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 large and interesting order contains eighteen British genera, amongst which are the Salvia, Dead Nettle, Sage, Thyme, Mint, Marjoram, Bugle, and Calamint. Most of them, if not all, produce honey at the base of the ovary.
In few flowers is the adaptation of the various parts to the visits of insects more clearly and beautifully shown than in the common white Dead Nettle (Lamium album), (Fig. 109).
The honey occupies the lower contracted portion of the tube, and is protected from the rain by the arched upper lip and by a rim of hairs. Above the narrower Tower portion the tube expands, and throws out a broad lip (Fig. III m), which serves as an alighting place for large bees, while the length of the narrow tube prevents the smaller species from obtaining access to the honey, which would be injurious to the flower, as it would remove the source of attraction for the bees, without effecting the object in view. At the base of the tube, moreover, at the point marked ca, Fig. III, there is a ring of hairs which prevent small insects from creeping down the tube and so getting at the honey. Lamium, in fact, like so many of our other wild flowers, is especially adapted for humble bees. They alight on the lower lip, which projects at the side, so as to afford them a leverage, by means of which they may press the proboscis down the tube to the honey; while, on the other hand, the arched upper lip, in its size, form, and position, is admirably adapted not only as a protection against rain, but also to prevent the anthens (Fig. a, a) and pistil (Fig. III, st) from yielding too easily to the pressure of the insect, and thus to ensure that it should press the pollen which it has brought from other flowers against the pistil.
Fig. 109. - Lamium album.
The stamens do not form a ring round the pistil, as is so usual. On the contrary, one stamen is absent or rudimentary, while the other four lie along the outer arch of the flower, on each side of the pistil. They are not of equal length, but one pair is shorter than the other; the inner pair in some species, the outer pair in others being the longest. Now, why is this? Probably, as Dr. Ogle has suggested, because if the anthers had lain side by side, the pollen would have adhered to parts of the bee's head which do not come in contact with the stigma, and would therefore have been wasted; perhaps also partly, as he suggests, because it would have been deposited on the eyes of the bees, and might have so greatly inconvenienced them as to deter them from visiting the flower. Dr. Ogle's opinion is strengthened by the fact that there are some species, as for instance the Foxglove, in which, as shown in Figs, 100 - 102, the anthers are transverse when immature, but become longitudinal as they ripen.
Fig. 110. - Flower of Lamium album.
Fig. III. - Section of ditto
But to return to the Dead Nettle. From the position of the stigma which hangs down below the anthers (Fig. III st) the bee comes in contact with the former before touching the latter, and consequently generally deposits upon the stigma pollen from another flower. The small processes (Figs. no, III m) on each side of the lower lip are the rudiments of the lateral leaves with which the ancestors of the Lamium are provided. Thus, then, we see how every part of this flower is either - like the size and shape of the arched upper lip, the relative position of the pistil and anthers, the length and narrowness of the tube, the size and position of the lower lip, the ring of hairs, and the honey - adapted to ensure the transference, by bees, of pollen from one flower to another; or, like the minute lateral points (m), an inheritance from more highly-developed organs of ancestors. If we compare Lamium with other flowers we shall see how great a saving is effected by this beautiful adaptation. The stamens are reduced to four, the stigma almost to a point; how great a contrast to the pines and their clouds of pollen, or even to such a flower as the Nymphsea, where the visits of insects are secured, but the transference of the pollen to the stigma is, so to say, accidental. Yet the fertilisation of Lamium is not less effectually secured than in either of these.
Lamium maculatum has a somewhat longer tube (15 - 17 mm.) than L. album, and only bees with a long proboscis can therefore suck it. B. terrestris, however, obtains access to it by force, and B. rayellus, according to H. Muller, uses the holes made by B. terrestris. In L. purpureum the tube is somewhat shorter.
Lamiiim amplexicaule, in addition to the normal flowers, also produces cleistogamous ones (Figs. 36, 37), which appear in the early spring and again in autumn.
In this genus it would appear, as already mentioned, that the pistil matures as early as the stamens, and that cross-fertilisation is obtained by the relative position of the stigma, which, as will be seen in the figure, hangs down slightly below the stamens, so that a bee bearing pollen on its back from a previous visit to another flower would touch the pistil and transfer to it some of this pollen, before coming in contact with the stamens.
In other species belonging to the same great group (Labiatae) cross-fertilisation is secured by the fact that the stamens come to maturity, shed their pollen, and are shrivelled up, before the stigma is mature. The genus Salvia was described by Sprengel, and more recently by Hildebrand and Ogle (Pop. Sci. Rev. July, 1869). Fig. 112 represents a young flower of Salvia officinalis in which the stamens (f) are mature, but not the stigma (p), which, moreover, from its position js untouched by bees visiting the flower, as shown in Fig. 113. The anthers, as they shed their pollen, gradually shrivel up; while on the other hand the pistil increases in length and curves downwards, until it assumes the position shown in Fig. 114 st, where, as is evident, it must come in contact with any bee visiting the flower, and would touch just that part of the back on which pollen would be deposited by a younger flower. In this manner self-fertilisation is effectually provided against. There are, however, several other points in which S. officinalis differs greatly from the species last described.