118 HYDROSTATIC PRESSURE. [CH. V 



epidermis of the under-surface : to get good results 

 it is necessary to use closely adjacent parts of the 

 epidermis taken from the midrib. De Vries makes 

 parallel incisions IJ or 2 mm. apart in the epidermis of 

 the midrib : the areas so marked out can • then be freed 

 by a surface-cut with a razor. The fragments of the 

 epidermis must remain in the solutions for at least an 

 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 hydrostatie pressure in 

 turgescent tissue^. 



Take an actively growing flower stalk such as that of 

 the cowslip (which must be in the budding condition): 

 mark off 100 mm. near the upper end and place the stalk 

 in 5 "/o 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 contraction 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 theTurgidity 

 "orthe tissuea 



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

 with cork-plates and the clamp is slipped behind a couple 



1 De Vries, Unterstichungen uber die mechanischen Ursachen der 

 ZelUtreckung (1877), p. 118. 



