USING THE KEYS ix 



USING THE KEYS 



, The determination keys are essentially "dichotomous." At 

 each point it is necessary to decide between two rarely three 

 or four very distinctly contrasted characters, and in each 

 case these contrasts are grouped under a single number in the 

 key. The first few choices are between differences that can be 

 seen without touching the plant. Since poison ivy, poison oak 

 and poison sumach are very poisonous to the touch, it is advis- 

 able to have the first two, which are common everywhere, 

 pointed out by someone who knows them, and to regard any- 

 thing with compound leaves as suspicious until these three 

 are well known. A few examples will show the simplicity of 

 using a key, and the directness with which it leads to the 

 name of a plant. 



Wishing to become acquainted with one poisonous species 

 as quickly as possible, I go to a "vine"-covered fencepost and 

 without touching the plant am able to see readily that it is 

 thin-leaved, therefore probably deciduous; with one leaf at a 

 node, the leaves therefore alternate; and that each leaf is 

 compound, made up of three rather large wavy-margined leaf- 

 lets coming from the end of the leaf-stalk, and therefore digi- 

 tate, or palmate. Turning to the Synopsis of Groups (p. xii), 

 I find that it is to be sought in Key D on p. xli. In this key, 

 beginning as always with no. 1, the characters that I have 

 seen already take me through the key by the following refer- 

 ences: no. 1, to 9; no. 9 to 12; no. 12 to 13; no. 13 to 25, 

 with a caution that this group contains poisonous species; no. 

 25 compels me to look at the plant a little more closely, still 

 without touching it, and I see that it does not support itself 

 by coiling about the post, and that it has no tendrils though it 

 has fastened itself by short roots coming from between the 

 nodes. The conclusion is* inevitable that it belongs to the genus 

 Rhus. In the key to species of this genus (p. 84), I find, 

 under no. 1, that there are three species with only 3 leaflets to 

 each of the compound leaves ; although the characteristic white 



