Crafts et al. —136— Water in Plants 



The preceding discussion can lead to no definite conclusions as to the 

 existence of active control of water in frost or drought resistance. The 

 explanations listed above are highly speculative ; they must await new 

 methods for substantiation ; only as the molecular structure of protoplasm 

 in the living state is investigated and described may we hope to build a 

 clear picture of the forces maintaining its structure and accounting for its 

 remarkable properties. 



Evidence from Animal Physiology: — Animal cells dififer from plant 

 cells in two distinct ways: they possess neither a semi-rigid wall, nor a 

 large central vacuole. In many ways, however, they are functionally 

 similar. 



LucKE and McCutcheon (1932) consider plant and animal cells to 

 behave as osmotic systems if experimental conditions are carefully con- 

 trolled so that injury does not result in loss of semi-permeability, and if 

 correction is made for osmotically inactive contents. For example, they 

 found normal uninjured erythrocytes to swell and shrink in hypo- and 

 hypertonic solutions to a lesser degree than would be expected of an ideal 

 osmometer, but when a correction for osmotically inactive material was 

 made approximate agreement obtained. Brooks and Brooks (1941) make 

 the same general conclusion — that only osmotic forces are acting, but point 

 out that some anomalous results are still unexplained. 



Ponder (1944) discusses three possible explanations: a) the anomalies 

 are due to varying amounts of bound water, b) the cells lose or gain solutes 

 during the experiments, c) elastic structural forces residing in the proto- 

 plasm oppose uptake of water from hypotonic solutions as well as loss of 

 water to hypertonic solutions. The last explanation was considered the 

 most plausible. 



Correction for non-solvent volume in marine invertebrate eggs did not 

 produce the expected agreement in the pressure- volume relations of the 

 cells (Leitch, 1934). For some cells the calculated amount of osmotically 

 inactive material, based on the total protein and fat content, appeared to be 

 too low ; for others it was too high. He suggested that several factors might 

 affect water exchanges ; where volume changes were below expectation 

 surface forces might prevent penetration of water. Brooks and Brooks 

 (1941, p. 75) conclude that "most eggs do not behave as perfect osmometers 

 even after allowance is made for the known amount of water and solids in 

 them." 



Differential behavior of animal cells and tissues toward water has been 

 reported in many instances. Krogh (1939), in a monograph on osmoregu- 

 latory processes in aquatic animals, points out that most animal cells are 

 in osmotic equilibrium with their bathing solutions ; when the concentra- 

 tion is modified swelling or shrinking must occur. Some cells, however, 

 are able to maintain hypo- or hypertonicity with their liquid environment. 

 Since rigid walls are absent, neither turgor nor tension is great enough to 

 be important in the attainment of a steady state with respect to water. 

 Such a state can exist only because osmotic regulatory devices requiring 

 energy are able to eliminate water or solutes as the case may be; these 

 processes involve the utilization of metabolic energy. 



Though the over-all evidence for active water regulation by the lower 

 aquatic animals is inconclusive, several cases may be mentioned that are 

 strongly suggestive. Noctiluca miliaris, a marine protozoan has a lower 

 specific gravity than sea water, differing from most other members of the 



