THE AXGIOSPERMAE : LEAVES 989 



4. LiGULES and other scale-like appendages of certain leaves, such as the 

 intravaginal scales in some Monocotyledons. 



The morphology of these structures and their relation to foliage leaves 

 is still, in many cases, obscure, but some have been investigated in detail. 



Cataphylls are in most cases homologous with leaves, but only in a 

 minority of cases is a scale equivalent to a whole leaf or has a recognizable 

 likeness to the fully developed leaves, though in some examples, e.i^., bud 

 scales of Syringa vulgaris (Lilac), a series of forms may be traced between 

 the outer typical bud scales and the juvenile leaves within the bud. Another 

 condition is that in which the scale corresponds to the leaf base, and this is 

 probably the commonest case. There is a difference in the rates of develop- 

 ment in the upper and lower zones of the scale rudiment, the upper zone, 



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Fig. 977. — Ribes saimiiineum. A series showing the continuous 

 transition from the outermost bud scales consisting of leaf base 

 onlv, to the mature foliage leaves with expanded leaf blades. 



corresponding to the petiole and leaf blade, failing to develop beyond the 

 rudimentar\' state. In some buds, however, a succession of forms may be 

 found, showing the gradual development of the lamina at the apex of the 

 scale, the proportion of lamina to leaf base increasing towards the centre of 

 the bud until the normal form of the leaf is reached. Such intermediate 

 stages are well shown during the opening of the bud in such woody plants as 

 Rihes (Fig. 977) and Aesaihis. 



The divergence from typical leaf development begins at a very early stage 

 of scale formation. In opposition to the usual idea of reduction is the fact 

 that there is no diminution of growth rate in the development of a bud scale 

 such as that of Aesculus. Actually the development of the scale rudiment is 

 faster than that of the leaf. There is more rapid vacuolization and maturation 

 of the cells, but growth is limited to the margin and soon ceases. Scale 

 formation and leaf formation are two distinct periodic phenomena, and are 



