SELF POLLINATION (AUTOGAMY) Self pollination is effected in various ways. In the simplest case the anthers are close to the stigmas, covering these with pollen when they open ; this occurs in a number of small annual plants, also in Narcissus, Crocus, etc. In snowdrop and other pendulous flowers the anthers form a cone around the style and the pollen falls on to the underlying stigmas, or in erect flowers the pollen may fall on to the stigmas which lie directly beneath the opening anthers (e.g., Narthecium). In very many cases the pollen is car ried to the stigma by elongation, curvature or some other move ment of the filament, the style or stigma, or some other part of the flower, or by correlated movements of two or more parts. For instance, in many flowers the filaments are first directed outwards so that self pollination is not possible, but later incline towards the stigmas and pollinate them (e.g., numerous Saxifragaceae, Cruciferae and others) ; or the style which first projects beyond the anthers, shortens later on, so that the anthers come into con tact with the stigmas (e.g., species of Cactaceae) ; or the style bends so that the stigma is brought within the range of the pollen (e.g., species of Oenofhera, Epilobium, most Malvaceae, etc.). In Mirabilis Jalapa and others the filaments and styles finally be come intertwined, so that pollen is brought in contact with the stigma. Self-pollination frequently becomes possible towards the end of the life of a flower which during its earlier stages has been capable only of cross-pollination. This is well seen in the case of the flowers of the Compositae, where the stigma lobes later bend round and come in contact with the pollen held by the brush of the style below.
anthers and the pollen tubes pierce the walls and so reach the stigmas and the ovules are fertilised. Some plants such as Salvia cleistogama produce only cleistogamous flowers.
classical compendium on flower pollination in English is P. Knuth, Handbook of Floral Pollination (Oxford, 1906, original German edition, 1898-1905). where a great mass of data is brought together. Good brief reviews.of the subject are given in M. Skene, The Biology of Flowering Plants (1924) ; Kerner and Oliver, The Natural History of Plants (1895) ; the earlier editions (e.g., the 2nd of 1904) of J. C. Willis, A Dictionary of Flowering Plants. Most text books of botany give some review of the subject. The earliest work on the subject was C. K. Sprengel's book entitled Das entdeckte Geheimnis der Natur in Ban und in der Befruchtung der Blumen (Berlin, 1793) ; Sprengel's first observation in this field was that of the presence of hairs in the throat of the corolla of a species of Geranium, which, he concluded, were for the purpose of preventing the nectar being washed out by rain; from this he was led on to consider the function of corolla markings such as the yellow centre of forget-me-not which he interpreted as honey guides to the insects. For the distinction of flower classes see H. Muller, The Fertilisation of Flowers (London, 1883). For the colour sense and olfactory sense of bees see K. von Frisch, Der Farbensinn und Formensinn der Biene (Jena, 1914) and tiber den Geruchsinn der Biene (Jena, 1919). For a review of ornithophilous flowers see F. Werth, "Kiirzer Vberblick fiber die Gesamtfrage der Ornithophilie," Bot. Jahrb. (1915).
(V. H. B.) POLLIO GAIUS 6