THE PHYSICS OF THE ASCUS JET 243 



parts, beginning with the front end, having been ejected from 

 the ascus mouth with successively diminishing velocities, it will 

 reach a stage when the relationship of its length to its diameter 

 becomes such that surface tension must cause it to break up 

 into eight separate parts. Each tiny column of fluid, which at 

 first must connect adjacent spores, will become unstable when 

 its length exceeds three times its diameter, and upon slight 

 further elongation it will snap into two in the middle. Since 

 the eight spores ejected from an ascus, on their first appearance 

 in the beam of light, usually form a more or less vertical series 

 in which the highest spore is several millimetres above the lowest, 

 it seems to me probable that the segmentation of the ascus jet 

 takes place almost immediately after it has left the ascus 

 mouth. 



If the ascus is regarded as a mechanism for discharging a jet 

 in such a manner that the jet elongates and becomes segmented 

 by surface tension into eight separate parts, each part containing 

 a spore, then the structure of the ascus becomes more intelligible. 

 The long cylindrical form of the ascus and the size of the spores 

 are such that the spores must lie in a single row one behind the 

 other. This arrangement favours the production of a long jet in 

 which the spores are situated in a row. The oval shape of the 

 spores, and the fact that their long diameters are wider than the 

 ascus mouth, must necessitate their being ejected through the 

 ascus mouth end-wise (Fig. 79, G). Whilst the first half of a 

 spore is passing through the contractile ascus mouth, the velocity 

 of movement of the spore is probably slightly diminished ; and 

 whilst the last half is passing, the velocity is probably slightly 

 increased. This would lead to a separation of the spores in the 

 jet at the very beginning of its formation. The advantage in an 

 ascus containing a number of spores instead of one is probably 

 to be found in the fact that less energy would be required to 

 shoot up several spores from one ascus at one time than would 

 be required to shoot up the same number of spores if each were 

 contained in a separate ascus. The production of exactly eight 

 spores in each ascus rather than a few more or less may have 

 been determined in the first place by convenience in nuclear 



