4 EDMUND W. SINNOTT AND IRVING W. BAILEY 
modifications among herbs are often of very recent origin and that 
the important steps in the foHar evolution of the Dicotyledons took 
place while they were practically all woody in habit. 
Evidence from various sources as to the relative antiquity of 
the six leaf types may be outlined as follows. 
Palaeobotanical Evidence 
Analyses of leaf type in various Cretaceous and Tertiary dicoty- 
ledonous floras have been made and are presented in the following 
table.* Owing to the frequent impossibility of determining whether 
an impression is that of a palmately or pinnately compound leaf, all 
the compound species have been thrown together. For simplicity's 
sake the few pinnate lobed forms have been included with the pinnate 
simple ones. 
It will be noted that the percentage of compound leaves in the 
fossil floras is smaller than in the modern ones. This is probably due 
in part to the fact that compound leaves are usually broken up in 
fossilization into their component leaflets, which have often been 
described as complete leaves. The most striking difference, however, 
between ancient and present day floras is the much greater abundance 
in the former of palmate leaves (32 per cent as against 13 per cent 
in living species). In the Cretaceous the palmate-lobed forms were 
as numerous as the simple ones. In the Tertiary the lobed type was 
^ The following fossil floras have been examined: 
Cretaceous: The Lower Cretaceous Deposits of Maryland, Berry (and others). 
Maryland Geological Survey; The Upper Cretaceous and Eocene Floras of South 
Carolina and Georgia, Berry. Professional Paper 84, U. S. G. S. ; Flora of the Da- 
kota Group, Lesquereux, Geol. Surv. of the Territories, Vol, VIII; Cretaceous Flora 
of the Territories, Lesquereux, Geol. Surv. of the Territories, Vol. VI; Flora of the 
Raritan Formation, Berry, Bull. 3, Geol. Surv. of New Jersey; Flora of the Amboy 
Clays, Newberry, Monograph XXVI, U. S. G. S.; Cretaceous Floras of Southern 
New York and New England, Hollick, Monograph L., U. S. G. S.; Flora Fossilis 
Arctica, Heer, including the Arctic, Patoot and Atane floras (Cretaceous) and those 
of the Polar Lands, Greenland, North Greenland, and Spitzbergen (Tertiary). 
Tertiary. The Tertiary Flora of the Western Territories, Lesquereux, Geol. 
Surv. of the Territories, Vol. VII; The Flora of the Laramie Group, Ward, 6th 
Ann. Report, U. S. G. S. ; The Flora of the Bad Lands, Lesquereux, Geol. Surv. of 
the Territories, Vol. VIII; Flore Fossile des Travertins Anciens de Sezanne, Saporta, 
Mem. Soc. Geol. de France (2), VIII; Tertiarfloren der Oesterr. Monarchic, Etting- 
hausen, Vienna, 1851; La Flore Fossile du Japon, Nathorst, Bih. Svensk. Vetensk- 
Akad. Handlingar, B. 20, No. 2. 
