HISTORY OF PILOBOLUS 29 



solution. The treated haK of the cell was allowed to project into 

 the air while the untreated half was immersed in water. Under 

 these conditions the treated half excreted drops. These drops were 

 of so large a volume (62 per cent, of the cell volume when alcohol 

 vapour was employed) that it could be concluded with certainty 

 that, while the treated half of the cell had been excreting water, the 

 untreated half had been absorbing water. In this experiment, 

 however, while the absorbed water was doubtless pure, the excreted 

 water may well have been nothing more than escaped cell-sap 

 becoming more and more diluted. 



The excretion of water from the sporangiophore of Pilobolus is, 

 as we have seen, not general over the whole surface but at certain 

 definite places on the wall. These places are very small in area and 

 may be over one hundred in number. They become covered by 

 as many drops, each of which seems to issue at a point. The drops 

 do not run down over the surface of the sporangiophore because the 

 wall is covered with a thin layer of fat.^ According to Lepeschkin's 

 theory of excretion : (1) the plasma-membrane just beneath each 

 point of the cell-wall where a drop exudes must be regarded as 

 exceptionally permeable for water and dissolved substances ; and 

 (2) the driving force causing exudation is the osmotic pressure of 

 the cell-sap. 



As Lepeschkin's analyses indicate, the drops excreted by 

 Pilobolus are more watery than the cell-sap. In 1921, V. H. 

 Blackman 2 showed that " the claim of Lepeschkin that the osmotic 

 pressure of the stronger cell-contents is responsible for the exudation 

 from the cell of a weaker solution cannot be substantiated " and 

 pointed out that Pfeffer himself ^ in 1890 had withdrawn this 

 hypothesis as physically unsound. Blackman suggested other 



1 No proof has been offered that the sporangiophore is covered with a layer of 

 fat or wax. There is the possibility that the drops do not run down the sporangio- 

 phore because the surface of the latter bears numerous fins crystals of calcium oxalate. 



2 V. H. Blackman, " Osmotic Pressure, Root Pressure, and Exudation," The 

 New Phytologist, Vol. XX, 1921, .pp. 106-115, with three diagrams. 



3 W. Pfeffer proposed this hypothesis (along with others) in 1877 in his Osmotische 

 Untersuchungen, Leipzig, pp. 224-225, and repudiated it first in 1890 in his " Zur 

 Kenntnis der Plasmahaut und Vacuolen," Abk. K. Ges. d. Wiss., Leipzig, p. 302, 

 and again, in 1897, in his Pflanzenphysiologie (English edition, Oxford, 1900, 

 pp. 271-272). 



