USING THE KEYS. 
As in the Summer Manual, the keys are essentially dicho- 
tomous, bringing together in couplets alternatives with usu- 
ally sharply contrasted differences. In each instance, these 
-contrasts are grouped under a single number. A few exam- 
ples will show the simplicity of using such a key, and the 
directness with which it leads to the name of a plant. 
Caution should be observed with all alternate-leaved 
shrubs until the poisonous species of Rhus have become 
known. 
Having a disinclination to come into unexpected contact 
with the poison ivy, I decide to familiarize myself with its 
essential winter-characters at once. Remembering where I 
saw it last summer, I go to a post or tree trunk over which 
it is climbing, and without touching it I am able to see that 
it climbs by the aid of numerous short roots that fasten them- 
selves to the support, but has no tendrils, and that its leaf- 
scars are distributed singly along the stem, or alternate. Cau- 
tious examination under a lens, still without touching the 
plant, shows that these leaf-scars are half-round or somewhat 
shield- or heart-shaped or 3-sided, and after looking at several 
of them I have satisfied myself that each scar is marked by a 
number of dots—more or less evidently in 3 groups, each of 
which is a bundle-trace corresponding to one of the woody 
strands that passed from the stem to make up the framework 
of the leaf last season. Over each leaf-scar is a single bud, 
slightly elongated or stalked below its leaves. No sign of 
stipules, or scars left after their fall, is evident near any of 
the leaf-scars. I have noticed everything essential to naming the 
plant without having exposed myself to danger from its poi- 
son. Turning to the key (p. xi), I find only five contrasts— 
no. i to 140; no. 140 to 150; no. 150 to 151; no. 151 to 157— 
before I stop convinced that it belongs to the genus Rhus. 
Ex 
