62 



THE FUNGI WHICH CAUSE PLANT DISEASE 



When covered, the chamber in which they are borne is called a 

 perithecium, Fig. 39; when on an open disk the disk is called an 

 apothecium, Fig. 101. 



According to their length of life spores are classed as: 1. rest- 

 ing spores whose function is to tide over unfavorable conditions, 



hence the common name 

 ' '^ "winter spore," and 

 in contradistinction: 2. 

 "Summer spores" which 

 are produced in abun- 

 dance in warm weather, 

 germinate immediately, 

 and can ordinarily live 

 but a short time. 



In some species the 

 spores that are to func- 

 tion in water possess cilia, 

 and the power of motion. 

 These are zoospores or 

 swarm spores, Fig. 44. 

 At sporing time many kinds of fungi produce special structures 

 for the bearing of spores. The fungous threads interweave to 

 form a firm, or even a densely soUd, mass and constitute a false 

 parenchyma. Such are the stalks and caps of the mushrooms and 

 of the shelving toadstools, the skin of the puff ball, etc. A cross 

 section of such a structure appears much as a true parenchyma, a 

 longitudinal section shows it to be merely a mass of interwoven 

 fungous threads. 



Sexual Spores are formed by the union of sexual elements, 

 gametes. They are most conspicuous among the Oomycetes 

 where the antheridium carries the sperms into the oogonium, 

 fertilizes the obsphere and produces an oospore. Figs. 53-55. 



As a rule the sexual spores are produced toward the end of the 

 vegetative period of the fungus. The asexual spores are produced 

 eariier and for a longer period. Sexual spores are commonly 

 resting spores. 



Germination of spores. Under suitable environment mature 

 spores germinate and eventually give rise to vegetative bodies 



FiQ. 38. — Basidia of various ages. After Schenck. 



