Minnesota Plant Diseases. 



199 



have already been mentioned as living in an unequal partner- 

 ship with root fungi in which the green plants are the dominant 

 partners. They are not however found as parasites on other 

 green plants. 



Higher seed plants. A number of Minnesota species of the 

 higher seed plants are known as true parasites on other leaf- 

 green plants and a few of these are of economic importance. 

 When a race of plants which was originally self supporting by 



FIG. 101 Twig of a witches'-broom of spruce, showing the parasitic plants of the mistle- 

 toe which cause the "brooming" of the branches. The mistletoes are seen as very 

 small plants, scarcely larger than the spruce leaves; they are tipped with an egg- 

 shaped body which is the fruit of the mistletoe and contains a single seed. (See also 

 Figs. 24 and 25.) Photograph by the author. 



means of a leaf-green apparatus, enters upon a parasitic life, the 

 leaf-green mechanism falls into disuse and may suffer reduction 

 or may even entirely disappear. Hence we find in confirmed 

 parasites of this group more or less of a bleaching of the para- 

 site. They are often, therefore, yellowish in color and the leaves 

 are reduced to mere scales or are wanting entirely. 



