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. 1 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. 



ix 



