418 



PLANTS AND MAN 



regions of the western United States. They cause a swelling of 

 the attacked portion and the development of a very bushy 

 growth called a "witches broom." Ultimately the portion of 

 the stem beyond the infection dies; thus young trees would be 

 killed and other trees would suffer a reduction in growth. The 

 green leafy mistletoe, so popular during the Christmas holiday 

 season, is another stem parasite, but depends upon its host only 

 for water and mineral nutrients, being able to manufacture its 



n 



Fig. 256. — Mistletoe plants arc scmi-parasitcs, absorbin 



from the host tree. 



water and minerals 



own food (fig. 256). Leafy mistletoes attack both deciduous and 

 coniferous tree species; their chief damage is in causing a break- 

 age of the stems and limbs due to increasing weight, since they 

 grow from one year to another. Mistletoes all produce a fleshy 

 fruit, which is eaten by birds. The seeds, however, are not eaten 

 and are wiped off" the birds' beaks onto the tree stem where they 

 adhere due to a sticky surface, and germinate to produce a new 

 mistletoe plant. Fortunately many of the more destructive 

 mistletoes are highly specific as to their host. Thus, if the seed 

 from a ponderosa pine mistletoe were placed upon the bark of 

 a red alder tree, the seed would germinate, but the mistletoe 



