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THE FERTILISATION OF CERTAIN PLANTS
(DIDYNAMIA).

BY WILLIAM OGLE, M.D. Oxon, F.R.C.P.
LECTURER ON PHYSIOLOGY AT ST. GEORGE'S HOSPITAL.

[PLATE LVI.]

IN

the July Number of this Review I described the fertilisation of Salvia and of some other flowers.* I showed how in each case arrangements existed either entirely to prevent selffecundation, or, at any rate, to render it of rare occurrence. I wish now to show that the same fact is observable in all didynamia, and that the ordinary didynamous structure is indeed nothing more than a contrivance to facilitate intercrossing, and finds in its adaptation to this object its raison d'être.

I will begin with a flower abundant enough in our fields, the common red rattle (Pedicularis sylv.). This has a labiate corolla, and, as is the general rule in this form of irregularity, the tube furnishes a sweet secretion to attract insects, while the lower lip forms for them a convenient landing-place. The upper lip or hood is remarkably flattened, as though the flower had been pressed in a book. The upper end of this hood is closed for some little distance in front completely, and the stigma projects from it just at the point where the complete closure ceases. Below this again, from the stigma down to the projecting point, marked a in the illustration (fig. 1), the hood is practically closed by the close approximation of the two sides, so that the stigma is effectually shut off both above and below from the interior of the hood. Below the projecting point the hood is open in front, but the fissure is very narrow because of the flattening already mentioned. The style rising from the two-celled ovary runs at the back of the corolla in the closest contact with it, following its contour exactly, and thus the curved outline of the closed part of the hood causes the style to bend round, and brings the stigma into the position it occupies. The stamens are four in number, two rather longer

viz. Mallow, Lopezia, Larkspur.

than the others. Their anthers are lodged in the closed part of the hood, filling completely all the vacant space. They are slightly attached by their external surface to the walls of the hood, while their dehiscence is on their internal surface; so that, if you split open a flower longitudinally, as in fig. 1, you expose the interior of the anther cells on one side to view. The dehiscing surface, therefore, of the anthers of the right side is face to face with the dehiscing surface of the anthers of the left. Now the edges of the open anther cells on one side exactly correspond with the edges of the open anther cells on the other, just as the edges of one valve of an oyster exactly correspond with the edges of the other. It will thus be seen that the pollen grains of the opposite anther cells lie in a common cavity, and so long as the opposing edges of the cells are held in close contact, the pollen is kept securely imprisoned. How is this holding together brought about? As to the upper anthers there is no difficulty, for the part of the hood in which they lie is so excessively flattened as necessarily to press them together. The two lower anthers are in rather a wider space, and might perhaps retreat from each other, and let the pollen escape, were it not for a number of stout hairs underneath them, which by their elastic pressure prevent the separation. These hairs spring from the filament of the upper anther on either side, which passes underneath the lower anther; and it is to be noted that no such hairs grow on the filaments of the lower anthers, where their presence would serve no useful end. The four anthers with the style so completely fill up the closed part of the hood that all is held tightly packed, and it is next to impossible for the parts to get disadjusted.

We have now to consider what is the use of this exquisite workmanship. It is very simple. The flower is visited by large humble bees, which are attracted by the nectar of the tube. As the bee approaches the mouth of the tube, it strikes the projecting stigma with its head, and then settles on the lower lip. It cannot now reach the nectary without inserting its head into the fissure of the hood; but this, owing to the flattening already described, is so narrow that the broad head dilates the hood, and forces the two sides more widely asunder. The necessary result of this can be foreseen. The anthers are attached, as I have said, externally to the inner surface of the hood; consequently, the widening of the hood draws the opposite anther cells apart. A little fissure is formed between the edges, which were before in close approximation, and through this the dusty pollen falls in a little shower on the back of the bee's head. With this adhering to it the bee flies off, and, striking the stigma of some other flower with its pollen-daubed head, fertilises it. I am unable to assert positively that the bee

in its exit from the flower does not touch the stigma, and so fertilise the flower with its own pollen. The bee is so large comparatively, and so abrupt in its departure, that though I watched repeatedly, I was unable positively to assure myself on this point. But it appeared to me that it did not, and I fancy that the projecting point below the stigma serves as a safeguard against this (a, fig. 1). At any rate, it is easy to convince oneself by watching that not all the pollen is left on the stigma of the same flower, but that much is carried off to other blossoms. Moreover, when the bee leaves a flower, the stigma is already occupied with the foreign pollen which the insect left there in entering, and there is, consequently, little or no exposure of viscid surface to which the fresh pollen may adhere.

The structure of yellow cow wheat (Melampyrum) (fig. 2) so closely resembles that of red rattle that I need not describe it in detail. Here also the opposite anthers form one common receptacle, which is kept closed till a bee opens it by dilating the hood. There are some small differences to suit the arrangement to the somewhat different form and position of the flower, which is set horizontally, and not erect as in red rattle, but the main features are the same.* One little point is, however, worth noticing, and that is that the narrow hood fissure through which the insect must pass its head is made narrower still by the filaments of the front pair of stamens. The insect's head passes between these, pushing them asunder, and thus the dilating force acts still more directly on the opposing anthers than is the case with the other flower.

The cow wheat is fertilised by the small buff-bodied humble bee. The large humble bees also visit the flower, but apparently the fissure is too small for their heads, for they adopt the common treacherous habit of bees when they are unable to get in at the mouth, and make a hole in the tube just above the nectary, and so reach the secretion. The very small calyx admits of this robbery, whereas in red rattle the large leafy calyx acts as a safeguard.

What practised thieves these large bees are, is plain from the fact that, on gathering 100 open blossoms, I found the hole in 96, only 4 being intact. It occurred to me that I might use this habit of the bees to throw light on a point of some little interest, the period, namely, at which a flower begins to secrete its nectar. If there be nectar in the tube before the flower opens,

In order that the pollen may escape and fall upon the bee, the fissure between the opposing anther cells must occur at their lowest part. This, in the erect Pedicularis, is on the side turned towards the tube; in the horizontal cow wheat, on the side turned towards the inferior lip. In each case the fissure occurs where it is required.

then these marauding bees would not wait for that occurrence, but would make holes in the buds. I therefore gathered 100 flower buds, as nearly open as might be, in the same place and on the same day as had furnished the 100 open blossoms. In 89 of them there was no hole, in 11 only was one present. It seems a fair inference from this, that nectar is not secreted until the flower is on the point of opening. It is plain how accordant this is with the view I have taken of the use of nectar; namely, that it attracts insects to enter by the mouth of the flower. Among the figworts there are other species nearly allied to the two I have described-bartsia, for instance, and yellow rattle (Rhinanthus)-in which there is a very similar arrangement of the anthers and a similar method of fertilisation.

The coherence of the anthers, and the mechanism for fecundation in the species as yet mentioned, may be called exceptional. In most didynamia there is a simpler arrangement, and this must now be considered.

The flower which I shall take as an example-the woodsage, or Teucrium Scorodonia-though not very attractive in appearance, presents some points of interest. Figs. 3, 4 will give a sufficient notion of its shape. It will there be seen that the flower differs from most of its tribe in having no hood formed by the upper lip. The stamen and pistil are, however, protected while the flower is in bud by the lower lip, which is so bent back as to cover them in and shut out wind and rain, while it also prevents the premature visits of importunate insects. So soon as the anthers are ripe, this lip turns over and becomes the landingplace of insects. The stigma is at this time immature, and lies behind the stamens out of reach. The stamens, on the other hand, incline forwards (fig. 3), so that the ripe anthers, which dehisce in front, are in such a position that the bees, which visit the flower in abundance, cannot fail to rub the back of their heads against the pollen. So soon as the stigma is ripe, the stamens, which have now shed all or most of their pollen, retire backwards, bending over the upper lip; while the style comes forwards, and occupies the position which before was held by the anthers (fig. 4). It is plain that this arrangement brings about the fertilisation of the older by the younger flowers, an occurrence of which we have numerous examples.

Let me now notice another point. The stamens in this flower are of unequal length. Two are long, two short, or, in botanical language, the flower is didynamous. Can any use be ascribed to this arrangement? I think there can. Were all the stamens of equal length, the hinder pair would be impeded by the front pair, and the pollen of the former would be wasted on the posterior surface of the latter, and never come into contact at all with the bee. This evil is avoided by their longitudinal

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