Diseases of the Potato 187 



to extract their food, they will germinate under favorable 

 conditions of temperature and moisture. Indeed most 

 spores will germinate under favorable conditions without 

 the presence of the host plant. These conditions vary 

 with different fungi, but, in general, a fairly warm, moist 

 condition is favorable for the germi- 

 nation of most spores. The sprout, 

 or germ-tube (Fig. 16), produced by 

 the germinating spore enters the tis- 

 sue of the host directly through the 

 wall which it is able to dissolve, .^^^ ,^^:™;-- 



through wounds or through the ing germ-tube entering 



breathing pores. The root-hairs of ^^ ' 

 higher plants have very tender walls, and these may serve 

 as a point of infection by parasitic fungi of the soil. After 

 a fungus gains entrance to a plant, it lives upon the cell 

 products and usually brings about the death and discol- 

 oration of the tissues at the point, so that the infested 

 area may be seen as a spot, rot, blight or canker. The 

 fungus, sooner or later, puts forth fruiting bodies on the 

 outside of its host, and on these bodies the spores are 

 borne. 



Bacteria are plants of a lower order than fungi, many 

 kinds consisting of but a single minute cell. In the 

 presence of oxygen, water, food and suitable temperature 

 they reproduce very rapidly. They do not produce 

 spores like fungi, and are unable to penetrate unaided 

 the outer tissues of plants, but may enter the plant 

 through insect and other wounds or through other open- 

 ings. Thus, parasitic bacteria may be carried from plant 

 to plant by insects and be introduced into their tissues 

 during dry weather when fungi would be unable to do 

 so. Bacteria also live entirely within the host plant and 



