A synopsis of the palms of Puerto Rico

by O. F. Cook

2. The leaf segments are much narrower (3.6 cm.) in the tall than in the short species (4.8 cm.).

3. Practically the difference in width is still greater because the segments of Thrincoma are never fully expanded but remain deeply channelled, thus decreasing the area of exposure to the wind and increasing the rigidity of the leaf.

4. Resistance to the wind is also reduced in the tall species by the separation of all the segments to more than two-thirds their length, while in Thrinax praeceps the median segments are united more than half way up. In the latter, as in the other members of the group, the separation begins at the point of greatest width of the segment, but as if to show that the deeply divided leaves of Thrincoma are an adaptation, the greatest width is located near the longitudinal middle of the segments, 10 cm. or more above the bottom of the cleft.

5. The texture of the leaf of Thrincoma is thicker and firmer so that the segments generally remain straight to the tips while in Thrinax they often droop after the leaves have become fully expanded.

6. The color of the leaves of the tall palm is a very dark green while those of Thrinax praeceps are uniformly of a much lighter, fresher tint.

7. The veinules of the firm leaves of Thrincoma are more numerous and closer together than those of Thrinax.

8. The veinules are also subequal in size, giving an appearance of uniform pattern, while in Thrinax praeceps from 3 to 5 of the veinules of each side of the midrib are distinctly larger than the others, the larger veinlets being separated by from 3 to 10 smaller ones.

9. In Thrincoma the cross-veinules are scarcely visible to the naked eye; under a lens they are still obscure, never equalling in size the smaller of the longitudinal veinules, which they seldom appear to cross. In Thrinax praeceps, on the contrary, the cross-veinules are as large as the finer longitudinal ones; they are obvious without a lens and give the fabric of the leaf a peculiar marbled effect on account of the fact that they are generally oblique or wavy and commonly appear to cross several of the longitudinal veinules.

10. The margins of the segments are thickened in both species, and on the upper side there is a groove inside the marginal rib. In the short species the margin is flat below and does not become decurved in drying. In the other the thin edge is closely folded under, and on drying the sides of the segments uniformly roll under, giving the dried leaves of the two species an appearance even more dissimilar than in the fresh state.

11. The lower surface of the leaf of Thrincoma has a silvery white layer of fine closely appressed hairs, all lying parallel to the veins and forming a continuous covering. The fibers seem not to be attached merely at one end, but along the side. They are firmly adherent and are to be removed only by scraping or rubbing; the surface underneath is deep green like the upper side, but the fibers remain in the grooves between the veins. In Thrinax praeceps the lower surface of mature leaves is smooth and glaucous, a comparatively very slight hairy covering present in young leaves being evanescent, though traces of it are usually to be found in the deeper basal grooves. The glaucous appearance is due to the presence of numerous white or hyaline points arranged in rows (stomata?). The hairiness of one leaf and the glaucous character of the other are probably to be looked upon as different adaptations for the same purpose—the reduction of transpiration.