LOCALIZED STAGES IN DEVELOPMENT. 97 



Rhus toxicodendron. The seedling of the Poison Ivy (^Hhus toxicodendron L.) has 

 two evenly rounded, fleshy cotyledons (PI. 16, fig. 18). The first nej)ionic leaves are 

 trifoliolate and opposite. Succeeding leaves are similar but alternate. The terminal 

 leaflet has a short stalk, but lateral leaflets are sessile as in the adult. Lateral leaflets of 

 these early leaves present a small lobe on their proximal borders. The leaves of the 

 adult differ little from those of the seedling except in the lengthening of the stem of the 

 terminal leaflet and the usual absence of lobes on lateral leaflets. While this is true at 

 West Chop, Mass., where the soil is barren, in fertile soils growth is more luxuriant and 

 lobation is commoner. Als(j in vigorous adults there may be several lobes instead of 

 a single lobe in individual leaflets. Simple leaves are occasionally seen in adults. 

 Leaves near the base of the adult vine (nepionic area) have a tendency to produce lobed 

 leaves similar to those of the seedling. \i\ seedlings grown in fertile soil nepionic leaves 

 are occasionally more lobate than in the specimen figured. Lateral leaflets may have 

 two lobes and the terminal leaflet a lobe on either side. Such extra lobation is not 

 common, however, and tlie specimen figured is typical of most of the seedlings seen. 

 Primary (nepionic) leaves in this genus show considerable variation, according to Lub- 

 bock ('92) being scale-like, simple, or, as he says in one species, Rhus typldna, tri- 

 foliolate. In R. toxicodendron, the trifoliolate nepionic leaves, so similar to those of 

 the adult, mark it as a species with an accelerated development, a feature common in 

 degradational species. The degradational or specialized character of this species is indi- 

 cated by its creeping habit, its development, and perhaps by its toxic properties as well. 



ViTis. In the grape, Vitis (garden varieties), the cotyledons are opposite, ovate, 

 entire. The nepionic leaves are cordate, slightly trilobed. In later growth, leaves 

 become more lobate, and tendrils, which are absent in the young, begin to appear. In 

 adult vines the young leaves, when they first appear, are strikingly like the seedling, not 

 taking on the full lobate character until later growth. Nepionic leaves and young leaves 

 of new growths resemble fossil species of Vitis in their trilobed form and general sim- 

 plicity of outline. 



Ampelopsis quinquefolia. In the Virginia Creeper (Amjyelopsis quinquefolia 

 Mich.) the seedling (PI. 16, fig. 19) has ovate cotyledons resembling those of A. tricu- 

 spidata. The first two succeeding nepionic leaves are trifoliolate, alternate, as in A. tri- 

 cuspidata, but they differ from that species somewhat in form; compare Fig. 19, and PI. 

 17, fig. 20, The third leaf has attained adult characters. The adult has compound 

 leaves of five leaflets, not unifoliolate leaves, as in A. tricusjndata. The adult does not 

 present localized stages, excepting that occasional leaves are reversionary in being trifo- 

 liolate. These leaves may be borne on any part of the plant, and occasional branches 

 bear many such leaves. This is especially seen in late terminal branches. 



