CURRENT LITERATURE 
BOOK REVIEWS 
Ascent of sap 
Drxon’s' volume on transpiration and ascent of sap clearly puts the 
evidence in favor of the cohesion theory, along with the weak points of the 
other theories. The chapter headings give an idea of the organization and 
contents of the work: (1) nature of transpiration and the ascent of sap; (2) 
ascent of sap in stems; criticism of physical theories; (3) ascent of sap in 
stems; criticism of vital theories; (4) cohesion theory of ascent of sap in 
stems; (5) tensile strength of the sap of trees; (6) estimate of the tension 
required to raise the sap; (7) osmotic pressure of leaf cells; (8) the thermo- 
electric method of cryoscopy; (9) method of extracting sap for cryoscopic 
observations; (10) osmotic pressure in plants; (11) energy available for raising 
the sa p- 
The cohesion theory of the rise of sap assumes that the water is drawn up 
through the stem by a pull applied at the top of the water column, and that 
this pull is transmitted downward through the cohering water. The pull, at 
least in high transpiration, is due to the evaporating power of the air, but it is 
limited in amount, as experim iments indicate, by the osmotic pressure of leaf 
considerable magnitude exist in this column. Drxon has apparently answered 
the first three points adequately, and has brought some evidence for the 
fourth; while RENNER has shown the existence of oC of 10-20 atmospheres 
in the water of the stem tracheae of transpiring plan 
Drxon’s new measurements of the tensile Saari of the sap of trees as 
exhibited in capillary tubes are of great interest. The values range from 45 to 
207 atmospheres. The latter, according to Dixon, is the highest experimental 
t Drxon, H. H., Transpiration and ascent of sap in plants. 8vo. viii+216. 
Sigs. 30. London and New York: Macmillan. 1914. 
74 
