HELIOTROPISM OF ASCI IN DISCOMYCETES 291 



" Puffing is probably not due (as de Bary supposed) to the mere 

 withdrawal of water from the asci. Solutions of grape sugar, 

 glycerine, sodium chloride, and potassium nitrate, which merely 

 withdraw water from the ripe asci of Aleuria vesiculosa, do not cause 

 their explosion. On the other hand, solutions of many poisonous 

 substances, e.g. iodine, mercuric chloride, silver nitrate, acetic acid, 

 and alcohol, give rise to marked puffing. Two alkahes — sodium 

 hydroxide and sodium carbonate — kill the asci without causing 

 them to discharge their contents. It seems probable that puffing 

 is caused by a stimulus given to the protoplasm in contact with the 

 ascus lid.^ 



" The physics of the ascus jet in Aleuria vesiculosa has been 

 discussed. It seems probable that the separation of the eight 

 spores of an ascus during their upward flight into the air is due 

 to considerable differences in the initial velocities given to the 

 individual spores upon their discharge. Surface tension probably 

 plays but a minor part in breaking up the ascus jet. When an ascus 

 is regarded as an apparatus for squirting out a jet in such a manner 

 that the jet immediately breaks up into eight parts so that each 

 part contains a spore, its structure becomes more inteUigible. 



" The eight spores in an ascus of Aleuria vesiculosa are loosely 

 attached together, and the row of spores is anchored to the ascus 

 hd by a special protoplasmic bridle. De Bary's hypothesis of 

 currents is unnecessary in accounting for the means by which the 

 spores are caused to take up their characteristic positions in the 

 ascus." 



Whilst making the investigation just summarised, I observed 

 that some of the asci of the fruit-bodies of Aleuria vesiculosa were 

 curved whilst some were straight ; but I did not then know the 

 cause of the difference and, in my illustrations (Vol. I, Fig. 79, 

 p. 241), represented only straight asci, such as are actually found 

 at the centre of the base of a well-formed fruit-body. The curvature 

 of the curved asci, as we shall see in the next Section, is not a mere 

 accident attributable to the kinetics of sUcing the living fruit-bodies 

 but is due to the developing asci bending heUotropically. 



^ For a discussion of the cause of the bursting of asci when a fruit-body puffs 

 under normal conditions vide supra, pp. 230-232. 



