22 RESEARCHES ON FUNGI 



breaks across transversely into two or three pieces. Each piece 

 encloses a single spore and appears as a short-cylindrical, wrinkled, 

 amber-coloured structure (F, g). As a sporangiophore collapses, 

 certain portions of it do not decrease appreciably in diameter 

 and these become the nodular thickenings so characteristic of the 

 species (E, e). 



The Excretion of Drops by the Sporangiophore and its Cause. — 

 Everyone who has studied Pilobolus has had occasion to notice the 

 numerous drops of clear hquid which are excreted from the sporan- 

 giophore (Fig. 12). 



Wilson,^ in 1881, working in Pfeffer's laboratory, observed : 

 (1) that, if sporangiophores of Pilobolus crystallinus are allowed to 

 stand for a short time in a dry atmosphere, the drops vanish and 

 in the place of each there can be seen a group of radiating crystals, ^ 

 visible to the naked eye ; (2) that, if the same plants are then placed 

 in a moist atmosphere, many of the drops reappear and precisely 

 in those places where the crystals were ; (3) that, if the sporangio- 

 phores are very carefully washed with the most delicate brush and 

 distilled water and are then placed in moist air, the drops, as a rule, 

 do not quickly reapppear and often do not reappear at all ; and 

 (4) that, if the sporangiophore is washed and afterwards minute 

 particles of sugar are placed upon its wall, large drops of water 

 quickly form about the sugar particles. Wilson came to the con- 

 clusion that the formation of the drops is due, not to the internal 

 pressure of the cell-sap, but, as in nectaries, to the osmotic action 

 of substances which are present on the surface of the cell-wall. 



In 1897 Pfeffer,^ in a discussion of the excretion of water from 

 uninjured plants, remarked: "Bearing in mind all these possi- 

 bilities it is impossible to say whether the drops which appear upon 



^ W. P. Wilson, " The Cause of the Excretion of Water on the Surface of 

 Nectaries," Unters. a. d. hot. Inst, zu Tubingen, 1881, p. 15. 



2 Each drop contains a colloidal gelatinous substance {vide infra, Figs. 42 and 43, 

 pp. 86 and 87) and, when the drop dries up, this substance contracts so as to form 

 a tiny, often angular, lump of solid matter. I have never seen the " radiating 

 crystals " of which Wilson speaks nor, apparently, did he realise that a drop contains 

 colloidal matter. It seems probable that he mistook angular lumps for groups of 

 crystals. 



3 W. Pfeffer, Pflanzenphijsiolwfie, 1897, English edition, Oxford, Vol. I, 1900, 

 p. 275. 



