256 



Dr. A. Arber. On the Development and 



apex is solid, and cylindrical in section (fig. 5, C and F). I have seen evidence 

 of the glandular function which Baillon attributes to this apical region in 



A B C D E 



Fig. 5. — Solid tips of plumular leaves. Fig. 5, A-C, Phoenix dactylifera, L. Series 

 of transverse sections from below upwards through apex of first foliage leaf to 

 show dying out of invaginations ( x 14) ; bundles consist chiefly of fibres. 

 Fig. 5, D-E, Pritchardia filifera, Lind. Series of transverse sections through 

 apex of first foliage leaf ( x 14) ; fig. 5, E, shows that the apex is slightly hooded ; 

 c, cavity. 



the case of Bentinckia nicoiarica, where the tip of a plumular leaf bore 

 conspicuous stomates and contained a plexus of tracheids. After cutting 

 successive sections of the distal region of these leaves, I have come to the 

 conclusion that the solid tip cannot be treated as if it were a separate organ, 

 for it appears to owe its existence merely to the fact that the invaginations 

 die out before they reach the distal end of the leaf. In Pritchardia filifera 

 the solid apical part of the leaf is, as it were, slightly undercut by the 

 termination of the median invagination, so that a minutely hooded apex is 

 produced (fig. 5, E). The solid tips of the seedling leaves of Pritchardia and 

 Phcenix seem to me exactly comparable with that of such a leaf as Crocus* 

 in which, also, the form of the apex is the result of the failure of the grooves 

 to reach to the extreme end. 



(4) The "Ligule" and "Dorsal Scale." 



The origin of the outgrowths which occur at the base of the limb in the 

 Fan-palms, and are commonly known as the " ligule " and " dorsal scale," 

 cannot be studied satisfactorily in seedlings, since it is only in the later leaves 

 that they arrive at their full development. The " dorsal scale " is seldom 

 conspicuous, but the "ligule," though it is very variable in size and shape in 

 different species, is often a striking object ; fig. 6 shows its appearance in the 

 case of the mature leaf of Trachycarpus excelsus. In order to follow the 

 developmental history of these structures, I cut serial transverse sections 

 through the apical bud of a well-grown shoot of Rhapis humilis. Fig. 7, A-D, 

 represent the transition from stalk to limb in one of the young leaves of this 

 bud. It will be seen that the invaginations, which are responsible for the 



* Arber, A. (1921), fig. 57, B, G, p. 324. 



