This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
This subclass contains those Dicotyledons in which the perianth is double, the petals separate, and the stamens either perigynous or epigynous.
This order contains one British species, the Common Spindle Tree, Euonymus Europeans. The flowers secrete honey, and are frequented by Diptera (Flies) and Hymenoptera, especially the former. They are proterandrous.
The Vine and the Virginian Creeper belong to this order; but we have only one British genus, Rhamnus,
(the Buckthorn) with two species, Rliamnns cathar ticus (the common Buckthorn and Rhamnus frangula), the Alder Buckthorn. The two species differ considerably. In Rhamnus frangula, the sepals, petals and stamens are five in number; the petals are very small. The stamens open before the stigma is fully developed, and probably even before it is capable of fertilisation. The pistil is in the centre, and insects which visit the flower for the sake of the honey necessarily touch the stamens with one side of the proboscis and the pistil with the other. They must, therefore, often convey the pollen from one flower to another. In the absence of insects, however, R. frangula is capable of self-fertilisation.
In R. catJiarticus, on the contrary, the flowers have four petals, and are dioecious; the male flowers have a rudimentary pistil, and the female flowers bear minute stamens. The individual flowers are very small, they are rendered conspicuous by being associated in clusters, while those of R. frangula are in twos or threes.
R. lanceolatus, which, however, is not an English species, has been shown by Mr. Darwin to be dioecious (Jour. Linn. Soc, v. vi., 1862, p. 95.)
This is a very extensive order, containing eighteen British genera; the Peas, Vetches, Brooms, Clovers, Furze, etc. belong to it.
It is probable that all flowers which have an irregular corolla are fertilised by insects. The advantage of the irregularity is that it compels the insects to visit the nectary in one particular manner. In the present group the result is that insects necessarily alight on a particular part of the flower, when their weight in many cases causes certain mechanical effects by which the pollen is transferred to the body of the insect, and thus carried from one flower to another. The corolla in the Leguminosae consists of five petals; an upper one, usually called the "Standard," two lateral ones, or "Wings;" and two lower ones, united at their edges into a boat-shaped organ, or "keel."
Fig 66. - Lotus corniculatus
The bases of the stamens coalesce into a hollow tube (Fig. 70 and 71 t) the inner walls of which, at their base, secrete honey in some species, though not in all. In the former, one or more of the stamens is detached, as in the Lotus (Fig. 70 b),or atrophied, so as Fig. 67.
Fig. 67. - Flower of Lotus comicnlatus seen from the side and in front.
Fig. 6. - Ditto, after removal of the standard.
Fig. 69. - Ditto, after removal of the standard and wings.
Fig. 70. - Ditto, after removal of one side of the keel.
Fig. 71. -Terminal portion of Fig. 70 more magnified.
E, entrance to the honey; d, the free stamen; c, the place where the wings lock with the keel; /', expanded ends of stamens; /, filaments of stamens; g, tip of keel; po, polen; st, stigma.
To leave a space through which bees can introduce their proboscis into the tube. In those species which do not secrete honey this is unnecessary, and the stamens are all fully developed and united.
In the Common Birdsfoot Trefoil (Lotus comicula-tits) the anthers burst and emit their pollen before the flower opens, and indeed before the petals have attained their full size. At this time the ten stamens form two groups, five of them being longer than the others; but by the time the flower opens they are all of the same length, though the five outer ones are somewhat swollen at the end; a difference which subsequently becomes still more marked. The pointed end of the keel is now filled by a mass of pollen (Fig. 70 and 71 po), while the anthers, having discharged their contents, commence to shrivel up. The free ends of the five outer stamens continue, however, to increase in size; so that, with the pollen mass, they completely fill up the cavity of the keel. When the flower opens the pistil, stamens, and pollen occupy the position shown in Figs. 70 and 71.
The five inner stamens, having discharged their pollen have become useless, shrivelled up, and lie in the broader part of the keel; the five outer ones, on the contrary (Fig. 70 e), which still have an important function to perform, lie behind the pollen mass, and keep it in its place.
Insects do not generally alight directly on the keel, but rather on the wings, which are more conveniently situated; the two, however, are relatively so arranged, that when a bee alights on the wings, she presses down the keel, which is locked with the two wings by a projection and corresponding depression, as shown in Figs. 68 and 69 c; thus a portion of the pollen and also the tip of the pistil are forced out at. the point of the keel, and against the breast of the bee, until on the removal of the pressure the elasticity of the flower causes the various organs to resume their former position; an obvious advantage, which prevents the pollen from being wasted. The union of the stamens at their base has probably reference to this, as Sprengel has suggested. From the manner in which these flowers are evidently arranged with a view to the visits of insects, we can hardly doubt but that these visits are of importance to the plant.