20 MORPHOLOGY OF THE ANGIOSPERMS 



There is little evidence to support the selection of an ancestral type 

 for the monocotyledon leaf. On the assumption that the monocotyledons 

 arose from dicotyledonous stock, the primitive monocotyledon leaf prob- 

 ably resembled that of the dicotyledon in gross structure. But, if the 

 angiosperms are polyphyletic, the ancestral monocotyledon leaf may 



have differed greatly from that of the dicotyledon 

 and comparisons with modern types would therefore 

 be valueless. 



In the morphological study of the angiosperm 

 leaf, recognition of shoot and root as the basic 

 structural categories in the plant body raises the 

 question of the fundamental nature of the leaf and 

 its relation to the stem. The lower vascular plants 

 suggest that there are two morphologically distinct 

 types of leaves: microphijUs, which are lobes or 

 outgrowths of the stem; and megaphijlls, which are 

 modified minor branch systems. Leaf gaps are as- 

 sociated with the traces of megaphylls but not with 

 those of microphylls. The angiosperms are mega- 

 phyllous. 



As a part of the development of the telome theory 

 of the nature of the plant body, considerable at- 

 tention has been given to the interpretation of the 

 make-up of the angiosperm leaf and sporophylls. 

 These appendages have been interpreted as systems 

 of dichotomizing axis tips, united laterally. But 

 nodal structure — number, position, and relation of 

 traces and gaps — and ontogeny of the appendages, 

 especially development by apical and marginal 

 meristems, do not support a telomic make-up. 



All appendages in the angiosperms have long 

 been interpreted as having basically an odd number 

 of traces, but it has been shown recently that, in the 

 primitive node, the appendage has two traces asso- 

 ciated with one gap (Fig. 5). Branches of the shoot also have two 

 traces with a single gap; leaf and branch are alike in the origin of their 

 vascular supply. The nature of the leaf as a basic part of the shoot sys- 

 tem is apparent, and the term "partial shoot" well expresses its funda- 

 mental nature. 



A two-part trace system throughout the shoot may suggest ancestral 

 dichotomy. Evidence in support of this view has been seen in veinlet 

 branching, cotyledonary lobing, and the forking of some simple stigmas, 

 but phyllotaxy and branching habit do not show dichotomy. (The 



Fig. 5 

 showing 

 tion of 

 leaf of 

 letja. 



Diagram 

 vasculariza- 

 node and 

 Austwbai- 

 Unilacunar 

 nodal structure with 

 two traces which 

 continue as two 

 bundles through 



petiole and leaf 

 blade. {After Bailey 

 and Swatny.) 



