Page:EB1911 - Volume 22.djvu/18

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4
POLLINATION


b Bird-pollinated, Ornithophilae.—Humming-birds and honey suckers are agents of pollination in certain tropical plants; they visit the generally large and brightly-coloured flowers either for the honey which is secreted in considerable quantity or for the insects which have been attracted by the honey (fig. 7).
EB1911 Flower - catkin of Corylus avellana.jpg

Fig. 5.—Catkin of Male Flowers of Hazel.

EB1911 Pollination - Grass Flower.jpg

Fig. 6.—Grass Flower showing pendulous anthers and protruding hairy stigmas.

c Snail or slug-pollinated flowers, Malacophilae.—In small flowers which are crowded at the same level or in flat flowers in which the stigmas and anthers project but little, slugs or snails creeping over their surface may transfer to the stigma the pollen which clings to the slimy foot. Such a transfer has been described In Various Aroids, Rohdea japonica (Liliaceae), and other plants.
Pollination7.jpg
(From a drawing in the Botanical Gallery at the British Museum.)

Fig. 7.—Flower of Datura sanguinea visited by humming-bird Docimastes ensiferus.

d
EB1911 Pollination - anther and pollen grain of Althaea rosea.jpg

Fig. 8.—1, anther; 2, pollen grain of Hollyhock (Althaea rosea) enlarged. The pollen grain bears numerous spines, the dark spots indicate thin places in the outer wall.

Insect-pollinated, Entomophilae, a very large class characterized by sticky pollen grains, the surface of which bears spines, warts or other projections (fig. 8) which facilitate adhesion to some part of the insect's body, and a relatively small stigma with a sticky surface. The flowers have an attractive floral envelope, are scented and often contain honey or a large amount of pollen: by these means the insect is enticed to visit it. The form, colour and scent of the flower vary widely, according to the class of insect whose aid is sought, and there are also numerous devices for protecting the pollen and nectar from rain and dew or from the visits of those insects which would not serve the purpose of pollen-transference (unbidden guests).[1] The following subdivisions have been suggested
A Pollen Flowers.—These offer only pollen to their visitors, as species of anemone, poppy, rose, tulip, &c. They are simple in structure and regular in form, and the generally abundant pollen is usually freely exposed.
B Nectar Flowers.—These contain nectar and include the following groups:—
1.  Flowers with exposed nectar, readily visible and accessible to all visitors. These are very simple, open and generally regular flowers, white, greenish-yellow or yellow in colour and are chiefly visited by insects with a short proboscis, such as short-tongued wasps and flies, also beetles and more rarely bees. Examples are Umbelliferae as a family, saxifrages, holly, Acer, Rhamnus, Euonymus, Euphorbia, &c.
2.  Flowers with nectar partly concealed and visible only in bright sunshine. The generally regular flowers are completely open only in bright sunshine, closing up into cups at other times. Such are most Cruciferae, buttercups, king-cup (Caltha), Potentilla. White and yellow colours predominate and insects with a proboscis of medium length are the common pollinating agents, such as short-tongued bees.
3.  Flowers with nectar concealed by pouches, hairs, &c. Regular flowers predominate, e.g. Geranium, Cardamine pratensis, mallows, Rubus, Oxalis, Epilobium, &c., but many species show more or less well-marked median symmetry (zygomorphism) as Euphrasia, Orchis, thyme, &c., and red, blue and violet are the usual colours. Long-tongued insects such as the honey-bee are the most frequent visitors.
4.  Social flowers, whose nectar is concealed as in (3), but the flowers are grouped in heads which render them strikingly conspicuous, and several flowers can be simultaneously pollinated. Such are Compositae as a class, also Scabiosa, Armeria (sea-pink) and others.
EB1911 Pollination - pollination of Salvia pratensis.jpg
(From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.)
Fig. 9.—Pollination of Salvia pratensis.

1, Flower visited by a humble-bee, showing the projection of the curved connective bearing the anther from the helmet-shaped upper lip and the deposition of the pollen on the back of the humble-bee.

2, Older flower, with connective drawn back, and elongated style.

3, The same, when disturbed by the entrance of the probosces of the bee in the direction of the arrow; f, filament; c, connective; s, the obstructing half of the anther.

4, The staminal apparatus at rest, with connective enclosed within the upper lip.

5.  Hymenopterid flowers, which fall into the following groups: Bee-flowers proper, humble-bee flowers requiring a longer proboscis to reach the nectar, wasp-flowers such as fig-wort (Scrophularia nodosa) and ichneumon flowers such as tway-blade (Listera ovata).

The shapes and colours are extremely varied; bilaterally symmetrical forms are most frequent with red, blue or violet colours. Such are Papilionaceous flowers, Violaceae, many Labiatae, Scrophulariaceae and others. Many are highly specialized so that pollination can be effected by a few species only. Examples of more special mechanisms are illustrated by Salvia (fig. 9). The long connective of the single stamen is hinged to the short filament and has a shorter arm ending in a blunt process and a longer arm bearing a half-anther. A large bee in probing for honey comes in contact with the end of the short arm of the lever and causes the longer arm to descend and the pollen is deposited on the back of the insect (fig. 9, 1). In a later stage (fig. 9, 2) the style elongates and the forked stigma occupies the same position as the anther in fig. 9, 1.

  1. See A. Kerner, Plants and their Unbidden Guests.