42 MORPHOLOGY OF THE ANGIOSPERMS 



as the leaf opens, the leaflets or pinnae so formed are at first held to- 

 gether at their tips by the marginal band but are soon freed by the 

 abscission of the band, together with the apex of the blade where the 

 veins meet. From each leaf hang two straplike structures, the reins, 

 with the apex, the hook, attached to one of them. These structures are, 

 morphologically, a part of the blade, and their presence is related to 

 the dissection of a lamina in which outermost veins of a pinnate-parallel 

 series enclose the inner, distal veins. By the formation of the reins and 

 hook and their abscission, leaflets are cut out of the central part of the 

 blade. The leaflets have no normal margins or apices and differ in this 

 way from other leaflets. The reins and hook vary greatly in structure, 

 often persisting as tough, fibrous strips (sometimes green), suspended 

 from the base of the blade for months; in palmate-leaved genera, they 

 may be threadlike and ephemeral. 



In the palms, the palmate leaf has clearly been derived from the pin- 

 nate (see Palmae, Chap. 11). That the compound leaf of the palms was 

 derived from the simple was accepted by morphologists in the middle 

 of the nineteenth century, but its method of origin was not understood 

 for one hundred years. 



There are two wholly different ontogenetic origins for the compound 

 leaf. The palm type is known only in the palms and their close relatives, 

 the Cyclanthaceae. (Plicate leaves resembling those of palms are 

 present in Curculigo and species of Pennisetum, but the folding in these 

 genera is the result of differential growth, with no dissection in- 

 volved.) In the ontogeny of the compound leaf in all other angiosperms, 

 so far as now known, the leaflets arise as lateral structures of the elon- 

 gating rachis primordium, much as the simple leaf arises (Fig. 2). The 

 existence of two types of compound-leaf origin strengthens the theory 

 that the basic lines of angiosperm stock had diverged far from one 

 another in early angiosperm history. 



The ontogeny of modified leaves — btid scales, cataphijlls — has received 

 some attention as a part of the investigations of apical meristems and 

 leaf development. The studies have been largely morphogenetic and 

 have raised the question whether bud scales represent modified or 

 transformed foliage leaves or are, ancestrally, appendages of different 

 rank. Bud scales are like leaves in vascular supply and major features of 

 ontogeny — apical and marginal meristems, axillary buds — and many 

 taxa show stages transitional to the foliage leaf; they are surely of leaf 

 rank and are homologous with the entire leaf or a part of the leaf. 



The ontogeny of floral appendages is like that of leaves at many 

 and at all important stages. Differences believed to exist, which 

 have been used to demonstrate that the floral shoot apex is mor- 

 phologically unlike that of the vegetative apex, are those of degree 



