Chapter VIII — 135 — Active Relations 



between the degree of hydrophily developed by hardening cells and the 

 amount of "bound" water in the protoplasm is in question. Levitt con- 

 cludes from study of the literature that the reduced rate of water loss 

 sometimes observed in hardy plants can usually be explained on the basis 

 of some morphological character, rather than on bound water. It is well to 

 emphasize again (Qiapter VI) that knowledge concerning the water hold- 

 ing mechanisms in protoplasm is limited. While the evidence for water 

 binding in dead systems such as expressed saps may be questioned, in proto- 

 plasm we are dealing with a system possessing active forces, a system that 

 is able to perform osmotic work with regard to solutes, and probably also to 

 water. 



Several observations indicate that protoplasmic volume increases dur- 

 ing hardening. Levitt and Scarth (1936) found this to be true for 

 cortical cells of Catalpa, where the protoplasm occupied about 50 per cent 

 of cell volume. In this amount it cannot help but exert a marked influence 

 on water control. The work of Kessler and Ruhland (1938) indicated 

 a greater volume of protoplasm in hardened tissues. The fact that smaller 

 cells, with greater relative volume of protoplasm, are the more hardy, is of 

 significance. On the other hand, Scarth (1941), on the basis of an ob- 

 served lower refractive index and a greater permeability of the ectoplasm, 

 has suggested that the increased hydration on hardening may be limited 

 to this outer protoplasmic layer. 



Levitt (1941) finds it difficult to reconcile the decreased water con- 

 tent of hardy plants with increased hydration of the protoplasm ; he sug- 

 gests that the walls may dry. A better explanation might picture the 

 protoplasm as varying its hydrophily allowing reduction of water in 

 vacuole and walls at the same time that its own water content increases. 

 Such a process might involve unfolding of protein chains or opening of 

 ring structures to present more points for hydration, the more active struc- 

 ture being maintained by metabolic energy. 



Another explanation for increased hydrophily is possible. It has been 

 reported that the respiration of hardened cells is of lower intensity than 

 in non-hardened cells {see review by Levitt, 1941, p. 124). This might 

 suggest that respiratory energy is utilized in masking hydration points, 

 and that a low rate would permit greater hydration. There seems to be 

 no agreement however on the relation between respiratory rate and degree 

 of hardiness. Regardless of the type of bonding involved the retention of 

 water, as a part of protoplasmic structure, must be influenced by metabolic 

 forces. The possibility must not be lost sight of that water may be held 

 in living protoplasm by forces so intimately related to the living state 

 that they cannot be studied in dead or injured cells, or in extracts. 



In many respects drought resistance is similar to frost resistance. It 

 has seemed logical to many investigators that a considerable amount of the 

 water of true xerophytes must be bound to colloids. In spite of the nu- 

 merous criticisms {e.g., Weismann, 1938), methods for estimating bound 

 water are still being employed in studies of drought resistance (Whitman, 

 1941, MiGAHiD, 1945). The latter author has aptly stressed the point 

 that where sap is expressed, bound water can be determined only when 

 the press cake residue is also taken into account. He concludes that the 

 bound water content of xerophytes is significantly higher than that of 

 mesophytes. In several instances values in the order of 17 per cent of 

 total leaf water was reported as bound. 



