542 Cook : A Synopsis of the 



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 Thrin- 

 coma being almost diamond-shape, while that of Thrinax is merely 

 lenticular. 



4. These differences of proportion of ligule and stalk are obvi- 

 ously 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 thick- 

 ened 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 thick- 

 ness. 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. 



2. The leaf segments are much narrower {3.6 cm.) in the tall 

 than in the short species (4.8 cm.). 



