PALMAE 443 



largement in cell size and change in shape, without new-cell formation, 

 and is always of brief duration. ) The unbranched trunk habit is typical 

 of most arborescent genera. Branching may be the result of injury to 

 the terminal meristem or of the replacing of axillary inflorescences by 

 vegetative shoots. One species of Hifphocne branches sparsely in a way 

 that resembles dichotomy. This has been called true dichotomy "per- 

 sisting in the angiosperms," but other species of the genus are un- 

 branched, or rather freely branched without suggestion of dichotomy. 

 It seems improbable that within one genus both ancient and modern 

 branching types could exist. (Superficial form is not sujfficient in deter- 

 mining dichotomy; only ontogeny and anatomy can determine the nature 

 of this branching. ) 



When genera witli a variety of habits are compared, the unbranched- 

 tree habit appears to be a specialized form. Similar conditions are 

 present in some woody Liliales. (Trees of simple form have existed in 

 all major taxa of land plants. In gross habit, simplicity in form may 

 represent reduction from complexity. Examples are seen in the lepido- 

 dendrids, cycads, calamites, ferns.) 



The Leaf 



The leaves of the palms are probably the largest of all seed plants, 

 attaining lengths, in pinnate genera, of up to 50 feet (Raphia) and, in 

 palmate genera, of up to 30 feet (Corijpha). In form, palm leaves 

 range widely. Simple leaves characterize a few taxa and the early leaves 

 of many genera, but compound leaves are typical for the family. The 

 two well-known compound forms, pinnate and palmate, are connected 

 by many transitional, "costa-palmate" types; these, although palmate 

 in general outline, have a strong midrib, which is a remnant of the 

 rachis of an ancestral pinnate type. The simple leaf is apparently primi- 

 tive; the compound leaf has been derived by dissection of the simple 

 lamina (Chap. 1). Leaflet formation in the compound leaf is by onto- 

 genetic dissection at a very early stage; leaflets in other angiosperm 

 families (except the related Cyclanthaceae ) are formed independently 

 by lateral lobes of the foliar primordium (Fig. 2). All palm leaves, 

 except the simple types, are characterized by a "folding" of the blade 

 in early stages, a folding that seems unique in the palms and the related 

 Cyclanthaceae (Figs. 18, 19, 20, and 148). 



The method of compound-leaf formation by ontogenetic dissection 

 (Fig. 17) involves the freeing of marginal strips of the lamina, which, 

 as the "reins" or lorea, are a prominent part of the young leaf in many 

 taxa (Fig. 148). Like other parts of the leaf, the reins have undergone 

 strong modification, a reduction from prominent ribbonlike bands to 

 ephemeral filaments in the specialized palmate leaf. 



