A synopsis of the palms of Puerto Rico

by O. F. Cook

What is believed to be the same species was collected in a similar situation on the side of a mountain overlooking the town and valley of Lares.

Thrinax Ponceana sp. nov. [Plate 43]

Trunk 5–8 cm. or more in diameter, columnar, or slightly tapering or enlarged upward, 1–4 m. high; surface coarsely and irregularly rimose longitudinally. Leaf-bases separating into abundant rather loose light grayish or brownish fibers. Leaves numerous, large, drooping or pendant; petioles 65 mm. long, 1.5–2 cm. wide; segments attaining 75 cm. in length and 3.5 cm. in width, united for half their length. Seed smooth, mahogany-brown, 5 mm. in diameter. Type specimen no. 1005.

This species apparently exists in much larger quantities than any other yet known from Puerto Rico, being the predominant plant on several square miles of territory along the range of dry limestone hills which skirt the southern coast of the island, to the west of Ponce. Many of the palms are scattered among the taller shrubs and trees wherever there is sufficient soil and water to permit these to grow and yet not enough to give them exclusive possession, but on many of the drier and more sterile higher slopes the advantage is with the palms.

This abundance of living material deserves more careful study than could be given during a very brief visit to this almost uninhabited part of the island, but one note of systematic interest was made. Several species of Thrinax, of which T. Morrisii Wendland may serve as an example, have been described chiefly with reference to the relative size of the leaf segments and the extent of their separation. If the palms under observation near Ponce belonged, as was believed, all to one species, it is not only true that the individual Thrinax passes all the stages from the narrow and grass-like, almost completely separated segments of the very young plant, to the more than half united leaf of the large tree, but it also appears to be true that under unfavorable conditions a Thrinax may not be able to attain to full maturity of size and form but may at the same time produce flowers and seeds. In the narrow chinks and crevices of the bare rocks were very small, stunted trees, obviously of great age, while but a few feet distant a deeper fissure might hold vegetable débris and moisture sufficient to nourish vigorous specimens several times the size of their less fortunate companions. The stunted trees retain in proportion to their size, but apparently with little reference to their age, the small deeply divided leaves of young plants and have short few-branched inflorescences, another difference of supposed systematic importance.

In Thrinax Ponceana the leaves of well grown trees have the middle divisions united to about the middle; the smaller the leaves, the more deeply they are divided. A further correlation with size is that of the “fullness” of the leaf. The basal sinus is not closed by the overlapping of the lateral divisions as in some species, but the area is too great for a plane circle and there are one or more folds, more numerous and deeper in large leaves. The lateral divisions do not lie in the plane of the others but project upward or backward nearly at right angles with the plane of the middle divisions.

The middle divisions of large leaves may measure 75 cm. in length by 3.5 and sometimes nearly 4 cm. in width, while the narrowly grass-like lateral segment is only .8 cm. wide and about 30 cm. long. The lowest segment is not divided at the tip but is produced into a slender hair-like seta, 6 or 8 cm. long, making it nearly as long or longer than the next segment above.

The normal segments are split at the apex to the distance of from 2 to 8 cm. and the tips are usually markedly divaricate, owing to the fact that the young leaves of this species suffer two impressions from the bases of older leaves, one near the middle, the other near the end. The pressure causes the curvature of the unopened leaves, which in turn causes them to split apart when the leaf expands.

Old leaves are smooth and glaucous on the lower side, but in the younger state more or less remains of the delicate appressed hairiness present on the lower surfaces of the newly opened leaves. The lower surface is distinctly grayish and glaucous, but under a lens it can be seen that this appearance is due to the presence of numerous whitish points (stomata?) among which are scattering brownish spots of larger size, the nature of which remains a question.

The free stalks of the largest leaves attain 65 cm. in length and are 2 cm. wide near the base, 1.5 cm. near the apex. The cross section is lenticular above, but the upper surface becomes flat toward the base.