2 DISSEMINATION OF PARASITIC FUNGI 



or the like. Nevertheless several instances of extensive spread in this manner 

 are known — 



Phytophihom wfeskms (the cause of the potato blight) has reached 

 practically all parts of the world, in which potatoes are grown, 

 as an internal mycelium within the potato tuber. 



The blister-rust of white pines {Peridermium Strobi) has been introduced 

 into the United States as an internal mycelium in the stems of 

 seedling pines obtained in vast numbers from Europe. 



The mildew of Euonymia jafonicits survives the winter in Europe as 

 an external mycelium on the leaves, and doubtless came from Japan 

 or elsewhere in this condition, when first introduced into Europe 

 some 15 years ago. 



The spores are, however, the usual means of spread. For this they are 

 much better fitted than the mycelium, being often long-lived bodies, able to 

 stand drying or extiemes of heat or cold. The spores of bunt of wheat have 

 been germinated after 8 years, and have been kept in a solid lump of ice for 

 3 months in Canada, without the slightest injury, though the temperature 

 fell to 20'' F. more than once. They are more susceptible to heat : bunt spores 

 can be killed by immersing them in hot water at about 130° F. for 10 minutes 

 and yellow rust of wheat in India by 5 minutes in water at about 120°F. ; the 

 dry spores can stand considerably higher temperatures. So far from being 

 injured by drying they can usually be best preserved in a dry condition and 

 it has often been noticed that thorough drying favours their subsequent germin- 

 ation.^ Owing to their small size also, often appi oxima+.ing to that of vary 

 fine dust, they aie readily blown about, and as they are usually detached as 

 soon as ripe, air currents play the most important part in their dissemination. 

 Many spores are provided with arrangements for sticking them to objects with 

 which they come into contact. Thus many rusts and smuts have fine spines 

 on the spore wall, the anthracnoses liberate their spores with a mucilaginous 

 substance which is adhesive on drying, the cilia of Pestalozzia serve the same 

 purpose, and so on. Not only, therefore, are spores cast into the air in vast 

 numbers,^ but their small size and tendency to adhere cause them to attach 



1 There are some exceptions to this, as the ' summer spores ' (sporangia) of the downj' 

 mildews, which are often unable to survive even after 24 hours' thorough drying. 



* It has been estimated that a small spore horn of the chestnut bark disease fungus 

 (Endothia parasitica) may contain 115 million spores, a bean pod affected with the bean anthrac- 

 nose (Glomerella Lindemuthianv.m) may produce from 500 to 1,000 million spores during the sea 

 son, while Lycoperdon bovista, one of the (harmless) puff balls, may contain 7,000,000,000.000, 

 and the common edible mushroom can shed spores at the rate of 40 millions per hour. 



