Chapter VIII 



— 119 



Active Relations 



water separates in the form of small vacuoles, and it is this liquid which is believed to 

 compose a portion of injury sap, 



c) death sap, sap expressed from the killed residue after the vacuolar sap has been 

 expressed, and originating almost entirely in the protoplasm. 



Phillis and Mason, while in accord with Bennet-Clark and Bexon 

 that vacuolar sap may be expressed by rapid increments of pressure, differ 

 with reference to the nature of "injury sap." On the basis of volume of 

 liquid obtained, and of the chlorine content of successive fractions, they 

 assume the absence of filtration effects, and prefer to consider the sap a 

 mixture of protoplasmic and vacuolar solutions. 



It is not at all clear, if solutes are filtered out at low pressures, why 

 some filtration would not occur at more rapidly exerted high pressures. 

 Similarly, even if fissures are produced in the protoplasm of some cells, it 

 does not appear likely that all cells would be aft'ected in the same way. And 

 if the protoplasm yields liquid when low pressure is applied for a rela- 

 tively long period, it seems possible that some protoplasmic sap would also 

 be expressed by quick pressure. 



More accurate methods for separating the liquid present in the various 

 cellular phases are needed. Until these are found, sap from frozen and 

 thawed tissue may be preferred over that from fresh tissue for use in com- 

 parisons with plasmolytic values. Since it generally contains more solutes 

 than that from living tissue, there can be Httle claim that vacuolar sap from 

 frozen cells is diluted by filtration. Furthermore, there is less variation 

 in concentration with different pressing techniques. The fact remains 

 however that it is whole cell sap from many cells — a mixture, that is worth- 

 less in interphasal distribution studies. 



While fluid expressed from living tissue usually exhibits a significantlv 

 lower osmotic pressure than that from killed tissue, there are some plants 

 for which this is evidently not true. Examples are given by Walter 

 (1931&, p. 36), and his data are presented in Table 35. The plants rep- 

 resented are of a type which one might expect to behave in this manner. 

 Rich in colloids, they hold relatively large amounts of water by imbibitional 

 forces. The equality in osmotic pressure between saps from living and 

 dead tissue could in part be due to the liberation of colloidally bound 

 water on heating or freezing. It could also reflect the anatomy of succu- 

 lent plants, made up of large thin-walled cells rich in water. Such cells, 



Table 35. — Effect of killing tissue on the osmotic pressure of expressed sap 



(data of Walter, \93lb) : — 



