1921] JOHNSON—POLY PODIUM 241 
which are then washed down to the epiphyte, must first be carried 
above the epiphyte by the water vessels inside the tree. The 
epiphyte is to this extent dependent on a physiological process of 
the living host, the upward conduction of water, which involves a 
considerable expenditure of energy. The mineral food demands 
made upon the tree by the epiphyte are thus somewhat equivalent 
to those made by the “half-parasite” of its host. The chief 
difference is that the mistletoe exacts its quota of salts (and of 
water also) from within the living host, before they have been 
Fic. 3.—Trunk of pane Prinus bearing at x and y (north side of ee two tufts 
of P, oh vulgare; upper tuft 9 feet above soil, others above this are 18 or 20 
feet from ground; Xs. 
used by the host itself, while the air plant gets its salts from the 
surface of the tree after they have served their function within it. 
The water obtained by the epiphyte of course has never been 
inside the supporting tree. If the mistletoe is to be called a “water 
(and salts) parasite,’ the epiphyte is a “‘salts saprophyte”; that 
is, it secures its mineral food from the dead and no longer functional 
portion of the supporting plant. 
ORIGIN OF TEMPERATE ZONE EPIPHYTES.—SCHIMPER (1888) 
announced the very important generalization that the vast majority 
of all vascular epiphytes are of tropical origin. Of extratropical 
epiphytes he believed that only the relatively few types found in 
