Morphology of the Leaves of Palms, 



253 



series does not reach high enough into the leaf to show the final form of 

 the limb, but it may be supplemented by reference to figs. 3, A-D, which 

 were drawn from a set of sections cut from the third plumular leaf of 

 another Feather-palm, Oreodoxa regia, H. B. et K. Fig. 3, A, shows an 

 early stage in the grooving of the petiole ; this section is interesting for 

 comparison with the corresponding stages in the two Palms already 

 considered, because the first grooves, in this case, are more or less parallel 

 to the upper surface of the petiole, instead of being approximately at right 

 angles to it. 



Fig. 3. — Oreodoxa regia, H. B. et K. Series of transverse sections through upper 

 part of petiole, and limb, of third plumular leaf (first foliage leaf, which was 

 preceded by two scale leaves) of seedling ( x 14) ; fig. 3, A, early stage of 

 invagination of petiole. 



As we have indicated on p. 250, two alternative explanations have hitherto 

 been put forward to account for the resemblance of the young Palm leaf to 

 a folded fan : (1) that there is a splitting of the tissues ; and (2) that the 

 rudimentary organ becomes folded in the course of development. I find 

 it impossible to accept the first of these views, since my observations on 

 the arrangement of the elements in the neighbourhood of the grooves 

 entirely confirm the conclusion — already drawn by Hirmer* from a study 

 of the detailed histology in certain genera — that no actual splitting takes 

 place. But I am, on the other hand, unable to accept the second view, 

 which Hirmer himself supports, namely, that the apparent plication is due 

 to the folding of an originally simple leaf-rudiment (" Faltung der urspriing- 

 lich einfachen Blattanlage "). The word folding necessarily implies something 

 which is the converse of the eventual unfolding of the blade. But no 

 process takes place in the leaf-rudiment which can justly be described by 

 such a term. I think it will be recognised that the way in which the 

 grooves penetrate between the vascular strands, as shown in figs. 1-3, puts 

 the folding hypothesis entirely out of court. My observations thus fail 

 to support either the splitting or the folding interpretations ; but they have 

 led me to a third view, namely, that the plications originate by invagination, 

 or, in other words, by differential growth in the course of which the outer 



* Hirmer, M. (1919). 



