2OO Minnesota Plant Diseases. 



Special kinds of sucker roots are frequently produced which 

 penetrate the host plant tissues and absorb the manufactured 

 food stuff. Some of these parasites have only half learned the 

 parasitic habit and still retain some of their leaf-greerr apparatus. 

 A typical parasite of this group of plants is the little mistletoe 

 which occurs in great abundance on spruce trees in the north- 

 ern part of the state. This little plant lives in the twigs and 

 larger branches of the spruce and induces the formation of 

 witches'-brooms. Badly diseased spruces therefore show 7 a very 

 irregular contour and may eventually be killed. The common 

 dodder is another confirmed parasite. It starts life from the 

 seed as a little leaf-green-possessing seedling but as soon as it 

 comes into contact with a suitable host plant it abandons its 

 leaf-green apparatus and coils itself closely around the support, 

 sending in its sucker roots which also serve to fasten it to its 

 support. The twining stem grows rapidly, bears very small and 

 reduced leaves, and the whole plant is yellowish in color. The 

 dodder is common on many wild swamp plants and is also occa- 

 sionally abundant on clover where considerable damage may be 

 caused. Of some interest are also those few forms which are 

 root parasites. The toad flax, which is a common Minnesota 

 plant, has partially learned this habit of parasitism. Here the 

 plant is apparently a typical leaf-green herb, but its roots may 

 be found penetrating the roots of other plants and there obtain- 

 ing nourishment. Parasitism is here an auxiliary process. 

 Other Minnesota plants, members of the broom-rape family, 

 have completely learned the root parasite habit and have con- 

 sequently lost all of their leaf-green. The stems are usually 

 small, reaching but a short distance above the ground, and bear 

 a few colorless reduced leaves and spikes of flowers. Several 

 species of cancer roots and broom rapes occur in this state. 

 They are not abundant, however, and produce no far-reaching 

 or destructive disease in plants. (Figs. 24, 25, 101.) 



