GENERAL SUMMARY 459 



the swelling gives a greater stimulus to the motor region of the stipe 

 than the spot farther away and, as a consequence, the stipe bends toward 

 the source of light which has formed the lower spot of light and away 

 from the source of hght which has formed the higher spot of light. As 

 this process continues, the lower spot of light moves toward the base of 

 the swelUng and the upper spot away from it. This results in the lower 

 spot giving a greater and greater, and the upper spot a lesser and lesser, 

 heliotropic stimulus to the stipe. Finally, the lower spot of hght comes 

 to rest directly on the perforate red protoplasmic septum at the top of 

 the stipe. Thus heliotropic equiUbrium becomes established and the 

 sporangium faces one of the two beams of light (the one that produced 

 the lower spot of light in the first instance) and one only. 



A model for illustrating the Pilobolus fruit-body in its relations with 

 light has been described. 



The diurnal periodicity in the development of the fruit-body of 

 Pilobolus enables the fruit-body to use the morning sun for directing its 

 axis toward the source of strongest light and therefore toward an open 

 space. Thereby the dispersion of the sporangia, which takes place 

 during the noon hours, is accomplished under favourable conditions. 



The second special function of the subsporangial swelling, that of 

 acting as part of a squirting mechanism for the discharge of the sporangium, 

 has been discussed. When a Pilobolus gun explodes, about one-haK of 

 the volume of the cell-sap in the stipe and subsporangial swelling is 

 shot out of the opening in the top of the swelling. The cell-wall of the 

 sweUing is highly elastic, and its most contractile part is a band, called 

 the collar, situated around what is to become the mouth of the swelling. 

 The abscission level, i.e. the level at which the swelling is destined to split 

 across, is indicated before the sporangium is discharged. As a Pilobolus 

 fruit-body explodes, a jet of cell-sap is shot out of the mouth of the 

 sweUing. The first and largest drop of this jet carries off the sporangium 

 and travels with it through the air. The other drops are smaller than 

 the first one and have shorter trajectories. 



Sometimes, when a Pilobolus gun explodes, the sporangiophore 

 breaks across, not — as normally — just below the sporangium, but across 

 the top of the stipe. When this happens, the projectile consists of a 

 sporangium, a subsporangial swelling, and a drop of cell-sap. 



By means of Barger's capillary-tube method it was found that the 

 osmotic pressure of the cell-sap of Pilobolus longipes is approximately 

 equal to 5 • 5 atmospheres. 



The efficient working of the Pilobolus gun is due to a number of 

 different factors, five of which have been specified. 



A chemical analysis of the cell-sap of Pilobolus longipes suggests that 

 the osmotic (turgor) pressure of the sap is largely due to phosphate and 

 oxalate ions, but is also due in part to potassium, sodium, chloride, and 

 sulphate ions, and in part due to some as yet unrecognised carbohydrate. 



