A synopsis of the palms of Puerto Rico

by O. F. Cook

2. The trunk tapers gradually from a diameter of 9 cm. near the base to 3.5 at the top, and thus possesses considerable flexibility in view of its great length, 11 meters, Thrinax praeceps and other related types not exceeding 4 or 5 meters.

3. In order to support the weight and strain of this greater height, the texture of the wood is extremely hard and firm, especially near the base of the trunk. Externally it is covered by a smooth shell or bark of very hard, brittle, dark colored material. The fibers of the interior which in Thrinax are merely imbedded in a soft pith like those of a corn-stalk are here thickened and cemented together, as in tall palms of other groups, into a dense hard wood. In the specimen cut by us all but a small area of the middle of the trunk was thus hardened, rendering it extremely heavy. The wood-fibers of Thrincoma are much coarser than those of Thrinax, and there appear to be none of the obliquely radial threads which are abundant in the wood of Thrinax Ponceana.

With reference to methods of leaf-attachment four differences may be noted:

1. In Thrinax praeceps the leaf-bases split below in the median line and remain long attached to the trunk. This adaptation is not confined to the old leaves but appears while the leaves are still very young, or as soon as they begin to be expanded by the pressure of those above them. In the tall species such pressure separates the fibers of the opposite side of the cylinder. The short species has the outside of the leaf-bases densely tomentose, and the tomentum is especially abundant along the edges of the split midrib of the young leaf.

2. The ligule of Thrincoma is notably larger than that of Thrinax and continues to lie in the same plane as the blade, and becomes brown with maturity. In old leaves of Thrinax the ligule stands nearly at a right angle to the blade and remains green.

3. For leaves of the same size the petioles, not including the sheathing base, are longer (75–80 cm.) in the short than in the tall species (60–65 cm.).

The petiole of the short species is of nearly the same width (1.2–1.5 cm.) throughout, while in the other it is distinctly broader at both ends than in the middle. The enlargement at the ligule is abrupt. The base widens gradually to about 2 cm. but is much thinner than in the short species. In the upper part of the petiole the reverse is true, the cross section of the leaf-stalk of the Thrincoma being almost diamond-shape, while that of Thrinax is merely lenticular.

4. These differences of proportion of ligule and stalk are obviously correlated with the different habits of the two species. The shorter and more robust trunk of the one enables it to withstand the strain of the relatively limited exposure to the wind. There is also a greater flexibility in the leaf itself, due to its thinner texture and to the smaller development of the ligule and adjacent thickened area, so that the leaves are often split to near the center. The narrow petiole of the tall species affords greater flexibility in the lateral plane while strength has been secured by the greater thickness. On the other hand the thinness of the base of the petiole of Thrincoma reduces resistance by permitting the petiole to be twisted when the leaf is opposed to the wind or blown laterally, thus avoiding the strain which would come upon the more rigid base of the petiole in Thrinax.

The more salient differences between the leaf-blades of the two species may be enumerated as follows:

1. Although the length of the middle segments of the leaves of Thrincoma are longer (62 cm.) than those of the other (55 cm.) the apparent size of the latter is much greater because they are fully expanded while those of Thrincoma remain more or less fan-shaped, generally opening less than a semicircle. This decreases the lateral expansion, since the shortest divisions are brought to the sides, and gives no projection below the ligule where in Thrinax more than one third of the foliar expanse is located.