22 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
(9) PARASITIC AND SYMBIOTIC PLANTS. 
Though not forming associations properly comparable with the fore- 
going ones, the parasitic plants of the Laboratory domain call for notice. 
Two species of mistletoe are widely distributed in this region, one of 
which (Phoradendron californicum) is of fairly frequent occurrence on 
Tumamoc Hill and the adjacent valleys, where it grows on Prosopis 
velutina, Parkinsonia microphylla, Cercidium torreyanum, Acacia con- 
stricta and A. greggu (plate 11). Of these P. microphylla and the mesquite 
are its most frequent hosts on the domain, where it has been observed 
but once on A. constricta, and once on Cercidium torreyanum. The other 
species, Phoradendron flavescens, has not been found within our limits, 
but it is of common occurrence in the valley of the Santa Cruz River 
in the vicinity of Tucson, growing on cottonwood and ash trees. 
In contrast with the limited number of hosts which are here affected 
by these species of mistletoe, Mr. J. C. Blumer states that in the Chiri- 
cahua Mountains, at altitudes of 5,000 to 6,000 feet, he has observed 
Phoradendron flavescens growing on two species of ash, on sycamore, 
on two species of cottonwood, and on two of willow, and in its form 
pubescens on many oaks. This is as might be expected from the far 
greater variety of hosts and the greater range of environmental con- 
ditions in the mountain region. 
Important observations and experiments have been made on both 
species by Cannon (1904), to which bare reference can here be made. 
These relate to germination and mode of penetration of the host and to 
the rate of transpiration of the parasite as compared with that of the 
host. In regard to the latter it was found that in the various experi- 
ments undertaken the parasite transpired more rapidly than the host, 
from which there results an unnatural draft on the resources of the host, 
which is often followed by the death of the infested branches, the con- 
sequent great disfigurement of the tree, and, in some cases, its ultimate 
destruction. It does not seem, however, that any of the hosts affected 
have been appreciably limited in their distribution by the action of these 
parasites. As for the distribution of the parasites themselves, it appears 
from the observation of the author referred to that it takes place from 
tree to tree by the agency of birds, but its distribution in a tree is such 
as to indicate that when once the higher branches have been infected, 
in due time seeds from this source reach the branches below without 
the agency of birds. 
An exceedingly interesting case of parasitism, in which a large number 
of hosts is involved, is that of Orthocarpus purpurascens palmeri, which 
in springtime grows so thickly on the sides of Tumamoc Hill as to give 
them a reddish tint noticeable at considerable distances. Dr. Cannon 
has identified some 20 species of plants on the roots of which this parasite 
fastens. 
