Piston Mechanism of Compositae.—The flowers of this f am ily show a very efficient pollination mechanism which, with the economy of material resulting from the massing of the flowers into heads and the effective method of seed dispersal, probably ex plains the dominant position of the family.
Fig.—The fig shows a very remarkable inter relationship between an animal and plant. The flowers in the fig are unisexual and are borne in numbers together on the inside of the hollow inflorescence, which opens to the outside by a pore. The swollen and fleshy infructescence (as the inflorescence be comes) is the edible fruit ; each "seed" being the product of a single flower and in reality a fruit. The female of a small wasp (Blastophaga) enters the inflorescence and deposits eggs in special "gall flowers" incapable of setting seed. The larvae are hatched out and undergo metamorphosis. The male wasps fertilise the fe male and then die without leaving the inflorescence. The female wasps leave the gall flowers and crawling out through the pore be come dusted with pollen from the male flowers in the neighbour hood of the pore. They then enter other figs and pollinate the female flowers, which set seed. The fig and wasp are thus mutually dependent. When Smyrna figs were introduced into California it was found necessary to introduce the caprifig (non-edible fig) containing the wasp Blastophaga.
Pollination of Yucca.—This is another case of the complete interdependence of a flower and a moth. The large white flowers of this plant emit their perfume especially at night and are visited by a moth (Pronuba yuccasella). The female moth (see YUCCA MOTH) collects pollen from the anthers of the flower and kneads it into a pellet about three times the size of its head. It flies to another flower and, piercing the ovary wall with its long ovipositor, lays a few eggs between the ovules. After this it climbs down the style of the hanging flower and presses the ball of pollen into the stigma ; by this means fertilisation is ensured. Only a certain pro portion of the seeds are destroyed by the developing insects, which, when mature, eat through the fruit wall, drop to the ground and remain dormant in a cocoon until the next flowering season, when the moth emerges. This seems to be the only method of pollina tion, for in the absence of the moth the plant is said to be com pletely sterile.
The orchids show many and com
plicated adaptations to pollination by insects. A great impetus to their study was given by the publication in 1862 of Darwin's monograph on the various pollination mechanisms exhibited by this group. As is well known, in this flower there is generally only one stamen, which is two-lobed, and the pollen is in the form of two stalked masses, the pollinia which the insect carries away stuck to its head. As the insect flies away, the pollinia, if not already properly oriented, execute such a movement as brings them into position to touch the stick stigma of the next flower that is visited. There are, however, a great many variations in the details of this process. Nectar is not usually secreted by the orchid flower, hut to obtain a sweet juice the insect has to pierce a special tissue, usually that of the labellum (the posterior petal), which is often spur-like.
In some cases the plant bears more than one type of flower. The primrose (Primula vulgaris) and the cow slip (P. veris) are dimorphic, i.e., some plants have flowers with a long style bearing a knob-like stigma at the mouth of the corolla tube and the five stamens stand half-way down the tube: in others the flower has a short style with a stigma half-way down the tube while the stamens stand at the top. These two types of flower are known as "pin-eyed" or long-styled and "thrum-eyed" or short styled, respectively. From their correspondence in position, the in sect tends to transfer pollen from the thrum-eyed to the stigma of the pin-eyed and vice versa. These two types of pollination are spoken of as "legitimate," and Darwin showed that this type pro duces more seed and more vigorous progeny than "illegitimate" pollination of thrum-eyed stigma by thrum-eyed pollen or pin-eyed stigma by pin-eyed pollen. In trimorphic plants such as the loose strife (Lyt/irum Salicaria), there are three types of flower, short styled, long-styled and those with styles of intermediate length. In each type the stamens are in two groups of different lengths; in the first type the stamens are long and intermediate, in the second type they are short and intermediate, and in the third they are short and long. Eighteen possible methods of pollination are possible, six being "legitimate" and 12 "illegitimate." Legitimate unions are found to yield a larger amount of seed than illegitimate.