MITES AFFECTING THE POISON OAK. 
H. E. EWING. 
According* to the plan of natufe animals are compelled either 
directly or indirectly to lay under tribute the plant world in 
order to obtain food ; and so completely have they done so that 
it is doubtful if any, among* the many thousands of species of 
the latter, are exempt from attach. Being thus exposed to the 
wholesale appetites of higher creation plants have been forced 
into the evolution of devices for warding off animal attacks. 
We are all familiar, and probably to our sorrow, with the way 
in which many plants protect themsebces from the larger ani- 
mals by means of thorns or spines, thus lancing or even lacer- 
ating the hungry herbivor that comes too near or reaches with 
open mouth to devour them. Other plants obtain much .protec- 
tion, especially from their arthropod enemies, by the very 
tough tissues that make up the bulk of their substance, or by 
a well developed layer of hard cells that cover most of the ex- 
posed parts. But of all the devices that nature has contrived 
through the guiding hand of natural selection, to protect plants 
from animal attacks, it is doubtful if any Is more successful 
than the development of poisonous properties. That these poi- 
sons do protect plants possessing them observations clearly 
show. Thus our poison ivy ( Rhus toxicodendron) is known to 
be almost exempt from insect attacks. Only four species are 
known to feed upon it. Other species of poisonous plants also 
are known to be alibost exempt from attack. 
We have on the Pacific slope, a species of Rhus (Rhus 
diversiloba ) , known as the poison oak, which is much more 
abundant there than the poison ivy is with us, and also its toxin 
is, I believe, far more injurious to man. I have a photograph 
showing a large portion of a sheep pasture, located near Cor- 
vallis, Oregon, that is so completely overrun by poison oak that 
little of it is left available for grazing. In connection with the 
toxic effects of the poison oak, I remember seeing a woman who 
had both eyes swollen shut and her lips puffed out over an 
inch on account of a slight contact with poison oak. But con- 
tact is not necessary in order to be affected with the poison. 
