30 BOOKS AND CURRENT LITERATURE 



dermal cells does not determine root hair production. Corn roots 

 were used, and measurements of both smooth and haired epidermal 

 cells were compared with those of adjacent cortical cells. The initial 

 swelling which results in a root hair protuberance is not due to the 

 position of the nucleus, but to the less resistant nature of part of the 

 outer wall as compared with the remainder of this wall, and to the 

 excess of internal pressure of the cell over the opposing pressm'e on the 

 external wall. The internal pressure was found by measurement with 

 sucrose solution to exceed the external pressure by about fom' atmos- 

 pheres. Reduced moisture content of the surromiding air decreased 

 the extensibility of the wall thus retarding root hair development. In- 

 vestigation of the structm'e and composition of the walls of various 

 root hairs (alfalfa, amaranthus, barley, corn, cabbage, daucus, morn- 

 ing-glory, pea, tobacco, tradescantia, and others) showed in every case 

 excepting corn an outer layer of calcium pectate and an inner one of 

 cellulose. Some of the root hairs investigated had in addition to 

 the two layers a callose layer at the tip. Corn root hairs had only 

 a cellulose layer. The pectin layer is closely related to the absorp- 

 tion and retention of water by the hair, and the cellulose layer strength- 

 ens the wall. The formation of cellulose mucilage in corn, and of 

 pectin mucilage in the other root hairs investigated, makes possible the 

 close relation that exists between root hair and soil particles. — J. G. 

 Brow^n. 



Tropical Agriculture.— Our increasing interest in tropical coun- 

 tries and their development will give a useful place to Wilcox's recent 

 book on the agriculture of the tropics.' A large and well selected 

 series of tropical crops is described with an amount of detail depend- 

 ing upon the commercial importance of each of them. Sugar, rubber, 

 sisal, coffee, the banana, and the coconut, for example, are given an 

 extended treatment covering the botanical description of the plants 

 which yield these products, their propagation, cultivation, harvesting, 

 yield, and preparation for use. 



The book is sufficiently elementarj^ to enlighten those who still 

 believe that bananas hang from the tree as they hang in the fruit shop, 

 and it- is so full of well arranged material that it is sure to contain a 

 great deal of fresh information for every reader in the temperate zone. 

 No partiality has been shown to any portion of the tropics in the 



1 Wilcox, E. v., Tropical Agriculture, pp. 373, pis. 33. New York, D. Appleton 

 and Company, 1916. ($2,50.) 



