SPORE DISCHARGE AND DISSEMINATION 



349 



distance above the aecia to which the spores were shot varied from 6 to 

 15 mm., being about 8 mm. for P. graminis. The exact mechanism of 

 aeciospore discharge is not known, but it probably depends largely upon 

 turgor pressure of the mature aeciospores, which are formed in chains. 

 The double wall between two aeciospores is at first flat, but near maturity 

 of the spores the osmotic pressure increases and tends to make these w^alls 

 convex. It is assumed that the adhesive force between the two walls is 

 suddenly overcome by the increased osmotic pressure, and the terminal 

 spore or group of spores is thrown outward into the air. 



Fig. 72. Discharge of the third basidiospore from basidium of Agaricus campestris, 

 following the formation of a drop of water at the tip of the sterigma. (After Duller, 

 Researches on Fungi, Vol. II, p. 12, 1922. Reproduced by permission of Longmans, 

 Roberts and Green.) 



Liberation of zoospores. In most of the aquatic Phycomycetes and in 

 some terrestrial forms which show definite aquatic affinity, zoospores are 

 the primary means of reproduction. The characteristic motility of the 

 zoospores may be more correctly considered as a means of local dissemina- 

 tion, but motility is also involved in the liberation from such large spo- 

 rangia as those of Saprolegnia and related fungi. 



INFLUENCE OF EXTERNAL CONDITIONS 



The effect of light upon the discharge of spores by Piloholus, Ascoholus, 

 Sordaria, and Sphaeroholus has been discussed briefly. Light is necessary 

 for the production of spores in a number of fungi in which it plays no 

 direct part in spore discharge. 



Favorable temperature is a prerequisite for all biological activity. 

 Its effect upon spore formation and discharge is often not clearly defined. 

 The maximum temperature permitting spore discharge is appreciably 



