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
