REVIEWS 721 



tension applied to the upper end of the water columns, which will be able to raise 

 the transpiration stream in a tree, must equal the pressure produced by a head 

 of water twice the height of the tree— that is, in a tree 100 metres high a tension 

 of 20 atmospheres— and the cohesion or tensile strength of sap, amounting as it 

 does to at least 200 atmospheres, is obviously in no way taxed by this tension. 

 Since the transpiring cells of the leaf normally remain turgid during transpiration, 

 the osmotic pressure keeping them distended must on this theory correspond in 

 magnitude to the tensions necessary to raise the sap ; and this is invariably the 

 case, the pressures developed being indeed far in excess of those demanded by 

 transpiration. Briefly, on this theory the flow of water up the highest tree is to 

 be regarded as due to the evaporation and condensation produced by the 

 difference between the vapour pressure in the soil spaces and that obtaining 

 around the leaves, the column of tensile water flowing under the action of this 

 difference from end to end of the plant. 



Many of the experimental methods and results given in this work throw light 

 upon other problems in plant physiology than that with which the author is 

 particularly concerned ; in this connexion particular mention should perhaps be 

 made of the methods devised for extracting cell-sap and determining its osmotic 

 pressure, and for investigating the conditions under which water flows in the 

 vessels of the wood. 



F. Cavers. 



Cocoa. By C. J. J. van Hall. [Pp. xvi + 515, with 140 Illustrations and 1 Map.] 

 (London : Macmillan & Co., 1914. Price 14J. net.) 



Dr. van Hall's position as Director of the Institute for Plant Diseases and 

 Cultures at Buitenzorg and his long experience in tropical agriculture would alone 

 give weight to his sound and emphatically expressed opinions regarding the 

 methods of cultivation of any useful plant grown in the tropics. He has, however, 

 had unusual opportunities for mastering the subject with which he deals, having 

 taken a large share in developing the cocoa plantations in Java, where the decay 

 of the coffee-culture about 1880, owing to the ravages of the leaf-disease fungus 

 Hemileia, was the direct cause of the starting of cocoa-growing. Cocoa has been 

 extensively planted in the old coffee fields, but the cocoa in its turn was badly 

 attacked by two insect pests, with the result that the crop was much reduced. The 

 planters in Java have had to resort to many expedients, and have been successful 

 in overcoming the difficulties which threatened to wipe out cocoa-culture there as 

 completely as coffee -culture was wiped out in Ceylon. 



The present work is based on the author's intimate knowledge of cocoa-culture 

 in the East ; but he has incorporated in it so much information about the growing 

 of this valuable crop in other lands that its range is monographic, if not encyclo- 

 paedic, in completeness. Following upon introductory chapters dealing with the 

 history of the cocoa industry, geographical distribution, climatic conditions, soils, 

 chemistry of cocoa, botanical characters of the cocoa plant and its varieties, the 

 cultivation of cocoa, fermentation and drying, and diseases, the longest chapter of 

 the book is devoted to a valuable survey of the cocoa-growing countries. In the 

 case of each country passed under review, the author gives not merely statistics, 

 but interesting data regarding climate, soils, methods of culture and curing, diseases 

 and enemies, labour conditions, etc. 



The enormous increase in the world's production and consumption of cocoa is 

 strikingly shown in a series of tables. The most remarkable increase in production 

 is shown by the Gold Coast, where the output rose from just under thirteen to 



