6 



BULLETIN" 317, U. S. DEPARTMENT OF AGRICULTURE. 



development of the larch, but decidedly favorable to the mistletoe 

 found upon it. This is at once evident to those familiar with the 

 environmental requirements of host and parasite. The region affords 

 a most instructive study of the advance and -predominance of a forest- 

 tree parasite on its host, showing this advance to be in as near an 

 exact proportion as the conditions for its optimum development 

 become more favorable. 



The problem of the mistletoes in their ecological relationships, re- 

 gardless of the fact that they are parasitic, is similar to that of all 

 chlorophyllaceous plants; hence, they respond to light, gravity, and 



Fig. 2. — Cross section of a part of a trunk of a larch tree, showing the regeneration of 

 branches from the same whorl to the fourth generation. (Tape in feet graduated in 

 tenths.) 



chemical influences, and in a far less degree to the influences of tem- 

 perature and moisture. How, then, do the ecological requirements 

 of the larch mistletoe hold with the climate of the region described, 

 over which the parasite is widely distributed? The great variation 

 in temperature, occasionally abnormally high, and the late, early, and 

 winterkilling frosts of some sections, although seriously injuring the 

 host, produce but little effect on the parasite. The uniform dryness 

 of the air at all seasons of the year throughout the region does not 

 greatly influence the mistletoe plant, which is essentially xerophytic. 

 On the other hand, the large percentage of sunshiny days and the ab- 

 sence of clinging fogs are directly favorable to the parasite, as it is 



