140 RESEARCHES ON FUNGI 



In a culture of Pilobolus Kleinii about twenty among several 

 hundred fruit-bodies that came up on fresh horse-dung balls dis- 

 charged their sporangia abnormally ; for, as was inferred from an 

 examination of their projectiles (Fig. 67), abscission in the spor- 

 angiophores took place not immediately under the sporangium but at 

 the base of the subsporangial swelHng or across the top of the stipe. 

 The sporangia with the subsporangial swellings attached were found 

 sticking to the under side of a glass cover situated several inches 

 above the horse dung on which the fruit-bodies had been growing. 

 Spores derived from the abnormal projectiles were sown and a new 

 generation of fruit-bodies was obtained, but none of the new fruit- 

 bodies in their mode of discharging their sporangia exhibited the 

 abnormality of their parents. 



The Osmotic Pressure of the Cell-sap of Pilobolus. — The force 

 which expands the highly elastic cell-wall of the Pilobolus gun and 

 is primarily responsible for the explosion of the gun and for the 

 discharge of the projectile is the osmotic pressure (turgor pressure) 

 of the cell-sap of the sporangiophore. It is therefore of considerable 

 interest to determine what this pressure is and how it compares 

 with osmotic pressures known to exist in the cells of higher plants. 



In an endeavour to determine the osmotic pressure of the sap of 

 Pilobolus, three methods were tried: (1) the classical plasmolytic 

 method of NageH, Pfeffer, and de Vries ; (2) extracting the sap of 

 Pilobolus and using it in an attempt to plasmolyse cells having a 

 known or determinable osmotic pressure ; and (3) Barger's capillary- 

 tube method. The freezing-point method was not employed owing 

 to the difficulty of collecting an amount of cell-sap sufficient for the 

 requirements of the Beckmann apparatus. 



(1) The plasmolytic method for determining osmotic pressures 

 was found unsuitable in its appHcation to Pilobolus : firstly, because 

 the sporangiophore is large and has a peculiar rounded form ; 

 secondly, because the sporangiophore wall is highly elastic and 

 contracts much more than the cell-walls of higher plants ; and, 

 thirdly, because there is an optical difficulty in observing the 

 primordial utricle just beginning to separate itself from the cell-wall. 



(2) Cell-sap was extracted from a number of Pilobolus fruit- 

 bodies and then small strips of a leaf of Elodea canadensis, cut with 



