36 



POISOX OAK. 

 films diversiloba Torr. & Gr. 



Other names : Poison ivy; yearaj California poison sumac. (Fig. 18.) 



Description and habitat, — The poison oak differs from the preceding 



species mainly in the character of its leaflets, which are somewhat 



thicker and smaller, more nearly 

 elliptical, and less sharply lobed. 

 Their similarity to the leaves of the 

 Western oaks gives the plant its 

 common name. The poison oak 

 thrives best on cool westward moun- 

 tain slopes and in ravines, but is 

 quite generalry spread throughout 

 the Pacific coast from Lower Cali- 

 fornia and Arizona to British 

 America. It does not, however, 

 frequent the higher mountains. 



poison sumac, 



films vernix L. 



Other names : Swamp sumac ; dog- 

 wood (Mass.) ; poison dogwood ; poi- 

 son elder (Ala.); poison ash (Vt.); 

 poison tree; poison wood; poison 

 swamp sumac; thunderwood (Ga., 

 Va.). (Fig. 19.) 



Description and habitat. — An ar- 

 borescent shrub 6 to 30 feet high, 

 with long, pinnate leaves having 

 from 7 to 13 entire leaflets. The wood has a faint sulphurous odor, 

 which, together with the leaf scars, which are very prominent, enables 

 one to distinguish the plant from other shrubbery in winter. It grows 

 in swamps and in damp woods from Florida to Canada and westward 

 to Louisiana. 



Each of these species produces about the same effect on the human 

 skin. It will therefore be sufficient to consider the most widely known 

 representative of the group, viz, Rhus radicans, the poison ivy. 



Poisonous character. — Xo plant of the. United States is more popularly 

 recognized as harmful to man than this. Its effects are familiar to 

 everyone, and, as its victims far outnumber those of all other plants 

 combined, it has come to be regarded as the poisonous plant of America. 

 Until recently no one was able to tell how its effects were produced or 

 to what principle they were due. At the beginning of the present cen- 

 tury it was supposed to be a vapor or exhalation from the plant; then, 

 with successive stages in the development of chemistry and biology, it 

 was attributed to a specific gas, a volatile alkaloid, a volatile acid, and 



Fig. 18.— Poison oak (Rhus diversiloba), showing 

 leaves, flowers, and fruit, one-third natural size. 



