Palms of Puerto Rico 541 



The trunk of Thrincoma differs in three adaptive particulars 

 from that of Thrinax praeceps^ Ponccana and similar species which 

 are merely columnar with very short internodes and an irregularly 

 rimose surface, not smooth and hardened. 



i. There are distinct internodes from 3.5 to 5 cm. in length. 

 These indicate rapid growth and would increase the chances of sur- 

 vival in the face of competition of quick-growing tropical vegeta- 

 tion. 



2. The trunk tapers gradually from a diameter ot 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, espe- 

 cially 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 Ponce- 

 ana. 



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 tomen- 

 tum 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 



