210 



sap. The present book will be cordially welcomed as It brings 

 together in one place a concise review of the literature and a 

 clear statement of the present status of one of the most difficult 

 subjects in plant physics. 



Referring to the earlier writings, which, almost as a foregone 

 conclusion, assigned the ascent of sap to "vital force," as now 

 chiefly of historic interest, the present monograph gives an 

 account "of a physical explanation of the rise of water in trees. 

 This theory rests on a knowledge of a property of liquids, which, 

 altheugh discovered in the middle of the last century, was little 

 recognized and seldom referred to in physical literature. It 

 now appears that a full appreciation of this property is essential 

 for a realization of the manner in which water is raised in plants 

 and of the meaning of the structure of trees as a mechanism for 

 lifting water." This property of liquids is cohesion. 



The first chapter, on "The nature of transpiration," supports 

 the thesis that the process of transpiration is not a purely physical 

 one, but involves the important vital activity of secretion. 

 This contention is supported by experimental evidence on the 

 transpiration of living and dead leaves, and on transpiration in 

 saturated spaces. The experiments demonstrate that the ele- 

 vating force is largely, if not wholly, confined to living leaves, 

 and that it is not evaporation but secretion. 



The next and following chapters are devoted successively, to a 

 criticism of the physical theories, a criticism of the vital theories, 

 the cohesion theory of the ascent of sap in stems (the author's 

 theory, the tensile strength of the sap in trees, estimate of the 

 tension required to raise the sap, osmotic pressures of leaf-cells, 

 the thermo-electric method of cryoscopy, methods of extracting 

 sap for cyroscopic observations, osmotic pressures in plants, 

 and the energy available for raising the sap. 



To summarize, the author claims to have demonstrated that, 

 "The transpiration stream is raised by secretory actions taking 

 place in the leaf -cells, or by evaporation and capillarity (im- 

 bibition) at their surfaces, drawing water from the trachae. 

 The state of saturation surrounding these cells determines which 

 of these agencies is effective." 



