446 



ELEMENTARY STUDIES IN BOTANY 



lives in the sap, cutting off the water supply and causing the 

 host plant to wilt and die. Since the first symptom of the 

 presence of these parasites is the wilting of the host, these 

 diseases are known in general as " wilts." There are a great 

 many kinds of wilt-producing fungi, but their relation to the 



host and their effect upon it are 

 the same. A few illustrations 

 will be given. 



Cabbage wilt. In this disease, 

 the water-conducting vessels are 

 invaded by bacteria that enter 

 through the " water pores " of 

 the leaves, which are minute 

 openings along the edges of the 

 leaves that are connected with 

 the system of water-conducting 

 vessels. They are in fact the 

 open terminals of this system. 

 The disease is often called the 

 " black rot of cabbage " (Fig. 

 96), but the rotting is not due 

 to the wilt-producing bacteria, 

 but to the decay of the leaves 

 or of the whole head which they 

 have been the means of killing. 



Cucurbit wilt. This is often 

 a very destructive disease among 

 In this case there is no natural 

 opening for the entrance of wilt-producing bacteria, as in 

 cabbage, but the infection is through wounds produced by 

 the bites of insects. From the point of entrance the in- 

 fection extends through the vessel system. If the infection 

 is at the tip of a branch, the wilting is gradual ; if it is in 

 the main stem, the wilting is rapid. 



Fusarium wilts. Bacteria are not the only fungi that 



FIG. 95. Black knot: a, on plum 

 b, on currant. After MASSEE. 



melons and cucumbers. 



