spot from which to reach two drops of honey that seem to rest upon a knee-shaped bend in the upper petals (a), the leaf springs downwards (b), and the stamen is set free and flies upwards, dusting the lower surface of the insect with pollen. When the stamen has thus served its purpose, it gradually curves upwards out of the flower (c), and the style which was hitherto undeveloped grows gradually out of the flower in a horizontal direction, so as to form another alighting place (c)." Rapid movements in the stamens are not unknown. I described that of Medicago

-

many years ago, and now supply figures. Fig. 49, a represents the front view of a flower on expansion; b, the same after a bee has exploded it the staminal column has now arisen, curled upwards, and abuts against the b standard; c shows the curved position of the stamens, the corolla being removed. The stamens are inelastic, as they will not return to

Fig. 49.—Medicago sativa. (For de- a horizonal position without breaking across, if pressed downwards.

scription, see text)

Many other rapid movements of the filaments are too well known to need description, such as those of Berberis, Helianthemum, Sparmannia, Centaurea, and Urtica; while Orchids exhibit various movements in the caudicles of their pollinia.

Besides slow movements, the pistil often exhibits rapid ones on being touched, as are known to occur in Stylidium, Canna, Maranta and allied plants; while the flap-like stigmas of Mimulus,† and of several genera of orders allied to the Scrophularineæ, close together on being irritated mechanically. *Journ. Lin. Soc., vol. ix. p. 327.

† Mr. F. W. Oliver has lately investigated the mode of conduction

There is no need to describe a long series of movements, my object being simply to emphasize the fact that sensitiveness and irritability are pronounced phenomena in flowers, which point to a highly irritable condition of the protoplasm contained in the cells of all the floral members.* And, although we cannot now trace the progress of change in the floral organs under the mechanical and physiological impulses due to insect agency, the probability that these have been the actual influences to which the tissues have responded, and thence evolved the existing floral structures, will now, I trust, appear to the reader to be of a very high order.

of the irritation in the stigmas of Martynia lutea and M. proboscidea, and of Mimulus luteus and M. cardinalis. He believes it to be due to the continuity of the protoplasm from cell to cell. The tissue of the stigma consists of two lamellæ. The irritability is confined to several layers of prismatic cells on the inner side of the lamella, where the continuity of protoplasm was determined. (Quoted from Journ. Roy. Micr. Soc., 1887, p. 781. Ber. Deutsch. Bot. Gesell., v. (1887), p. 162.)

Mr. Oliver has also lately contributed a valuable paper to the Annals of Botany (vol. i., p. 237, pl. xii., 1888), on "The Sensitive Labellum of Masdevallia muscosa." Continuity of the protoplasm occurs in the irritable "crest" on the labellum, which rapidly rises on being touched; the mechanism being closely comparable with that of the pulvinus of Mimosa. The author corroborates Mr. Gardiner's observation that a large amount of tannin occurs in the cells with which such irritability is concerned. References are also given to descriptions of other Orchids remarkable for having irritable perianths.

*For further information on the effects of light and heat upon the opening and closing of flowers, the reader is referred to Sachs' Physiology of Plants, chap. xxxvi., p. 641, where the author gives an account of Pfeffer's investigations. It is not clear, however, how temperature acts. A casual discovery may perhaps supply a hint. On forcing air into the flower-stalk of the white Water-lily, I found that the petals instantly spread open. May not, therefore, a rise of temperature cause the air within the tissues to expand, and so at least help to produce the same effect?

CHAPTER XVIII.

IRRITATION OF THE POLLEN-TUBE-THE ORIGIN OF CONDUCTING TISSUES.

THE first effect produced by the action of the germination of the pollen-tube is the formation of the so-called conducting tissue or layers of specialized cells which nourish the tube in its downward growth. Like glandular nectaries, this tissue consists of small merismatic-like cells, highly charged with nutritive and saccharine substances. In some cases it is a metamorphosed condition of the epidermis alone, as

a

b

с

Fig. 50.-a, section of (epidermal) conducting tissue of Fumaria; b, that of Rubus; c. section of ovary of Crucifer (after Capes.)

M. Capes has shown in his researches,* as in Fumaria. Fig. 50, a, represents a section of the stylar canal, the lining epidermis having its cells charged with such matters, while

* Ann. des Sci. Nat., vii., 1878, p. 209.

three pollen-tubes are seen in section. Fig. 50, b, shows the formation of conducting tissue at the angle of the inflected carpellary edges of Rubus. The epidermal and subjacent cells form the conducting tissue in this case. The cells on the outskirts are charged with sphæraphids. Fig. 50, c, is a section of the ovary of a Crucifer. The replum or false dissipiment, as in the Papaveraceae, forms the machinery for conducting the tubes. The dotted lines show the original lines of fusion. Now, if my theory be true, that no structure exists which has not been brought into existence through some foreign action having been brought to bear upon it—either directly from without, as insect agency, light, etc., or indirectly through nutrition within the plant, then, the existence of this specialized tissue would never have arisen had it not been for the irritating action of the pollen-tubes. The analogous influence of the mycelium of a parasitic fungus here gives us the clue. As such causes hypertrophy to set in, and induces nutritive matters to accumulate upon which the fungus lives,-just as the irritation of the egg or pupa of a cynips or other insect causes a similar accumulation of richly nutritive substances to be made within the tissues of the gall upon which it feeds,—so the germinating power of the pollen-grain and the growth of the pollentube have actually brought about the formation of these highly nutritive conducting tissues of the style. The effect has then become hereditary, so that they are now in course of formation, at least, during the development of the flower in preparation for the ingress of the pollen-tubes.

The remarkably stimulating action of the pollen-tube had been observed more especially in Orchids. Hildebrand noticed that the influence of the pollen was twofold, in that it determined the growth of the ovary and the complete formation of the ovules before the process of fecundation had

In

taken place.* M. Guignard has described the effects resulting from his experiments.† Thus, in the case of Vanilla aromatica, he found the development of the ovary was very rapid after pollinisation. At the time of flowering, the placentas have only the rudiments of the papilla which will develop into ovules, and the conducting tissue formed by the epidermis and subjacent layers on either side of the placentary projections is still undifferentiated. the intervals which separate the bands of conducting tissue, corresponding to the midribs of the carpels, there is no appreciable modifications before fecundation; but as soon as that has taken place, a layer of elongated papillæ, filled with a granular substance, arises. With regard to the development of ovules, M. Guignard remarks: “La pollinisation et la germination du pollen sont indispensables à leur formation. L'ovaire d'une fleur non pollinisée ne s'accroît pas et tombe quelques jours après l'épanouissement."

As soon, however, as the pollen-tubes are formed, the ovules begin to grow, until the twentieth day, when the primine thickens (much more than in other orchids) and finally gives to the matured ovule a globular form.

In the mean time the embryo-sac and sexual apparatus have been forming, and are completed (excepting the fusion of the two members of each tetrad, which does not take place to form the secondary embryo-sac nucleus) in little more than a month after pollinisation. Five weeks after that

period, fecundation commences.

In following the progress of the pollen-tubes, it is not *Die Fruchtbildung der Orchideen, ein Beweis für doppelte Virkung des Pollen, Bot. Zeit., 1863. Bastardirungsversuche an Orchideen, Bot. Zeit., 1865.

+ Ann. des Sci. Nat., 1886, tom. iv., p. 202; see also Maury, Observations sur la Pollinisations des Orchidées, comp. rend. de l'Acad. des Sci., 2 Août, 1886; and also Guignard, do., 19 Juillet, 1886.