14 FUNGOID DISEASES 



as seen in the brain-like swellings on such trees as 

 the elm. In some cases the invaded tissues are killed, 

 in others only certain tissues or only certain areas, 

 while often the fungus simply robs the affected 

 tissues of certain food stuffs so that death does not 

 occur ; indeed the tissue may be stimulated to ab- 

 normally active growth. 



The injury due to the presence of the fungus 

 itself may be very slight but its consequences far- 

 reaching. The destruction of leaf tissues by many 

 fungi leads to starvation of the whole plant owing 

 to the destruction of the food manufacturing organs 

 (leaf tissues) ; so also a fungus may simply destroy 

 the root hairs (the fine feeding roots), the death of 

 the plant resulting from the destruction of these 

 water-absorbing organs. The rust of wheat {Pitc- 

 cinia grauiinis) is chiefly injurious in that it splits 

 the stems and so permits them to dry out. In 

 many cases the fungus secretes poisons which pass to 

 various parts of the plant causing various reactions. 



Some of the results of parasitic attack are men- 

 tioned below : — 



1. Sudden collapse of the host as above- 

 mentioned. 



2. Plant and parasite may live together for years, 

 with comparatively slight alteration of tissue, e.g. 

 species of Exoasctts on trees. 



- 3. Abnormal fruit, e.g. Exoascus pntni, causing 

 the disease known as ''pocket plums". 



