110 ROBERT TRACY JACKSON ON 



cal leaves of the species. After making the above drawings and observations I was much 

 interested to find that Prof. Lester F. Ward ('88) had made similar observations, and he 

 compares this lobate condition of modern Platanus with a sunilar basal lobate appendage 

 in Platanus hasilobata Ward, of the Laramie group (Cretaceous) . There is a difference, 

 however, as P. basilobata has much larger basal lobes and the upper part of the leaf is 

 trilobate instead of five-lobed. In the Cretaceous Aspkliojjhyllain dentafum Lesq. a 

 similar strongly marked basal appendage occurs. Ward ('88) and Lesquereux ('91) note 

 this striking similarity to leaves seen in living Platanus as described, and further the fact 

 that the nervation of A. dentatum is very similar to that of Platanus. All the evidence 

 would seem to show that the Cretaceous Aspidiophyllum is an ancient though somewhat 

 aberrant form of Platanus. It is quite possible that the extravagant development of basal 

 appendages in these several types represents parallel differentiation in related forms 

 rather than direct phjdogenetic association. 



The tj^pical leaves of adult Platanus occidentalis (PL 19, fig. 57) ai'e broadly pentag- 

 onal, five-lobed, dentate, the base of the leaf extending down shghtly on the stem form- 

 ing an obtuse angle proximally instead of an acute angle as in seedlings, young suckers, 

 and many fossil forms. This basal portion is adnate to the petiole and the midrib extends 

 to its base. As variations, leaves are frequently found which have none of the secondary 

 dentations, as in the specimen figured ; also the two proximal lobes are frequently wanting, 

 and the leaf becomes trilobate and resembles more or less closely leaf 3 of Fig. 56 and 

 fossil alUes as Platanus primaeva Lesq., of the Cretaceous. 



HicoRiA OVATA. In the Shag-bark, Hkoria ovata (Mill.) Britton, in normal germina- 

 tion the nut rests on the surface of the ground (PL 20, fig. 59) . The first leaves are 

 simple, oval, slightly serrate, with the apex pointed bluntly. The specimen figured has 

 lost the leaves at the lower portion of the stem, but I did not succeed in finding a speci- 

 men with these in place. The fourth leaf is distorted and besides the distal leaflet 

 has a small lateral leaflet on one side. The fifth and sixth leaves are trifoliolate. The 

 distal leaflets of these compound leaves are practically like the first simple leaf, but lateral 

 leaflets are shorter and smaller. 



In the adult of this species the leaves are compound, composed of five leaflets (PL 20, 

 fig. 61, leaf ]), and the distal leaflet bears a considerable resemblance to the first simple 

 nepionic leaves of the seedUng. At the tips of branches of adults there is characteristically 

 one (sometimes two or three) , trifoliolate leaves (PL 20, fig. 61, leaf 2) . Such trifoliolate 

 leaves are not borne normally on any other part of the branch and represent a localized 

 reversion, comparable to the later stages in development of the leaves of the seedling 

 (Fig. 59). 



The Shag-bark does not sucker freely ; but a sucker from the root of an adult (PL 20, 



