Page:EB1911 - Volume 22.djvu/19

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In Broom there is an explosive mechanism; the pressure of the insect visitor on the keel of the corolla causes a sudden release of the stamens and the scattering of a cloud of pollen over its body.
6.  Lepidopterid flowers, visited chiefly by Lepidoptera, which are able to reach the nectar concealed in deep, narrow tubes or spurs by means of their long slender proboscis. Such are: (a) Butterfly-flowers, usually red in colour, as Dianthus carthusianorum; (b) Moth-flowers, white or whitish, as honeysuckle (Lonicera periclymenum).
7.  Fly flowers, chiefly visited by Diptera, and including very different types:—
a Nauseous flowers, dull and yellowish and dark purple in colour and often spotted, with a smell attractive to carrion flies and dung flies, e.g. species of Saxifraga.
b Pitfall flowers such as Asarum, Aristolochia and Arum maculatum, when the insect is caught and detained until pollination is effected (fig. 10).
c Pinch-trap flowers, as in the family Asclepiadaceae, where the proboscis, claw or bristle of the insect is caught in the clip to which the pairs of pollinia are attached. Bees, wasps and larger insects serve as pollinating agents.
d Deceptive flowers such as Parnassia, where the conspicuous coronet of glistening yellow balls suggests a plentiful supply of nectar drops (fig. 11).
e Hoverfly flowers, small flowers which are beautifully coloured with radiating streaks pointing to a sharply-defined centre in which is the nectar, as in Veronica chamaedrys (fig. 12).
EB1911 Pollination - modified spadix of Arum maculatum.jpg

Fig. 10.—Spadix of Arum maculatum from which the greater part of the spathe has been cut away.

p, Pistillate, s, staminate flowers; h, sterile flowers forming a circlet of stiff hairs closing the mouth of the chamber formed by the lower part of the spathe.

EB1911 Pollination - flower of Veronica.jpg

(From Vines’s Text Book of
Botany, by permission.)

Fig. 12.—Flower of

k, Calyx.
u, u, u, The three lobes
 of the lower lip of
 the rotate corolla.
o, The upper lip.
s, s, The two stamens.
n, The stigma.
EB1911 Pollination - grass of Parnassus.jpg

Fig. 11.—Grass of Parnassus (Parnassia palustris).

1. One of the scales which form the coronet in the flower, enlarged.

Literature.—Joseph Gottlieb Kölreuter[1] (d. 1806) was the first to study the pollination of flowers and to draw attention to the necessity of insect visits in many cases; he gave a clear account of cross-pollination by insect aid. He was followed by Christian Konrad Sprengel, whose work Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen (Berlin, 1793), contains a description of floral adaptations to insect visits in nearly 500 species of plants. Sprengel came very near to appreciating the meaning of cross-pollination in the life of plants when he states that “it seems that Nature is unwilling that any flower should be fertilized by its own pollen.” In 1799 an Englishman, Thomas Andrew Knight, after experiments on the cross-fertilization of cultivated plants, formulated the conclusion that no plant fertilizes itself through many generations. Sprengel’s work, which had been almost forgotten, was taken up again by Charles Darwin, who concluded that no organic being can fertilize itself through an unlimited number of generations; but a cross with other individuals is occasionally—perhaps at very long intervals—indispensable. Darwin’s works on dimorphic flowers and the fertilization of orchids gave powerful support to this statement. The study of the fertilization, or as it is now generally called “pollination,” of flowers, was continued by Darwin and taken up by other workers, notably Friedrich Hildebrand, Federico Delpino and the brothers Fritz and Hermann Müller. Hermann Müller’s work on The Fertilization of Flowers by Insects and their Reciprocal Adaptations (1873), followed by subsequent works on the same lines, brought together a great number of observations on floral mechanisms and their relation to insect-visits. Müller also suggested a modification of the Knight-Darwin law, which had left unexplained the numerous instances of continued successful self-pollination, and restated it on these terms: “Whenever offspring resulting from crossing comes into serious conflict with offspring resulting from self-fertilization, the former is victorious. Only where there is no such struggle for existence does self-fertilization often prove satisfactory for many generations.” An increasing number of workers in this field of plant biology in England, on the Continent and in America has produced a great mass of observations, which have recently been brought together in Dr Paul Knuth’s classic work, Handbook of Flower Pollination, an English translation of which has been published (1908) by the Clarendon Press.

POLLIO, GAIUS ASINIUS (76 b.c.a.d. 5; according to some, 75 b.c.a.d. 4), Roman orator, poet and historian. In 54 he impeached unsuccessfully C. Porcius Cato, who in his tribunate (56) had acted as the tool of the triumvirs. In the civil war between Caesar and Pompey Pollio sided with Caesar, was present at the battle of Pharsalus (48), and commanded against Sextus Pompeius in Spain, where he was at the time of Caesar’s assassination. He subsequently threw in his lot with M. Antonius. In the division of the provinces, Gaul fell to Antony, who entrusted Pollio with the administration of Gallia Transpadana. In superintending the distribution of the Mantuan territory amongst the veterans, he used his influence to save from confiscation the property of the poet Virgil. In 40 he helped to arrange the peace of Brundisium by which Octavian (Augustus) and Antonius were for a time reconciled. In the same year Pollio entered upon his consulship, which had been promised him in 43. It was at this time that Virgil addressed the famous fourth eclogue to him. Next year Pollio conducted a successful campaign against the Parthini, an Illyrian people who adhered to Brutus, and celebrated a triumph on the 25th of October. The eighth eclogue of Virgil was addressed to Pollio while engaged in this campaign. From the spoils of the war he constructed the first public library at Rome, in the Atrium Libertatis, also erected by him (Pliny, Nat. hist. xxxv. 10), which he adorned with statues of the most celebrated

  1. Vorläufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden Versuchen und Beobachtungen, 3, 4, 6 (Leipzig, 1761).