PLANT DISEASES l8l 



the germ tube penetrates a stoma (Fig. 19, F) and in- 

 fects the wheat plant again, producing a mycelium which 

 forms uredospores, the type with which we started. 



Thus the fungus may live on alternate hosts (cf. the 

 bacilli of tetanus and gas-gangrene, p. 155), the wheat 

 and the barberry ; though it may also, by wintering 

 in the uredospore condition, live exclusively on the 

 wheat plant. Long before this complicated life his- 

 tory was worked out, farmers had noticed that if 

 there were barberry bushes in their hedges their wheat 

 was particularly liable to " rust/' and in some countries 

 it is a legal offence to allow the barberry on a farm. 

 Wheat rust often causes losses to the wheat crop of 

 the world represented by millions sterling. 



Puccinia graminis (the Black Rust) is rare on 

 wheat in this country. Puccinia gtumarum (the Yellow 

 Rust) is much commoner, but, so far as is known, 

 only two spore stages occur in the life-history of this 

 species uredo- and teleuto-spores. 



There are thousands of different kinds of parasitic 

 fungi known which infest wild and cultivated plants, 

 some living exclusively on one species of host, some, 

 like black rust of wheat, living on alternate hosts, and 

 others being less exclusive in their habits. They 

 often do severe and widespread damage to important 

 crops and cause heavy losses in money. In the 

 seventies of the last century, for instance, the coffee- 

 planting industry of Ceylon was literally destroyed 

 by a fungus (Hemileia vastatrix) parasitic on the coffee 

 plant. The study of the life histories and habits of 

 these parasitic fungi forms one of the principal branches 

 of plant pathology, which has become within recent 

 years a study of great practical importance. For it 

 is only by a careful and thorough study of the life 



