438 INSECTS AFFECTING VEGETATION 



Some fungi which produce disease survive by their thread-like 

 parts (mycelium) in \a manner similar to the survival of Canada 

 thistle, quack-grass and the mints among troublesome weeds by their 

 visible underground stems. A good illustration of this form of sur- 

 vival is found in the case of potato rosette ; in this disease the masses 

 of mycelium (sclerotia) remain upon the surface of the potato tubers 

 and unless destroyed by treatment of the seed will be ready for im- 

 mediate attack upon the growing plants (sprouts), even before these 

 have reached the outer air and taken on a green color. 



Similar survival may occur in cultivated soils, especially where 

 the same or closely allied crops are grown in succession. Thus the 

 same fungus as that of the potato disease first named, survives in 

 greenhouse soils or in celery soils outdoors. 



RESTING FORMS AMONG FUNGI. 



The active parasitic phases of fungi necessarily coincide with 

 the activity of the host plants ; it, therefore, follows in our temperate 

 climates with alternating periods of activity and rest of growth and 

 practical somnolence, that the parasites require to be mutually 

 adapted to intermittent activity. Some spores will survive the brief 

 rest period between harvest and seed time, as in a number of the va- 

 rious grain smuts and in grain anthracnoses. Here they are found 

 simply adherent to the seed grain. 



Seed infesting parasites like the loose smut of wheat, the an- 

 thracnose of pea and bean, and a variety of other vigorous species 

 survive as resting mycelium, which remains virtually inactive so 

 long as the parasitized seed is not exposed to conditions of moisture 

 and temperature such as bring about germination. 



There are endless gradations between these instances of resting 

 mycelium and the protected fruit cases of the higher type of fungi. 

 Thus the perithecia or closed fruit bodies of the wheat scab fungus, 

 develop shortly after harvest upon the infected glumes or culms of 

 wheat, and may be observed by the unaided eye, as black bodies 

 seated upon the pink mass of the summer form. These fruit bodies 

 in this case are the kind called perithecia, which contain within them 

 spore-sacs of a nearly fixed number and each sac contains a fixed 

 number of spores of definite form for each species. A great many 

 fungi develop these housed or protected forms during the dormant 

 period, and indeed, spore development may proceed in the periods 

 of lower temperature. 



With the perithecial or sporehouse form of wheat scab (Gibber- 

 ella), the spore sacs are formed during the later summer, in our 

 latitude, and these spore sacs disappear before midwinter. For each 

 genus or species under study, peculiar time relations of development 

 may be discovered. The perithecial or spore sac (acsigerous) form 

 just described, or some comparable development of the spores under 

 a definite cover-form, is viewed as a more or less ultimate stage in the 

 development of the higher fungi the summit in the cycle of their 

 development. 



The rot of stone fruits, such as peach, plum, cherry and the 

 like, is commonly known only in its conidial development called 



