8 MORPHOLOGY OF THE ANGIOSPERMS 



a part of the shoot, a nodal region transitional between stem and leaf — 

 a transition region. Because the term leafbase is used not only in this 

 way but also, loosely, for the proximal part of leaves— which is morpho- 

 logically various— the term leaf buttress would distinguish the part that 

 forms the nodal transitional region from that which forms petiole and 



blade. 



The angiosperm leaf blade, or lamina, is remarkable for its extra- 

 ordinary variety of form. Descriptively, two chief types are distin- 

 guished as simple and compound. From the phylogenetic viewpoint, 

 each of these has been considered primitive, but strongest support has 

 come for the view that the simple leaf is primitive. 



The leaves of the woody ranalian families — now recognized as show- 

 ing many primitive characters and probably the most primitive living 

 dicotylecions — are simple. The simple form is apparently more common 

 in the fossil leaves of the Cretaceous and Tertiary periods. Comparative 

 study shows many series leading from simple, through increasing dis- 

 section, to compound — in Acer, in Rubtis, in the Vitaceae. These series 

 can hardly be read in the other direction. Chief support for the com- 

 pound leaf as primitive has come from families where reduction of 

 leaflets to one is shown by comparison of many taxa and by the presence 

 of vestigial structure in leaflets, petiole, and rachis— Leguminosae ( Fig. 

 1), Rosaceae, Rutaceae. But the woody ranalian families, with their 

 highly primitive flowers and wood, have simple leaves. The decom- 

 pound leaves of some genera in families with primitive flowers — espe- 

 cially the fernlike leaves of some genera of the Ranunculaceae and 

 Fumariaceae— have been regarded as possible evidence that these taxa 

 retain in some measure the leaf form of an ancestral fernlike stock. 



Under the Durian theory of the habit of the primitive angiosperms, the 

 large, pinnately compound leaf is considered the form characteristic of 

 early angiosperms. The basis for this conclusion is that leaves of this 

 type accompany the growth habit believed primitive and that they in- 

 habit tropical forests, where today the most primitive living angiosperms 

 are found. But the woody ranalian families with simple flowers and 

 anatomy also inhabit these forests, and their leaves are simple. 



Though the simple leaf seems undoubtedly at least one primitive 

 type, the simple leaves of many taxa clearly represent modified 

 compound leaves; the lamina of the compound leaf has been re- 

 duced by loss or fusion, or by both loss and fusion, of leaflets. Un- 

 questioned reduction of these types is present in the Leguminosae, 

 Rosaceae, Oleaceae, Rutaceae, Proteaceae. Evidence of loss and fusion 

 of leaflets comes from comparison of closely related taxa, with structural 

 evidence in venation and in the presence of an articulation between 

 rachis and petiole below the solitary leaflet (Fig. lA). The genus Citrus 



