l82 PARASITIC FUNGI 



histories and necessary conditions of life of the different 

 fungal parasites that we can get the knowledge neces- 

 sary to enable us to devise methods of preventing or 

 mitigating their attacks. The cure of plant diseases 

 caused by fungi is seldom possible ; but preventive 

 measures, taken after a thorough knowledge has been 

 obtained of the parasite and of the conditions under 

 which the disease spreads, are often very successful. 

 Just as in the case of animal diseases, " prevention is 

 better than cure," but with plants it is almost the 

 only method which is of any use at all. 



Bacteria are the cause of some plant diseases, just 

 as we have seen that mycelial fungi cause certain 

 animal diseases. But the latter are far more wide- 

 spread as pathogenic plant parasites, just as bacteria 

 are by far the most widespread and destructive of 

 the parasites of animals. This difference is mainly 

 due to two causes. First the spores of fungi are pro- 

 duced and carried about mainly in air, while many 

 bacteria are largely carried about in the blood-stream 

 which is absent from plants ; and with this the second 

 cause is connected. The life processes of fungi are 

 adjusted to ordinary air temperatures, and the tempera- 

 tures of the bodies of plants exceed these by very 

 little. On the other hand, the life processes of patho- 

 genic bacteria go on much more rapidly at higher 

 temperatures, such as that which is maintained in the 

 body of a warm-blooded animal (about 37 C.). 



PRACTICAL WORK. 

 " WHITE RUST " (Albugo). 



(i) The white pustules on the shoots of the Shepherd's Purse 

 (Capsella) are caused by a parasitic fungus (Albugo) which often 

 deforms the inflorescence. The mycelium of the fungus sends 

 out hyphae which bud off chains of conidia just below the surface 

 of the host in such numbers that they burst off the surface 



