irregular. The flowers are almost horizontal, closely compacted against the axis, and consequently not readily visited on any side except from the front. The style and stamens curve upwards, so that "the smaller bees and flies thrust the head or proboscis from the front into the flower, and the upward curvature of the style and stamens causes the insect to enter by the lower half of the flower, and so to get dusted with pollen from above.” * Müller also notices, about this flower, that "the style, which even in the bud overtops the stamens, grows very markedly after the flower opens, as the flower itself does. As a rule, it attains its full length only after the anthers have completely shed their pollen, at which time also the four-lobed stigma reaches its full development." He gives five figures of Saxifraga Seguieri to show the progressive stages of development. In the first or female (protogynous) condition the stigmas only are mature, the anthers, petals, and sepals being far from having attained their full size. It is not until half the anthers have shed their pollen, and the others ready to do so, that the flower attains its complete dimensions.† I refer to these facts, which are equally applicable to many other flowers, to show that growth normally continues after insects have commenced to visit flowers; so that there is plenty of opportunity for the petals, stamens, etc., to respond to the insect's action before reaching maturity. Dr. F. Noll has investigated the various movements of zygomorphic flowers during growth, resulting in the external position of the flower; and he finds that the excess of weight on one side is, when necessary, counterbalanced by active tensions (see Jl. R. Mic. Soc., 1887, p. 612 and reffs.). *Fertilisation, etc., p. 379. + Ibid, p. 244. CHAPTER XIII. THE EFFECTS OF STRAINS ON STRUCTURES. VEGETATIVE ORGANS.-In explaining the origin of irregular flowers by insect agency, it will not be amiss to fortify the theory by describing other instances apart from flowers, and to add further results which I believe to accrue from the persistent action of insects on the one hand, and a ready response on the part of the organ on the other. Researches into the anatomy of stems have proved the existence of this responsive power. Thus, a tree will develop wood in a particular direction if it be compelled to meet special strains imposed upon it; for Andrew Knight found that when trees were allowed freedom in one direction only, and were thus made to oscillate in definite directions, either east and west or north and south, the stem became elliptical in section, the long axis corresponding to the direction of oscillation. Mr. Herbert Spencer has also described how Cactuses, if submitted to particular strains, develop wood to meet them. The various kinds of the supporting tissues of pedicels, such as collenchyma, sclerenchyma, the so-called liber-fibres as well as true woody fibre, are all so many contrivances of the stems to support the weight of the flowers and fruits, and to overcome gravity. So, again, in the case of apples and pears, if they hang vertically downwards they grow as symmetrically round the insertion of the stalk as an orange; but if the pedicel projects obliquely from the branch, they then thicken along the upper side, forming a sort of buttress running down into the stalk, which also itself tends to thicken. This enlargement, which gives the peculiar "lopsidedness" to several kinds of pears especially, and in a lesser degree to some sorts of apples, is simply due to the fact that the force required to counteract the resultant of the two forces, gravity and tension-which act vertically downwards and along the stalk, respectively-must be increased in proportion as the direction of the stalk approaches the horizontal one. The accompanying diagram (Fig. 38) represents the basal end of a Dr. Jules Guyot pear and in the position in which it hangs upon the tree. The letter w (weight) is in the line of gravity, t (tension) acts along the stalk, while r counteracts the resultant, which tends to tear the pear from the stalk at the upper side. This strain must be met, and the increased thickness Fig. 38.-Diagram of the end of a along this upper side enables the pear to resist it, and thus prevents the fruit, especially if it be a large and heavy kind, from being wrenched from the stalk. w Dr. Jules Guyot pear. t A somewhat similar development often occurs with plums and lemons; only, as there is no receptacular tube in either case, the weight of the fruit causes them to produce a thick fold in the carpel on the under side, together with some degree of hypertrophy on the upper, where the tension occurs. It is not uninteresting to notice how branches of trees similarly sustain the strain produced by their own weight. This is done by growing at an acute angle (originally caused by arising in the axil of a horizontally inserted leaf), much more often than in a strictly horizontal direction. The branch, after growing for a short distance upwards, generally bends downwards, assuming just the same curvature as of declinate stamens which have to support the weight of insects. If the vertical line in the adjoining diagram (Fig. 39) represent the trunk, and the curved line a branch, the insertion at ƒ supplies the fulcrum, w is the weight of f the branch, and acts in a vertical line, p is the power required to counteract the resultant of these two forces. W When the bough breaks, either Fig. 39.-Diagram of a tree and through an additional weight of snow branch, illustrating the distribution of forces. or by its own weight on decay, it snaps off at the point p, i.e. the place where the force acts, as it can no longer overcome the resultant of ƒ and w. REPRODUCTIVE ORGANS.-Applying these principles to floral structures, we have already seen in how many ways the strain to which parts of flowers are subjected, through the weights and pressures of insects, are met and overcome. In a large number of instances the organ becomes curved, and assumes the character of a spring, yielding on pressure, but recovering its position when pressure is removed. It is often so with the claws of the petals of papilionaceous flowers, the stamens of Dicentra, Corydalis, and Veronica Chamaedrys. Similar structures are seen in many styles, as those of Pansy (Fig. 54), and in genera of Polygalacea. All declinate stamens partake of it to a more or less degree. The distribution of the forces brought into play to support the insect is exactly the same as when a bough has to support its own weight, as will be easily understood from what has been described, and by referring to the diagram (Fig. 40a). Fig. 40a.-Diagram of declinate stamens, illustrating the distri bution of forces. If the tissue does not remain firm under pressure, then the lever-action of a spring may fail to be secured, and the organ will oscillate freely, as on a pivot. This I take to be another result of a constant, but of course unconscious, effort of the insect to push the organ in a certain direction. It is thus that anthers become versatile, and oscillate, and may become even inverted in position, when pollination is being effected by insects. Consequently anthers normally introrse can be made to assume a pseudo-extrorse position. This happens with some Crucifera as Cardamine pratensis, Tulips, etc. A similar cause I would attribute to the formation of the oscillating anthers of Salvia, and of the species of Calceolaria, as C. Pavonii, which form the section Aposecos of that genus, as shown in Fig. 32, a, p. 109. W As an example of an entire flower illustrating the distribution of forces, the accompanying figure of Lamium album (Fig. 40b) will explain how the forms of the calyx and corolla are adjusted to bear the weight of the insect. The bee alights on the lip and bum, showing distri- then partially crawls into the expanded mouth of the corolla, so that its weight now lies in the direction of w. and the resultant of these is in the opposite direction to r. This is where the strain will be felt; so that it is just at this Fig. 40b.-Lamium al bution of forces. The fulcrum will be at f, |