Figure 6.—Leaves of common poison-ivy, about half natural size. These may occur on the same or different plants: A, B, C and F, less common; D, with even margins, the most common and generally recognized form; E, with lobed margins, nearly nearly as often seen as the shape in D.
Figure 7.—Common poison-ivy vine with clusters of flowers in the axis of each leaf.
Common poison-ivy in full sunlight along fence rows or in open fields grows more in the form of a shrub than a vine. In some localities the common form is a low-growing shrub from 6 to 30 inches in height. This form, as well as the vining type, usually has a rather extensive horizontal system of rootstocks or stems at or just below the ground level. The vining form under some conditions later becomes a shrub. Plants of this type are shown in [figure 5], where the vine, supported on the fence, has extended upright stems that are shrublike. In some localities the growth form over a wide range is consistently either a vine or a shrub type, while in others what seems to be the same species may produce either vines or shrubs.
The leaves of the common poison-ivy are extremely variable, the three leaflets being the only constant character. It is impossible to describe the great range of variation in the shape or lobing of the leaflets. A study of the leaf patterns, especially the six shown in [figure 6], will give a fair range of types, although other forms may be found. One plant may have a large variety of forms of leaves or it may have all leaves of about the same general character. The most common type of leaf having leaflets with even margins is shown in [figure 6, D], and one with leaflets having slightly lobed margins is shown in [figure 6, E]. The other forms shown in [figure 6] are not quite so widespread, but may be the usual type throughout some areas.
Figure 8.—Common poison-ivy, about natural size: A, Flowers; B, mature fruit.