268 APPLIED BIOLOGY 



through some injured tissue, as a broken branch, gnawing by 

 animals, or careless pruning which leaves large surfaces 

 exposed. The reason for painting all injured surfaces of 

 trees with tar, cement, or paint is that these substances will 

 keep out mycelia. Spores are abundant in the air and may 

 at any time fall upon an injured surface, germinate, and form 

 a mycelium which penetrates between the cells of the wood. 

 It is an easy matter to keep mycelia from entering a tree 

 at an injured place, but quite impossible to check them when 

 once they have penetrated deeply into the plant. 



The " dry rot " which often attacks the foundation tim- 

 bers of houses and makes them unable to stand the strain of 

 supporting buildings is due to the mycelium of a near rela- 

 tive of common mushrooms. The remedy for " dry rot " 

 is ventilation of spaces beneath buildings and coating tim- 

 bers with tar, crude oil, or creosote. 



Some mushrooms have their mycelia penetrating the roots 

 of trees, causing decay and weakening so that heavy winds 

 uproot them. 



Puff-balls, named because they puff out a mass of spores 

 when ripe, are near relatives of the common mushrooms. 

 Their mycelia grow in rotten logs and humus (decaying 

 vegetable matter) . They contain numerous small cavities in 

 which spores are formed. Some of the puff-balls are edible 

 in the young state. One kind of puff-balls, called earth- 

 star, has its outer skin split to form a number of star-like 

 projections. 



References for pupils. In addition to those given in the text 

 above, Atkinson's " Elementary Botany," pp. 326-339 ; Marshall's 

 " Mushroom Book " (excellent illustrations) ; Coulter's " Plant 

 Structures," pp. 68-74. 



Yeast Plants 



250. The Cause of Fermentation. It is a well-known 

 fact that liquids containing sugar, such as juices of grapes, 



