260 



Dr. A. Arber. On the Development and 



fan-leaved ; in the Lepidocaryinere, again, there are two tribes differing in the 

 same character— the fan-leaved Mauritese and the feather-leaved Metroxyleee. 

 These considerations seem to show that the various types of Palm leaf may 

 safely be treated as homologous, whether they assume the palmate or the 

 pinnate form. 



As I have shown on pp. 251-4, the plicate limb, both in the Tan- and 

 Feather-palms, is not, as has hitherto been commonly assumed, the result of 

 the folding of a flat leaf-blade ; it takes its origin, on the contrary, from the 

 upper region of the leaf-stalk, through a series of invaginations which penetrate 

 the tissues of this organ, and elaborate its more or less radial structure into 

 a flattened and perfectly dorsiventral limb. My observations on the ontogeny 

 thus indicate that the Palm leaf is not, as has been generally supposed, 

 an organ whose structure is almost without a parallel, but that it falls into 

 line with the leaves of other Monocotyledons (e.g., certain Irids).* 



I thus regard the Palm leaf, as a whole, as a petiolar phyllode, and its 

 blade as a pseudo-lamina, analogous to, but not homologous with, the blade 

 of a Dicotyledon. The fundamental identity of the leaf of the Palms with 

 that of other Monocotyledons is, however, soon masked by secondary fusions, 

 and, a little later, by the disintegration and tearing into segments, which the 

 leaf undergoes in passing to its peculiar definitive form. 



IV. Summary. 



The evidence from ontogeny and comparative morphology, brought forward 

 in the present paper, leads to the following conclusions: — 



1. The leaf-stalk, which succeeds the basal sheath, is, both in the Fan- 

 and Feather-palms, the basal or proximal region of the true petiole. 



2. The " fan " or " feather " limb is not, morphologically, a lamina, but 

 is a modification of the distal region of the true petiole. The complex 

 plication of the limb arises through the development of a series of invagina- 

 tions which penetrate into the leaf-stalk tissue between the bundles. 



3. The " ligule " and " dorsal scale " of the Fan-palms are not morpho- 

 logical entities, but merely represent the adaxial and abaxial distal margins 

 of the uninvaginated proximal region of the petiole. The terms " ventral 

 crest " and " dorsal crest " are proposed as substitutes for the terms " ligule " 

 and " dorsal scale." 



4. The Palm leaf, regarded as a whole, is, on the present interpretation, 

 a petiolar phyllode with a pseudo-lamina — a conception which brings it 

 into essential relation with the leaves of other Monocotyledons. 



* Arber, A. (1921). 



