LOCALIZED STAGES IN DEVELOPMENT. 95 



the type, provided additional species are differentiated in the progressive line of evolution 

 of the group. 



Besides reversionary leaves, reversionary individuals ai-e not uncommon in this protean 

 species. Frequently trees are seen in wliich all tlie leaves are four-lobed (PI. 16, fig. 1), 

 similar to a stage attained early in an individual wliich has six-lobed leaves in the adult. 

 At the Arnold arboretum, there is an interesting tree which is highl\' reversionary in 

 character. All or essentially all of the leaves are rounded, distally truncate, with slight 

 angles at the lateral terminations (PI. 16, fig. 10), closely resembling the nepionic leaves 

 of the seedling (PI. 16, fig. 11, leaf 4). A branch of this tree which sliows a progressive 

 locahzed vai'iation is represented in PI. 16, fig. 9. It is progressive for the individual 

 because more specialized than the tj-pical leaves, although if it were on a normal tree of 

 the species it would be a regressive variation. Leaf 1 has one slight lateral lobe on the 

 middle half of each side ; leaf 2 has two slight lateral lobes corresponding to the two well- 

 developed lobes of a normal tree (Fig. 12). Leaf 3 is nearly orbicular, truncate, rever- 

 sionary, and comparable to early leaves of seedlings. A tree with similar leaves in the 

 Botanic garden in Washington M'as desci'ibed by Holm ('90). On Pearl street, Dorchester, 

 Mass., there is a tree in which the typical leaves have six slightly developed lobes as in 

 leaf 2 (PI. IG, fig. 9). In another tree at the Arnold arboretum, the tyjiical leaves are 

 four-lobed as in Fig. 1. This is not an uncommon variation, as noted, and many such 

 leaves occur on all typical trees. It ))ears a close I'esemblance to Liriodendron g'ujanteum, 

 Lesq., of the Cretaceous. 



In the development of L'lr'iodendron tid'ip'ifcra, there is a distinct progressive series of 

 leaf differentiation. Starting with the young seedling, we find first a leaf which is rounded, 

 distally truncate (PL 16, fig. 5); next the terminal angles become sharpened (Fig. 6); then 

 lateral lobes become gradually differentiated, at first slightly, then strongly marked (Fig 1) ; 

 then two additional lobes appear (Figs. 2 and 12), giving the full sjiecific characters. As a 

 variation in the direct line of progressive differentiation, two more lobes may be differenti- 

 ated (Fig. 3). In regressive leaves at the tips of branches, etc., as described, these same 

 characters are repeated, but in inverse order, a slightly regressive leaf drops out the two 

 basal lobes (Fig. 2a-b), a more regressive leaf drops out two more lobes (Fig. 2 c-d), an 

 extremely regressive leaf becomes rounded at the angles (B^ig. 2 e-f) and repeats the 

 characters of young seedlings. It is seen that progressive development jiroceeds by add- 

 ing characters at the proximal end of the leaf, and regression is marked by the dropping 

 out of characters at the proximal end. The distal end never suffers any considerable change. 



Turning to the fossils, we find that Linodeiidronprimaemint Newb., of the Cretaceous, 

 is an ancient form Avhich has a rounded leaf, distally truncate, resembling the leaves of the 

 young seedling (Fig. 8). Liriodendron meeki Heer, of the Cretaceous, has slight marginal 



