ of a line. As in thecase of the pores of cellular tissue, the canals of adja-cent cells correspond to each other.

The second constituent of the vascular bundle, whichI have termed wood, is composed of two organic systems,cellular tissue and vessels.

The cellular tissue of this woody portion consists ofcolourless parenchymatous cells, the walls of which arenot very thick. They are usually somewhat elongated,stand in vertical series one above another, with horizontalsepta; never lie in series diverging like a fan from thehindmost point of the woody mass, but form an irregularparenchyma, the cells of which, in the vicinity of thevessels, are arranged according to the form and positionof these latter. These cells never contain starch-granules;their walls are studded with large and small pores likethe cells of Cycas .

The woody mass, as already mentioned, always lies atthe inner side of the vascular bundle; but in the transi-tion of the fibrous bundle, devoid of vessels, into thecondition of vascular bundle, it is very frequent, andalmost the rule, for the woody mass to lie, not at theinner side, but in the middle of the liber-bundle. In thevascular bundles of the outer, hard layers of the stem,also, a narrow strip of liber-tubes often runs round theposterior face of the woody mass, so that this is completelysurrounded by liber-tubes.

In other cases the membrane of the wood-cells is itselfthickened, and thus they acquire a resemblance, at leastin the cross section, to the liber-cells; however, they are