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.
The general form of the flower indeed is very similar. We find again that, as generally in the Labiates, the corolla has the lower lip adapted as an alighting board for insects, while the arched upper lip covers and protects the stamens and pistils.
Fig. 112. - Salvia officinalis. Section of a young flower. Fig. 113. - Ditto, visited by a lies. Fig. 114. - Ditto, older flower.
In Salvia officinalis, however, the back of the upper lip shows an arch at the part x, and the front portion of the lip, containing the stamens, is loftier than in Lamium, and does not therefore come in contact with the back of the bee (Fig. 112). In evident correlation with this arrangement, we find a very remarkable difference in the stamens (Figs. 115-16). Two of the stamens (Fig. 112, f) are minute and rudimentary. In the other pair, the two anther cells (Fig. 115 a, a') instead of being, as usual, close together, are separated by a long connective (m). Moreover, the lower anther cells (a, a) contain very little pollen; sometimes indeed none at all. This portion of the stamen, as shown in Fig. 112, hangs down and partially stops up the mouth of the corolla tube. When, however, a bee thrusts its head into the tube in search of the honey, this part of the stamen is pushed into the arch (x), the connectives of the two large stamens revolve on their axis, and consequently the fertile anther cells (d) are brought down on to the back of the bee as shown in Fig. 113.
Fig. 115. - Stamens in their natural position.
Fig. 116. - Stamens when moved by a Bee.
In S. pratense the lower branch of the anther is comparatively short. The different species of Salvia differ indeed considerably from one another in this respect. One of them, .S. cleistogama, produces cleis-togamous flowers, as its name denotes.
Teucrium Scorodonia is very markedly proteran-drous. When the flower first opens the stigma stands behind the stamens (Fig. 117) and is not touched by the insect. Gradually, however, the stamens turn backwards, and the pistil moves forwards (Fig. 118), so that in older flowers, it stands where the stamens stood before, and in its turn comes in contact with the insect. This flower, though not conspicuous, is a favourite with insects.
Fig. 117. - Teucrium Scorodonia, in the first state.
Fig. 118. - Ditto, in the second state.
In Ajuga reptans the upper lip is very short, but the flowers stand close to one another, and the stamens and pistil of each are protected by the lower bract of the flower above. According to Delpino, Ajuga is proterandrous. The pistil is already mature when the flower opens, but then lies behind and is protected by the stamens. After a while the stamens separate a little, so that the stigma is in its turn exposed. In Ballota nigra the arrangement of the stamens and pistils is somewhat similar, and the flower is also slightly proterandrous.
In Galeobdolon luteum, the flower tube is eight mm. (but, as the upper end is dilated, practically only six mm.) in length. Though the stigmatic ends of the pistil diverge shortly after the opening of the flower, and appear to be then already mature, still they occupy a more prominent position at a later period. In this respect, therefore, it is intermediate between Lamium and Ballota.
Galeopsis tetrahit is a variable plant, and the tube varies in length in different specimens from 11 to 17 mm.; of which, however, the 4 - 6 upper millimetres are somewhat expanded. This variability is an interesting fact in relation to the theory of natural selection. The pistil, when mature, moves forward, as in the preceding species. G. ochroleuca agrees very closely with G. Tetrahit, but the tip of the pistil, instead of lying between the anthers of the two longer stamens, projects slightly beyond them. G. versicolor has a longer tube, while G. Ladanum has a somewhat shorter one; in most respects, however, they agree with G. ochroletica.
Stachys sylvatica is distinctly proterandrous, but has not lost the power of self-fertilisation. In S. palustris the tube is shorter than in S. sylvatica; the four stamens are of equal length; and when the flower opens, the anthers of the outer ones lie in front of the inner ones. When they have shed their pollen they turn outwards, thus exposing the inner ones, which in their turn shed their pollen, and then move outwards to make room for the pistil, which thus occupies the place which they had previously filled.
Betonica officinalis is also proterandrous; the pistil being comparatively short when the flower first opens, and not attaining its full length until the anthers have shed their pollen.
In Calamintha Clinopodinm the upper process of the stigma varies considerably in size. The stamens are still more remarkable in this respect, presenting variations which, as mentioned in the case of Galeopsis tetrahity are very interesting.
I have already in the introductory chapter referred to the Thyme (Thymus Serpyllitm, Figs. 32 and 33) as a type of a proterandrous flower. It is extremely rich in honey, much frequented by insects, and, according to Muller, has lost the power of self-fertilisation. Besides the ordinary flowers, which contain both stamens and pistils, there are other smaller ones, which contain a pistil only. In Italy, Delpino has observed not only these two kinds, but also a third in which the pistil is quite rudimentary. Ogle also in England has observed that in some flowers the pistil never becomes fully developed. On the contrary in Germany, Hildebrand, Ascherson, and Muller, have sought in vain for these male flowers.
This geographical differentiation, if it really exist, is very interesting.
H. Muller attempts to explain the presence of these small flowers by pointing out that where there is any variation in the size of the flowers, the smaller and less showy ones would be the last to be visited by the insects. Under these circumstances, as such flowers would be fertilised by the pollen derived from previous visits, the stamens of such smaller flowers would be useless, and would tend to become rudimentary. Further observations are, however, I think, required before this explanation can be regarded as satisfactory.
The Mint (Mentha arvensis) is also proteran-drous, and, like the Thyme, possesses, in addition to the hermaphrodite flowers, others which are smaller and merely female. Some species of the genus are dimorphous. The genus Mentha seems to be in some respects a connecting link between the typical Labiates, and the ordinary tubular form.
Origanum vulgare (the Marjoram) also has plants with large, proterandrous, bisexual flowers; and others with smaller female ones. In the secretion and position of the honey it agrees with the Thyme; but while on the one hand it is less sweet, it is, on the other, more conspicuous. These two differences nearly counterbalance one another; the flowers are consequently much visited by insects, and have also lost the power of self-fertilisation.
Nepeta glechoma (the Ground Ivy), like the three preceding genera, is proterandrous, and has small female flowers, as well as the larger hermaphrodite ones.
Prunella vulgaris also has the two kinds of individuals, but the female plants are comparatively rare. Axell says that, in the absence of insects, the larger flowers fertilise themselves, but this was not the case with those observed by Muller. If Prunella be really self-fertile this would constitute an argument against Muller's view of the origin of the small female flowers.
Lycopus Europceus is distinctly proterandrous. In this species, as in Salvia, two of the stamens are rudimentary. This is an advantage in Salvia, on account of the curious mechanical structure of the stamens. In Lycopus, the diminution is perhaps connected with the smallness of the size of the flower. Veronica, which has the smallest flowers of all the Scrophu-lariacese, has also only two stamens instead of four, or more.
The common Verbena officinalis is the only British species of this order. The calyx is five-toothed, the corolla distinctly tubular, and with five somewhat unequal lobes. The stamens are sometimes two, sometimes four, in number. It secretes honey at the base of the tube.
There are two British genera of this order, viz. Statice and Armeria. The genus Plumbago has already been referred to in the introductory chapter (ante, p. 10) as an illustration of an insect-fertilised flower, in contrast with Plantago major, which is wind-fertilised.