130 RESEARCHES ON FUNGI 



to, and its basal ball comes to face, the incident rays. During the 

 progress of the turning movement, one can observe that the spot of 

 light gradually moves down the side of the bulb (this can be made 

 evident by applying a sheet of white paper to it) until finally it 

 takes up a perfectly symmetrical position on the cotton- wool plug 

 which, in consequence, becomes brilliantly lighted. 



(4) To show that, if the cell-sap in the subsporangial swelling 

 were replaced by air, the swelling would not act as a lens. Hold the 

 flask in the beam of light so that its basal ball faces the light, as in 

 the first experiment. The light is refracted on to the cotton- wool 

 plug. Remove the water from the flask without removing the plug. 

 Now hold the flask in the beam of light in the same position as 

 before. The light is no longer refracted on to the plug. 



The Periodicity in the Development of Pilobolus Fruit-bodies. — 

 As we have seen, in a good natural horse-dung culture of Pilobolus, 

 a crop of fruit-bodies is produced daily for several days in succession, 

 and each crop takes about 24 hours to complete its development. 

 An individual fruit-body consists : at mid-day, of a tuber (tropho- 

 cyst) or swollen plasma-filled cell which becomes the basal swelhng 

 (Fig. 24, A, p. 60) ; in the afternoon, of a basal swelhng and stipe 

 which is growing in length (B and C) ; in the late evening, i.e. 11-12 

 P.M., of a basal swelhng. a stipe, and a sporangium (D) ; 

 early next morning, i.e. about 6 a.m., of a basal swelhng, a stipe, a 

 subsporangial swelling and a sporangium (G) ; and, finally, at or 

 before noon, of a collapsed sporangiophore and a discharged 

 sporangium (H). Normally the discharge of the sporangium, which 

 marks the climax in the development of the fruit-body, takes place 

 between 10 a.m. and early afternoon. 



The naked stipe, whilst growing in length during the afternoon, is 

 positively hehotropic and always points with great precision to the 

 source of the strongest incident rays of light (Fig. 24, B and C, 

 p. 60). It contains a mass of colourless protoplasm at its apex, 

 dense red protoplasm just below its apex, and less red protoplasm 

 farther down its shaft. Doubtless, when the stipe is growing 

 toward the Ught, the rays are refracted at its apical surface, 

 converge, and concentrate themselves in the sub- apical dense red 

 protoplasm ; and it may well be that this red protoplasm is 



