258 Dr. A. Arber. On the Development and 



other leaf-sheaths is obvious. In certain Feather-palms (Desmoncus, Calamus) 

 the' apex of the sheath is continued upwards into the so-called ochrea, which 

 clearly corresponds to the tubular ochrea of the Polygonacere, etc. 



The sheath is succeeded by a stalk, for which I have throughout this paper 

 used the word " petiole," but of which the homologies, at least in the case of 

 the Fan-palms, are a matter of controversy. Domin* takes the view that the 

 " ligule " of the Fan-palms is indeed a true ligule, corresponding to that of 

 the Gramineie, etc., and to the ochrea of the Eotangs ; and since ligules are 

 always defined as belonging to the leaf-sheath region, he draws the conclusion 

 that in the Fan-palms the leaf -stalk, which comes below the " ligule," cannot 

 be a true petiole, but must be of leaf-sheath nature. This view cannot but 

 seem strained and artificial when one realises that it consigns the leaf-stalks 

 of Fan- and certain Feather-palms to different morphological categories. 

 Gliick,-]- who also regards the ventral outgrowth as ligular, steers a middle 

 course by treating it as the free apex of an extremely elongated ligule, which 

 is fused with the petiole as far as the point of junction of stalk and limb. 

 This speculation is ingenious, but, as I hope to show, unnecessary. 



Both Domin's and Gliick's views as to the nature of the " petiole " in the 

 Fan-palms, stand or fall with the question of the homologies of the " ligule " — a 

 problem which we must now consider. I should like to point out, in the first 

 place, that it seems to me that any theory of the " ligule " must also take 

 account of the corresponding structure which often occurs at the back of the 

 leaf, and is known as the " dorsal scale." The latter is more variable than 

 the " ligule," and though in some Fan-palms it is scarcely developed at all, I 

 have observed in a Palm, grown under the name of Sabal filamentosa, that it 

 may reach much the same degree of conspicuousness as the ventral structure. 

 An inspection of the leaves of this Palm certainly suggests that no explana- 

 tion of the origin of the " ligule " can be accepted which does not also embrace 

 the " dorsal scale." But it is clear that the " dorsal scale " cannot be ligular, 

 since no ligule is ever located behind the leaf to which it belongs, and it seems 

 to me highly improbable that the ventral outgrowth should belong to a 

 category from which the corresponding dorsal outgrowth is necessarily 

 excluded. 



In a preceding section of this paper I have discussed the ontogeny 

 of the " ligule " and " dorsal scale," and the evidence there adduced has led me 

 to the view that these structures have no more claim to be treated as distinct 

 organs than have the solid apice of the plumular leaves described on pp. 255-6. 

 The invaginations of the limb, in their proximal region, burrow as it were, 



* Domin, K. (1911). 

 t Gliick, H. (1901). 



