134 RESEARCHES ON FUNGI 



swelling, after its contraction, is filled with sap and not with air. 

 Even after the sporangium has been shot away and the sporangio- 

 phore has been thrown backwards on to the substratum, the wall 

 of the sporangiophore continues to contract, although slightly, with 

 the result that cell-sap slowly issues from the open mouth of the 

 sporangiophore and there forms a rounded globule (Fig. 62, B andC). 

 As soon as the contraction ceases, the globule, owing to evaporation, 

 begins to diminish in size (D), and soon it disappears completely. 

 As more and more of the water of the cell-sap in the contracted 

 subsporangial chamber evaporates, the sides of the chamber fall 

 inwards and become irregularly folded (Fig. 62, E, F, and G). 



The length and breadth of a subsporangial swelling of Pilobolus 

 longipes were measured (1) when the swelling was fully turgid and 

 (2) after the swelling had been made to contract symmetrically by 

 rupturing the stipe below it ; and it was found that, as a result of 

 contraction, the length of the swelling had decreased from 1-17 mm. 

 to 0-8 mm. or by approximately 30 per cent, and that the breadth 

 of the swelling had decreased from 0-875 mm. to 0-65 mm. or by 

 approximately 26 per cent. These figures indicate that the cell- 

 wall of the subsporangial swelling is highly elastic and contracts 

 very considerably when the sporangium is discharged. 



The cell-wall of the subsporangial swelling, when distended by 

 the pressure of the cell-sap, is uniformly thin right up to the 

 columella. As it contracts and drives the cell-sap away at the 

 moment when the sporangium is discharged, it thickens, but not 

 uniformly : it thickens most just around the mouth of the swelhng 

 and in a zone which stretches a short way below the mouth (c/. 

 B with A in Fig. 63). From this differential thickening of the cell- 

 wall on contracting and also from the external appearance of the 

 top of a contracted swelling (Fig. 62, B-G), it may be safely inferred 

 that the most contractile part of the wall of the swelling is that 

 which lies immediately below the sporangium and eventually forms 

 the lip or collar of its open mouth. 



The collar of the contracted subsporangial swelling is shown 

 externally in Fig. 62 and in vertical section in Fig. 63. It is covered 

 externally by numerous small crystals of calcium oxalate which, 

 when it is immersed in water, give it a dark appearance, apparently 



