222 TEXT-BOOK OF FUNGI 



known as causing a disease on the plant in question, were 

 obtained from the seeds. 



The theory that fungus spores may be carried by air 

 currents for long distances, even from Europe to Australia, 

 cannot be entertained. 



Now, in the case of seeds, legislation has played little or 

 no part, yet it is almost certain that many of the most 

 destructive diseases have been introduced into new areas 

 through the spores of the fungus adhering to seed. 



A second source of dispersion of disease, which in these 

 days of rapid transit is easily accomplished, is where the 

 mycelium of a parasitic fungus hybernates in the vegetative 

 portion of a plant that is used for reproduction in place of 

 seed. Tubers and bulbs may be cited as examples. 



It is well known that when potatoes are attacked by 

 ' potato blight ' (Phytophthora infos tans\ the tubers are 

 usually more or less diseased, the external indication of 

 which is the presence of brown stains in the flesh of the 

 tuber. If a browned portion of a tuber is examined under 

 the microscope, the living mycelium of Phytophthora can 

 always be found ; in fact the mycelium of the fungus is the 

 definite cause of the browning. The point to bear in mind 

 is the fact that if such a diseased tuber is planted, the 

 mycelium present in its substance grows into the young 

 sprouts, follows up the inside of the haulm, and finally 

 appears as ' potato blight ' on the leaves, if conditions are 

 favourable for its development. It may be argued that 

 obviously diseased potatoes would not be planted ; perhaps 

 not, but potatoes often contain the mycelium of Phytoph- 

 thora when there is no evidence in the way of discoloured 

 blotches on the surface, and but scanty naked-eye evidence 

 when cut. Nevertheless the comparatively small amount 



