USING THE KEYS. 



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 advisable 

 to have the first two, which are common everywhere, pointed out 

 by someone who knows them, and to regard anything with com- 

 pound 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 of the poisonous 

 species as quickly as possible, I go to a "vine"-covered fence- 

 post 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. 10), 

 I find that it is to be sought in Key D on p. 38. In this key, 

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

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

 ences : no. i, to 7; no. 7, to 10; no. 10, to n; no. n, to 23, 

 with a caution that this group contains poisonous species ; no. 23 

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

 out 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. 117), I find, under no. I, 



