BIOLOGICAL FACTORS 



191 



cance, but they are always of interest because of their peculiarities 

 and relatively local distribution. A considerable range of degree of 

 parasitism is possible. 78 The common dodder (Cuscnta) is repre- 

 sentative of those parasites (holopar ashes) completely dependent 

 upon their hosts, but the mistletoes and others are termed partial 

 parasites because they are green and can manufacture food. Some 

 species are attached to their hosts at a single point of contact, often 

 by roots. A number of Scrophulariaceae are of this type. Others 

 twine or sprawl over the host plant and are connected to it at in- 

 tervals by absorbing structures called haustoria, whose conducting 

 systems may be in intimate contact with xylem and phloem of the 

 host. Still others may be contained within the host and show only 

 their reproductive structures externally. Effects upon the host are 

 obviously physiological, and reduction of growth and vitality are 

 usually apparent. Abnormal growth is also common in the pres- 

 ence of a parasite. It is often manifested as bushy masses, called 

 "witches brooms" or is occasionally found in twisted, flattened, 

 or distorted branches. Parasitic seed plants have little effect upon 



FIG. 89. A stand of scrubby oak infested with mistletoe (?horadendron 

 ftavescens).—U S. Forest Service. 



