12 Minnesota Plant Diseases. 



greater area of soil is drained of its nourishment. Root hairs 

 of flowering plants never branch, but new ones are constantly 

 being formed near the tip of the growing rootlets, thus effect- 

 ing a similar result to that of the branched mycelium of the 

 fungus. The absorptive system of parasites often consists of a 

 similarly branched mycelium which runs between the cells of 

 the plant upon which it feeds. Some of the branches of these 

 fungus threads are of a special kind and penetrate into the cells 

 of the host plant. Parasitic plants may not have such a richly 

 developed absorptive system as a mushroom, but in other re- 

 spects may be more highly specialized. The mycelium of a 

 highly organized parasite is usually only able to obtain nour- 

 ishment from certain species or* groups of species of plants. 

 For instance, certain rusts are capable of getting nourishment 

 only from one kind of grass plant. It will be seen from these 

 considerations that the absorptive system of this group of 

 plants, whether parasites or saprophytes, is, in general, well de- 

 veloped. 



Parasitism and saprophytism. Parasites are usually de- 

 scribed as those plants which obtain their nourishment directly 

 from living plants or animals. They are so organized that, 

 when their nourishing threads come into close contact with cer- 

 tain living plants or parts of plants, they answer to certain im- 

 pulses, sending special branches directly into the living tissues 

 and there absorbing nutrition. Saprophytic plants, on the 

 other hand, are not reacted upon by living plants and are com- 

 pelled to get their nourishment from the dead products of 

 plants or animals. The real substances which are absorbed by 

 parasites and saprophytes may not be different in their chem- 

 ical natures but the methods of obtaining them differ. The 

 parasite has learned to respond to certain impulses, which it re- 

 ceives when it comes near to another plant, and by this response 

 obtains nutrition. True saprophytes never respond to such an 

 impulse. They live on ground rich in leaf mold or in decaying 

 wood, or on dung of animals, on remains of animal life or on 

 still other products of living plants or animals, but never 

 upon the organism when the latter is still alive. 



Parasites are limited in their size by the size of their host- 

 plant, hence they are usually very small — often microscopic in 



