Grafts etal. —118— Water In Plants 



prove that such a process, dependent on metabolic energy, actually exists, 

 and suggested that high imbibitional forces might also be involved. 



In other sap-expression experiments designed to shed more light on the 

 proposed water-secretory phenomenon, Bennet-Clark and Bexon (1940) 

 distinguished betvi^een pressure slowly applied, and pressure quickly ap- 

 plied. Leaves containing anthocyanin in their vacuoles were found to be 

 suitable material. Slow increments of pressure applied to carefully stacked 

 leaves produced a colorless sap approaching pure water in composition, up 

 to a certain point which was termed the "break." Here the yield of sap 

 per pressure increment increased, the osmotic pressure was higher, and 

 anthocyanin appeared in the juice. The fraction expressed prior to the 

 break was interpreted to consist primarily of vacuolar water, from which 

 many of the solutes had been filtered by the cellular membranes, and pos- 

 sibly some protoplasmic water. Sap appearing after the "break" was at- 

 tributed to breakdown of the protoplast. 



Sudden increments of pressure, on the other hand, released a sap con- 

 taining anthocyanin. with a higher osmotic pressure than occurred with 

 slow increments. This was interpreted to be primarily vacuolar in origin. 

 Bennet-Clark and Bexon feel, with Mason and Phillis, that the vacuo- 

 lar contents are probably liberated through fissures formed in the proto- 

 plasm without serious injury thereto. Sap from the killed residue of leaves 

 was considerably more concentrated than the "vacuolar" sap. 



Cryoscopic determinations on these various sap fractions gave some 

 interesting results, especially when compared to plasmolytic values. A 

 sample of these is shown in Table 34. 



Table 34. — Comparison of osmotic pressure values determined plasmolytic ally and 



those determined cryoscopically on various fractions of expressed leaf sap 



{data of Bennet-Clark and Bexon, 1940) : — 



Fagus Beta Parthcnocissiis Gossypium 



sylvatica vulgaris quinquifolius barbadense 



atm. 



Mean plasmolytic value . . . 30-32 



OP cytoplasmic sap 18-24 



OP vacuolar sap 5-8 



OP entire cell sap 15 



It is evident from these results that even where entire cell sap (from 

 frozen leaves) is used for comparison, the cryoscopic value amounts to only 

 40-50% of the plasmolytic value. The discrepancies are even greater with 

 respect to "vacuolar" sap. If the inference drawn here is true, that the 

 water absorbing power of the cell markedly exceeds the osmotic pressure of 

 expressed sap, then the plasmolytic method and other procedures involving 

 equilibrium between living cells and bathing solutions are to be preferred 

 in measurements of diffusion pressure deficits but are not suitable for 

 osmotic pressure determinations. 



Phillis and Mason ( 1941 ) postulate three kinds of sap which may be 

 expressed from cotton leaves : 



a) vacuolar sap, obtained by rapid application of relatively high pressure to care- 

 fully stacked living leaves, 



b) injury sap, the result of rather low pressures for long periods of time, origi- 

 nating partly in the vacuole and to some extent in the protoplasm; they agree with 

 Lepeschkin (1937) that pressure first results in vacuolar contraction, but after a time 



