carried by the wind they are caught in the cup of depression formed in the surface film round a female flower. A very large number may be so caught. When by the action of waves the female flower is drawn down slightly the staminate flowers are forced against it, standing vertically to the (inclined) walls of the depression, and if the female flower is temporarily drawn under the surface, several male flowers are carried down with a bubble of air. They are then completely turned over the stigmas, and so pollinate them. Each male flower produces about 100 grains of pollen, and since 400 ovules may be fertilised in the ovary as much as the whole pollen of several flowers must adhere to the stigmas. The author suggests that possibly his plant and Kcerner's are not of the same species. The paper is illustrated with photographs, and is a useful reinvestigation of a process, hitherto thought to be sufficiently explained. The author would like photographs and drawings of the process in other countries. Since the plant is common enough in Indian pools of water perhaps some one reading this will take the matter up.
Physiology.
J. Locb in Bot. Gaz. for January, LXVIII, pp. 25 — 50, continues his account of experiments on apogeotropism and the formation of roots in stems of Bryophyllum.
In this paper he examines the influence of leaves on the curvature of a horizontally placed stem. The stems were suspended over water in a closed jar. Very striking differences were shown in the rate and amount of geotropic curvature according to the position of the leaf, if only one is left on the stem. If the leaf is at the apical end curvature and root-formation are both much greater than when a basal leaf only is left. Curvature is due to a growth of the cortex, which is naturally in a state of tension. So that when the cortex is removed from the lower side the curvature is very slight, but if removed from the upper side it is very great. He finds that conditions which favour root-formation (e.g., a leaf left at the apical end) favour also curvature, while those, which favour shoot-formation (e.g., a leaf left at the basal end) do not. Ke suggests therefore that each leaf has a tendency to send shoot-forming substances towards the apex, and root-forming substances towards the base of the stem ; and that probably geotropic curvature is due to a specific substance (hormone) which is associated with or identical with the root-forming hormone, and that these hormones have a tendency to collect in the cortex of the lower side of a horizontal stem. The common phenomenon of a lateral branch taking the place of a decapitated stem by growing erect, (which he refers to as if only in certain Firs) would then be explained by the flowing of the shoot-forming hormonies into a lateral branch near the (decapitated) apex. "After this the mechanical advantage due to the vertical position will favour the continued flow of these substances in this branch, which thus-becomes the new apex". The theory is an interesting one and falls into line with Sach's theory of special root-forming substances. It would be of value to have experiments on these lines with plants of other genera.
