CURRENT LITERATURE. 155 



The author thinks the extra floral nectaries of plants are safety valves 

 to prevent the rupture of the cell by excessive osmotic pressures. The floral 

 nectaries have had historically, perhaps, the same function, but have been 

 perpetuated by natural selection acting upon their usefulness to the plant in 

 securing insect pollination. 



L A. Kenoyir. 



Books. 



Agnes Arber, Water Plants, a Study of Aquatic Angiosperms 

 Camb. Univ. Press, 1920. £ 1-1 1-6* 



This is a very full and detailed treatise on water plants. Mrs. Arber is 

 well qualified for work of this kind having made many valuable investigations 

 into the structure and evolution of leaves and leaf-like organs; she has also 

 for several years made a special study of water plants. A short time 

 ago she enunciated the principle of the Law of Loss, according to which an 

 organ once lost in the process of evolution cannot be regained, but if the need 

 for such an organ again arises another must be adapted for the purpose 

 (vide p. 179 of this Journal 1 of last year) ; and the book ends with instances 

 of this principle. The book is divided into -four parts ; (1) Water Plants as a 

 biological group; (II) The vegetative and reproductive organs of water plants ; 

 (III) The physiological conditon of plant life in water ; (IV) Water plants con- 

 sidered from the phylogenetic and evolutionary stand points. 



In part I is described the life history of the families of water-plants, be- 

 ginning with the Alismaceae and ending with the marine angiosperms. The 

 last Dr- Arber considers to have been derived from fresh water plants, and not 

 to be the result of the gradual adaptation of sea-shore or other land plants to 

 marine conditions. For the special qualifications necessary to enable a 

 plant to grow in the sea — strong anchoring roots, a power of vegetating when 

 wholly submerged, hydrophilous pollination, and a tolerance of salt water— are 

 found in some at least of the fresh water members of the two families, Hydro- 

 charitaceae and Potamogeionaceae, to which the marine angiosperms belong, 

 it may be recalled that Guppy in the " Naturalist in the Pacific " argued that 

 the floating habit of seeds and fruits which is so characteristic of strand 

 plants was not evolved by adaptation from the coastal vegetation, but was 

 rather the condition necessary to enable inland species to reach the sea at 

 the mouths of rivers and become established on the shore. A very similar 

 argument is advanced by Mrs. Arber to interpret heterophylly. The ribbon- 

 like or divided leaf of the submerged part is regarded as a return to the 

 juvenile state and to be brought about by insufficient nutrition. That is, the 

 submerged form is not an adaptation to the medium, but the property of 

 having such a juvenile form and of returning to it when submerged has been 

 the condition necessary to allow of a species becoming a water-plant. 



In regard to Utricularia the author appears not to be acquainted with 

 the work of T. Ekambaram on the bladders of U. flexuosa, which actually 

 suck in the small organisms which touch the trigger hairs. (Agric. Journ. 

 India 1916.) 



Three chapters follow on the anatomy of water plants, the aerating and 

 other tissues being dealt with very fully. The author's leaning towards the 

 rigidity of inheritance, if one may call it that, is shown in an observation 

 that the differences which occur in the structure of related plants growing 



