BOOK REVIEWS. 



595 



cells at once pouring it out into them ? The fact is recorded by Sachs 

 that the more external the tissues of a stem are the greater is their 

 tension, while the pith is under the greatest contraction, so that 

 when the zones are freed by vertical sections the superficial tissues 

 contract while the pith expands ; hence these would seem to be just 

 the conditions for the water-bearing cells to discharge their supply 

 into the tracheae when transpiration is in excess. 



With regard to the question : What causes the sap to rise above 

 the height to which root-pressure can carry it ? the explanation by 

 the author is given in the following words : " Raising the Sap by 

 Evaporation. — Under certain conditions, when evaporation from the 

 transpiring cells removes water faster than their secretory powers 

 can provide it, the menisci formed in the substance of their walls 

 must support the tensile columns of water in the plant. Evaporation 

 from these menisci must provide the traction to raise the water. The 

 tension is transmitted downwards through the roots to the absorbing 

 cells. In these cells the actions which occur must be the converse 

 of those occurring in the mesophyll. At the root the entry of water 

 depends on the gradient of pressure on passing from the outside of 

 the root to the inside of the tracheae. The fall of pressure due to the 

 tension in the water is continuous all the way up the stem to the leaf. 

 Thus we may regard the flow of water up the highest tree as due to the 

 evaporation and condensation produced by the difference between 

 the vapour pressure in the soil spaces and that obtaining round the 

 leaves. The column of tensile water flows, under the action of this 

 difference, from end to end of the plant." (Pp. 203-4.) 



" Gardening for Beginners." By E. T. Cook. 8vo. 650 pp. 

 ("Country Life." London, 1914.) 12s. 6d. net. 



We have reviewed this excellent work before, and the fact that 

 it has now run into its sixth edition speaks for itself of its usefulness 

 and popularity. Mr. F. W. Harvey has added chapters on the Heath 

 Garden, the Wall Garden, the Bog Garden, Replanting or Renovating 

 Borders, &c., as well as a large number of new illustrations, amounting 

 to about 250, with plans, diagrams, and coloured plates, thus dealing 

 with every form of gardening, and making the book a treasury of 

 information, given in a pleasing style, and so that the whole is easily 

 understood. 



" The Garden under Glass." By William F. Rowles. 8vo. 

 368 pp. (Richards, London, 1914.) 65. net. 



However capable a man may be, it is quite impossible to deal 

 fully with such a vast subject as gardening under glass in a volume 

 of this size, but as it is written for amateurs the leading points of 

 culture of fruit, flowers, foliage plants, bedding plants, propagation, 

 forcing &c., the construction of glass erections, and the principal 

 plant pests and diseases are dealt with as fully as space will permit. 

 It is essentially an amateur's book, is well written, boldly printed, and 

 contains numerous illustrations and diagrams. 



