REVIEWS 319 



belong to two main types, viz. : the Nothofagiis forests of the South with/ 

 often pure, stands of one or other species of Evergreen Beech, whilst the 

 Rain forest of the North exhibits a vast assemblage of mostly evergreen 

 trees, in which Agathis australis, Beilschmiedia spp., Dacrydium spp., and 

 Podocarpus spp. are prominent. But though so different in composition 

 from our own, they exhibit much the same architecture, built up of trees, 

 shrubs, herbs, climbers, and epiphytes, and the individual species mostly 

 possess dull-coloured and inconspicuous flowers. 



Each type of vegetation is made to tell its story, from each there is 

 much of interest to be learnt, and we cannot too highly commend the action 

 of the Minister of Internal Affairs in publishing this volume as a manual 

 of the New Zealand Board of Science and Art at a price within the means 

 of all. E. J. Salisbury. 



Hydration and Growth. By D. T. MacDougal, Ph.D., LL.D. [Pp. vi + 176, 

 with 52 figures and 124 tables.] (Washington : Carnegie Institution, 

 1920.) 



Increase in size, the visible component of growth, is mainly the outcome of 

 absorption and adsorption of water consequent upon the osmotic and im- 

 bibitional capacity of the cell units of the organism. As the author of these 

 pages justly observes, our knowledge of the imbibition of colloidal substances 

 is largely based on the behaviour of gelatin, which, being a nitrogenous sub- 

 stance, is comparable rather to the protoplasm of animals rich in nitrogen 

 than to that of plants in which, normally, colloidal carbohydrates prepon- 

 derate. 



Bearing these facts in mind, the author utilised the carbohydrate Agar 

 for the purpose of many of his experiments either alone or with a small 

 proportion of added protein, salts, etc., so as to simulate as far as possible 

 the conditions in a phytocolloid. 



The results show that, whereas the maximum imbibition of gelatin and 

 colloids rich in nitrogen is attained in an acid medium, that of colloids in 

 which the proportion of carbohydrates is relatively high is attained in a 

 neutral relatively salt- free condition. 



The effect of dilute monobasic amino-compounds was to increase the hydra- 

 tion capacity of pentosan colloids, although, in higher concentrations than those 

 encountered in plants, they produce the reverse effect. 



Prepared colloidal plates and plant sections, alike, showed a marked 

 decrease of imbibitional capacity in swamp-water, whilst in bog- water with 

 a higher acidity but lower proportion of salts the swelling was nearly equal 

 to that in distilled water. With colloidal plates containing a high proportion 

 of nitrogen, the swelling in the swamp-water was greater even than in distilled 

 water, and it is suggested that possibly variation, in the relative proportions 

 of carbohydrate and protein colloids, may be one means adopted by plants 

 to meet the specialised conditions of particular types of habitat. 



Not the least interesting sections are those in which the author treats 

 of the varying imbibition capacity of sections of Opuntia under different 

 conditions of growth and at different times of the day, changes ascribed to 

 the variations in acidity, protein content, the proportion of mineral salts 

 and pentosans. Experiments are described on Opuntia, Mesemhryanthemum, 

 Helianthus, and Phaseolus, from which the author concludes that the fluctua- 

 tions in growth bear a direct relation to the varying hydration capacity of 

 the growing cells. 



These pages provide a valuable addition to our knowledge of imbibition, 

 none the less important even if we feel that the author somewhat over- 

 emphasises the role of this factor in relation to growth. 



E. J. Salisbury 



