With reference to the continuous flow of nectar, I would draw some analogy from animal secretions. Mr. Darwin, in speaking of the cow, observes * : "We may attribute the excellence of our cows and of certain goats, partly to the continued selection of the best milking animals, and partly to the inherited effect of the increased action, through man's art, of the secreting glands." This fact, recorded in the last sentence, which I have italicized, is only one example of the general principle of increase of growth by use, which I take to be strictly analogous to what takes place in the vegetable kingdom. And we may notice, in its special application to the formation of glands and other structures by mechanical irritation, that it is none other than a mechanical irritation which keeps up the secretion of milk for prolonged periods.

The common or physical basis of vegetable life, namely protoplasm, is very nearly † indistinguishable in its properties from that of animals. Their behaviour is every day being proved to be not only similar but identical in the two kingdoms. The effects, under mechanical irritations and strains, of nutritive matters of the same kind, of poisonous substances, of electricity, etc., all show that the bond which unites the animal and vegetable kingdoms together is of one and the same nature, and that the links of the chain are forged out of this common basis of life.

It is not to be wondered at, then, but rather to be antici

same direction. In the case of the eye, I take that cause to be light. In the case of an irregular corolla or the pitcher of Nepenthes, I assume it to be insects (Tr. Lin. Soc., xxii., p. 415; Ann. Sci. Nat., 4 sér., xii., p. 222).

Conversely, in the absence of light the eye vanishes; in the absence of insects, corolla, honey, etc., go; so that negative evidence tends to support the positive in all cases alike; see Or. of Sp., 6th ed.,

*Anim. and Pl. under Dom., ii., p. 300. † See Journ. Roy. Micr. Soc. 1887, 771.

p. 110.

pated, that tissues will behave alike in both kingdoms; that organs will grow with use and degenerate with disuse; that they will develop processes to meet strains put upon them, as the limbs of animals have done and as stems* will do by forming special tissues; and, on the other hand, that they will atrophy if not called upon to display their powers, as parasitic organisms abundantly show in both kingdoms; and as plants degenerate in water, which saves them the trouble of supporting themselves.

All this is exactly what one finds to be the case in every department of the animal and vegetable kingdoms alike, whenever we search diligently into the anatomy and meaning of the histological details of all parts of organisms.

CORRELATIONS OF FLORAL NECTARIES WITH POLLINATION.There is yet another point observable in glands. As the position of a gland or nectary is just where it is most easily accessible to the particular insects which visit the flower— a fact abundantly illustrated throughout the floral world,and since the sole use of it to the plant, as far as we can see, is that it should attract insects which transfer the pollen from one flower to another, one naturally looks to see if the positions of the anthers and stigmas are in any way correlated to that of the honey-gland. Such is, in fact,

* I would throw out a suggestion that the anomalous stems of climbers, which often develop supernumerary collateral axes, but all coherent in one common stem, may be due to a response to the strains to which these stems are subjected, occurring in various directions, as they hang dependent on other trees. Other peculiar features, as of innumerable vessels, feeble wood tissues, etc., I take to be due to degeneracy, through these stems not being self-supporting, so that they have assumed very much the anatomical characters of subterranean roots. Again, just as the pericycle plays so important a part in the structure of many roots, it will be found that this same active layer is the parent of at least several of the above-mentioned supernumerary tissues in climbers, as in the tendrils of Cucurbita, Bryonia, etc.

invariably the case; so that one cannot but infer that a common cause has brought about their correlated positions. This close correlation is, of course, especially observable in the more highly differentiated flowers. In regular flowers, accessible on all sides, the glands are placed symmetrically round the flower-whether on the sepals, as in Lime; on the petals, as in a Buttercup; or on the receptacle, as in Geranium pratense,‚—or else there is formed a disk, as in so many "discifloral" plants. As soon, however, as a flower begins to show some tendency to irregularity, or the flower is visited in one way only, the honey-secreting organ at once becomes more restricted in localization; as in the Wallflower, where it forms two cushions, out of the middle of which the shorter stamens arise, while the petals form two pseudo-tubes leading down to those two glands. Again, in the Labiata, so markedly zygomorphic, the honey-gland is often restricted to the anterior side, on which the proboscis is inserted. Similarly in Antirrhinum majus, "the honey is secreted by the smooth green fleshy base of the ovary, whose upper part is paler in colour and covered with fine hairs; it remains adherent

to the nectary and to the base of the anterior stamens. The short wide spur permits the insect's proboscis to reach the honey from below; above and in front it is protected by a thick fringe of stiff knobbed hairs on the angles of the anterior stamens.'

It is hardly worth while giving other cases to prove the universal rule, that the position of the honey and its gland is always where it is most accessible; and the position of the anthers is, at the same time, just where they will be most conveniently struck by the insect; while the style and stigma supply a third correlation, so that the latter organ invariably hits the insect where the pollen has been previously placed.

* Müller, Fertilisation, etc., p. 433.

One more point may be noticed in connection with the above-mentioned correlations, namely, the motility of many stamens. This is always in reference to fertilisation, and, if it be an adaptation to intercrossing, then the anther takes up such a position that the insect strikes it when searching for honey, as in the Aconite and Tropœolum. If, on the contrary, the motion is to secure self-fertilisation, then it is regardless of the honey, and may actually interfere with the access to it by insects, as in the Rosacea: for in members of this order, with an indefinite number of stamens, the further they spread away from the pistil the more readily is the honey accessible; but when they curve inwards, and crowd over the stigmas in the centre, they completely cover up and conceal the honey-disk.

The position of the anthers in relation to the honeysecreting organs will, I think, often be found to be the clue to certain anomalies in flowers. Thus in Geranium pratense it has been noticed that the petaline stamens stand ultimately externally to the calycine. Now, the position of the five glands in front of the sepals requires that a tubular space should exist above them, down which an insect may thrust its proboscis, as in the Wallflower. Consequently the five stamens in front of the sepals must be so disposed as not to interfere with this passage. This can only be secured by their bending well inwards towards the styles below, and then outwards, above, so as to bring the anthers again on the same vertical plane as those of the petaline stamens.

The more internal position of the calycine stamens, and the external position of the petaline ones, are immediately due to the gland, so to say, forcing the former inwards, while the buttress-like bases of the carpels thrust the latter outwards. This gives rise to the so-called obdiplostemony of the Geraniaceœ.

CHAPTER XVII.

SENSITIVENESS AND IRRITABILITY OF PLANT ORGANS.

GENERAL ILLUSTRATIONS-PROTOPLASMIC IRRITABILITY.-Having now stated on what grounds I believe that the cohesions and adhesions between them, as well as the forms of floral structures have arisen-namely, in response to the irritations set up mainly by insect agencies, coupled with the effects of nutrition, atrophy, hereditary influences, etc.,-it will be desirable to show briefly, not only how remarkably sensitive almost all parts, both vegetative and reproductive, are to the action of stimuli, but how they exhibit even visibly responsive effects, both in the protoplasm of the cells and in the tissues which are composed of them.

The sensitiveness of living protoplasm is one of its most marked and well-known phenomena. It exhibits changes in its distribution within the cell as well as motions, which are the direct result of external stimuli. These may be very various, such as light, heat, electricity, or a merely mechanical irritation, as well as organic and inorganic solutions.

Of the effects of stimuli upon the protoplasm, some may be beneficial, and partake of the nature of nutrition, as may be witnessed in the protoplasmic "aggregation" of insectivorous plants.* Very similar appearances follow electrical

* See Darwin's Insectivorous Plants, fig. 7, p. 40.