LOCALIZED STAGES IN DEVELOPMENT. 109 



ancestral features (lobed leaves) do not come until late in the growth of the seedling (PI. 

 19, fig. 50). As shown by Cope, t^pes may lose ancestral chai'acters b}^ retardation of 

 development. That is, features may appear at later and later stages in development until 

 they finally disappear. A common case is the late appearance or entire want of the last 

 molar or wisdom teeth in man. To apply this principle to the Sassafras : if we assume 

 that it is a reversionary type, repeating more or less fully the characters of types earUer 

 than the known Cretaceous lobed forms, then by the principle of retardation the lobed 

 features (Cretaceous) would appear later and later in the ontogeny (seedling) and be 

 present in latest growths of the adult (tips of branches), or absent, as is the case. While 

 this view is open to doubt, it is in accordance with the facts and seems worth consid- 

 eration. Associated genera, as Laurus, Persea, and Cinnamomum, are characterized by 

 leaves which are simple in outline Ijoth in fossil and living species. 



Platanus occidentalis. In Platanus orieiitalis L., according to Lubbock ('92), 

 the seedling (PI. 19, fig. 51) has cotyledons which are narrow, spathulate, tapering to the 

 base. The nepionic leaves are alternate, spathulate, cuneate, distally bluntly tridentate. 

 I have not seen seedlings of our species, Platanus occidentalis L. ; but we may assume 

 that it is similar to the same stage of the nearly allied European form. Suckers from the 

 base of an adult tree (PI. 19, figs. 55, 56) have leaves which are quite similar to nepionic 

 leaves of the seedling. The first leaves (Fig. ^)o) are elongate spathulate, distally den- 

 tate, laterally nearly or quite entire. While the tips are variable in outline they are fre- 

 quently tridentate as in leaf 1, Fig. 56, and some such leaves occur on all suckers. Leaves 

 at the base of the sucker (leaf 1) are more elongate and spathulate than later leaves, 

 thus making a closer approach to the character of the seedling. A noticeable feature is the 

 fact that the blade of the leaf extends down on tlie petiole, forming an acute angle basally, 

 as in seedlings and many fossil species. The trilobed leaves of the siTcker, as leaves 2 and 

 3, Fig. 56, closely resemble fossil species of Platanus, as P. primaeva Lesq. from the Creta- 

 ceous (Dakota group) . At the height of several feet from the ground suckers usually 

 develop very large strongly five-lobed leaves numerously and deeply dentate (PI. 19, 

 fig. 58) . The most striking feature, however, is the development of the base of the 

 blade into a lobed basal appendage which is more or less extensively developed. Nei'ves 

 run upon this appendage from the point of disappearance of the main veins of the leaf 

 proper, as shown in the figure. On the lower side of the leaf it is seen (Fig. 58a) 

 that this appendage stands out free from the petiole so that the leaf is perfoliate. At the 

 base of the petiole in the case of these vigorous leaves the branch is enveloped by an 

 extravagant development of the stipules (Fig. 58b), which fuse and form an encircling ruf- 

 fle. These characters are seen frequently also on vigorous leaves of adults especially those 

 growing in wet bottom lands ; but they are quite distinct from the characters of the typi- 



