Plant Associations at the Desert Laboratory. 89 



witliin our limits, but it is of common occurrence in the valley of 

 the Santa Cruz River in the vicinity of Tucson, growing on cotton- 

 wood 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 Chiricahua Mountains, at altitudes of 5,000 to 6,000 feet, 

 he has observed Phorade>idron 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 ranges of environmental conditions in the mountain 

 region. 



Important observations and experiments have been made 

 on both species by Cannon, 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 com- 

 pared with that of the host. In regard to the latter it was found 

 that in the various experiments undertaken the parasite trans- 

 pired more rapidly than the host, from which there results an 

 unnatural draft on the resources of the host, which is often fol- 

 lowed by the death of the infested branches, the consequent 

 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 para- 

 sites 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 purpura- 

 scens palmcri, 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. 



As far as present observations extend, the fungus flora of 

 the Laboratory domain may be characterized as insignificant. 



