48 NATURE-STUDY REVIEW 



way to the tip of the leaf it disappears altogether. Of course it is 

 not needed here. Indeed, it would be detrimental, for away 

 from the trunk flexibility is needed. In accord, then, with this 

 mechanical necessity, we find the shape of the organ changing in 

 cross-section. First, the buttress beneath disappears, then it 

 becomes rather triangular in shape, but with a strengthening 

 ridge running for a ways farther along the dorsal side. This 

 gives vertical rigidity but allows complete lateral freedom of 

 movement. About one-third of the way back from the tip of the 

 frond, in most individuals, this dorsal ridge disappears and the 

 tip is left free to bend and swing in every direction. Hence the 

 graceful curve seen at the tip of the leaf, and hence the individual 

 difference noticable between one palm and another as regards the 

 curving or erect attitude of the fronds. One can see in the ac- 

 companying sketches (figures 3 and 4) of cross-sections of a 

 midrib how the structure is gradually suited to every changing 

 need of each inch of length, and also how the bundles of conducting 

 and strengthening fibers run thru the interior. 



The clasping bases of the leaves have been mentioned as an 

 item in securing support for the leaf without compromising the 

 safety of the tree as a whole by too strong cohesion. Additional 

 support is afforded to the fronds by a very strong fibrous sheath 

 which envelopes them and binds them together and to the trunk. 

 It is an outgrowth of the vascular bundles from the margins of the 

 midribs and consists of two sets of interwoven fibers crossing each 

 other at almost right angles. So artificial does this webbing 

 appear that it is difficult to believe that it was not woven with 

 warp and woof on a crude loom. Outgrowing from the two 

 margins of the basal part of the midrib it forms a sheath com- 

 pletely enclosing the younger fronds. Thus, several of these, 

 one outside another, together form quite a strong, vaselike structure 

 thru which the upshooting fronds protrude (See cover) . As the 

 weight of the overarching fronds increases with size and expansion, 

 these fibers gradually give way from the top downward, thus 

 freeing the frond and allowing it to droop more and more. By 

 the time it is mature and ripe, as the time of its usefulness to the 

 tree draws to a close, the frond is allowed to hang low and out of the 

 way of the younger and out-standing fronds. Thus it is retained 

 as long as it has any usefulness whatever, but it is out of the way. 



