130 HYDROSTATIC PRESSURE. [CH. V 



hour before being examined. The condition of each is 

 noted as P. completely plasmolysed, H. P. half plasmolysed, 

 or N. P. not plasmolysed. 



The solution which produces the H. P. effect is taken 

 as osmotically equivalent to the cell sap. 



(151) Estimation of the hydrostatic pressure in turgescent 

 tissue 1 . 



Take an actively growing flower stalk such as that of 

 the cowslip (Primula veris), which must be in the budding 

 condition : mark off 100 mm. near the upper end and 

 place the stalk in 5% NaCl solution. As soon as it is 

 thoroughly flaccid it should be measured again, when it 

 will be found to be shorter, owing to the elastic con- 

 traction of the cell-walls, which were previously stretched 

 by the turgescence of the cells. If it can now be 

 ascertained what force is needed to stretch the shrunken 

 stalk to its original length, we shall know what was the 

 force exerted by the turgidity of the tissues. 



The bud of the cowslip is fixed in a screw-clamp lined 

 with cork-plates and the clamp is fixed to a horizontal 

 board, so that the stalk will be stretched when the other 

 end is pulled. The basal end of the stalk may be simply 

 knotted to a piece of cord, which passes over a pulley let 

 into the board, and supports a scale-pan. A millimeter 

 scale having been arranged so that the distance between 

 the marks on the stalk can be easily read off, weights are 

 added to the scale-pan until the marks are once more 



1 De Vries, Untersuchungen iiber die mechanischen Ursachen der 

 Zellstreckung (1877), p. 118. 



