REVIEWS 669 



and anatomy, and this in turn is followed by sections dealing with physiology 

 and reproduction. 



The chief plant groups are illustrated on the type system, which portion 

 of the work occupies about a quarter of the text. The last section treats of 

 various families of flowering plants, and there is an excellent chapter on 

 plant-breeding. There is a recognition of the economic aspects of the subject 

 that one would wish to be emphasised in all elementary textbooks. 



Having regard to the length of the book, the field covered is a wide one, 

 though this has resulted in a somewhat cursory treatment of certain aspects, 

 of which morphology is perhaps the most noticeable. The mere statement 

 that the terminal buds of Ulmus are replaced by lateral ones is scarcely 

 adequate guidance in what the student usually finds a difficult conception, 

 and the treatment of rhizomes, bulbs, etc., which can be made to teach 

 important lessons, is reduced to a minimum. In describing the differentiation 

 of the cell wall, the impression created of a uniform deposition of cellulose 

 is only corrected at a much later stage, and the statement that vacuoles in 

 the cytoplasm are water-drops (p. 47), though amplified some four pages 

 farther on, is calculated to perpetuate an all too common error. 



If the method of presentation in parts leaves something to be desired, 

 the matter is written in an interesting style, with a freshness of expression 

 which is particularly pleasing. The illustrations, too, are good, and the 

 diagrams especially exhibit a quite unusual standard of conception. 



Ji. J . o. 



Water Plants : A Study of Aquatic Angiosperms. By Agnes Arber, D.Sc, 

 F.L.S. [Pp. xvi + 436, with frontispiece and 171 text figures.] 

 (Cambridge : at the University Press, 1920. Price 315. 6d. net.) 



Whilst dealing with an ecological group of plants, Mrs. Arber has employed 

 them, not for study as a plant community, but as a conveniently limited field 

 for the study of morphological and evolutionary problems. 



Like Mrs. Arber's previous publications, the presentation is characterised 

 by marked lucidity and felicity of expression, which should go far to commend 

 it even to amateur botanists. 



In the first section the chief biological features of aquatic plants are 

 exemplified by means of a selection of typical life-histories, which prepares the 

 way for the subsequent matter on the vegetative and reproductive organs. In 

 this latter the authoress upholds the view that the submerged leaf type is the 

 juvenile and more primitive one, capable of being induced, however, by con- 

 ditions of malnutrition. Such an interpretation is in harmony with the views 

 of de Candolle and Henslow regarding the nature of the monocotyledonous 

 leaf, views which have been elaborated by Mrs. Arber and which are set 

 forth on these pages. 



In considering the anatomical structure of the leaf, it may be pointed 

 out that the facts adduced with regard to the straight and wavy epidermal 

 walls of the submerged and aerial leaves of Callitriche, though not explicable 

 on the hypothesis quoted, are quite consistent, as also those of sun and shade 

 plants, with the mechanical hypothesis of Haberlandt, to which, however, 

 no reference is made. 



Stress is pertinently laid on the necessity for recognising that lack of 

 lignification in the xylem vessels is no index of their incapacity to conduct 

 water, and a useful summary is furnished of work on the transpiration 

 stream in submerged species. 



It is noted as a curious fact, " as yet unexplained," that the water pores 

 of aquatics are often highly ephemeral. This feature is by no means confined 

 to aquatics, and it may be suggested that there, as in terrestrial plants, 

 the water pores probably have as their chief function the prevention of 

 excessive pressure in the juvenile stages of leaf development. 



