The stem or axis of a palm is a cylinder with a graceful plume of leaves at its summit, and a cone of white roots at its base. A section of the stem perpendicular to its axis exhibits a mass of cellular tissue with a number of dark-coloured spots irregularly scattered throughout the whole with the exception of an envelope of dense cellular tissue, which forms a kind of bark. The dark spots are the sections of bundles consisting of two or three very wide and large vascular ducts enclosed in extremely fine woody fibre and spiral vessels closely pressed together. These bundles after having formed the ribs and footstalks of the leaves, enter the upper part of the stem, approach towards its centre, bend down for a short distance, then turn towards the exterior, interlace with the bundles of the previous year, which have followed a similar course, and make a circle of rough marks on the surface of the stem. The stem then increases in length, and a new plume of leaves crowns its top; the same process is repeated; and, as this goes on indefinitely, the lower part of the stem is rough, and sometimes even rugged, as that of the Palmyra palm. In consequence of this manner of growth, a perpendicular section of a stem shows a series of curves intersecting each other and originating in points gradually ascending as the palm grows in height. These curves proceed from every point of the circumference of the stem, present their convexities to the centre, and bend round, and enter the leaves, which are long, lanceolate, and often pinnate with veins running through them longitudinally. In some palms the greater part of the centre of the trunk is altogether cellular tissue, but each species has an arrangement of its own. Some palms, as the date palm, are diœcious, one plant bearing male, the other female flowers. In these cases, the pollen is carried from one tree to the other by the wind or by insects, and, as much is lost in the transit, there is always more produced than is required to fertilize the female flowers.