34

ATTRACTION OF INSECTS BY HONEY. [CHAP.

conclusion that the secretion of honey is the result of developmental energy, which afterwards concentrates itself on the ovary.

No doubt, however, seems any longer to exist that Sprengel's view is right; and that the true function of honey is to attract insects, and thus to secure crossfertilisation. Thus, most of the Rosacea are fertilised by insects, and possess nectaries; but, as Delpino has pointed out, the genus Poterium is anemophilous, or wind-fertilised, and possesses no honey. So also the Maples are almost all fertilised by insects, and produce honey; but Acer negundo is anemophilous, and honeyless. Again, among the Polygonaceæ, some species are insect-fertilised and melliferous, while, on the other hand, certain genera, Rumex and Oxyria, have no honey, and are fertilised by the wind. At first sight it might appear an objection to this view, and one reason perhaps why the earlier botanists missed the true explanation, may have been the fact, that some plants secrete honey on other parts than the flowers. Belt and Delpino have, I think, suggested the true function of these extra floral nectaries.1 The former of these excellent observers describes a South American species of Acacia: this tree, if unprotected, is apt to be stripped of the leaves by a leaf-cutting ant, which uses them, not directly for food, but, according to Mr. Belt, to grow mushrooms on. The Acacia, however, bears hollow thorns, while each leaflet produces honey in a crater-formed gland at the

1 I, by no means, however, wish to suggest that we as yet fully understand the facts. For instance, the use of the nectary at the base of the leaf of the Fern is still quite unexplained.

11.]

PROTECTION OF PLANTS BY INSECTS. 35

base, and a small, sweet, pear-shaped body at the tip. In consequence, it is inhabited by myriads of a small ant, which nests in the hollow thorns, and thus finds meat, drink, and lodging all provided for it. These ants are continually roaming over the plant, and constitute a most efficient body-guard, not only driving off the leaf-cutting ants, but, in Belt's opinion, rendering the leaves less liable to be eaten by herbivorous. mammalia. Delpino mentions that on one occasion he was gathering a flower of Clerodendron fragrans, when he was suddenly attacked by a whole army of small ants.

I am not aware that any of our English plants are protected in this manner from browsing quadrupeds, but not the less do our ants perform for them a very similar function, by keeping down the number of small insects, which would otherwise rob them of their sap and strip them of their leaves.

Forel watched, from this point of view, a nest of Formica pratensis. He found that the ants brought in dead insects, small caterpillars, grasshoppers, cercopis, &c., at the rate of about twenty-eight a minute, or more than one thousand six hundred in an hour. When it is considered that the ants work not only all day, but in warm weather often all night too, it is easy to see how important a function they fulfil in keeping down the number of small insects.

Some of the most mischievous insects, indeedcertain species, for instance, of aphis and coccus― have turned the tables on the plants, and converted ants from enemies into friends, by themselves developing nectaries, and secreting honey, which the ants

36

PROTECTION OF PLANTS.

[CHAP.

love. We have all seen the little brown garden ant, for instance, assiduously running up the stems of plants, to milk their curious little cattle. In this manner, not only do the aphides and cocci secure immunity from the attacks of the ants, but even turn them from foes into friends. They are subject to the attacks of a species of ichneumon, which lays its eggs in them, and Delpino has seen ants watching over the cocci with truly maternal vigilance, and driving off the ichneumons whenever they attempted to approach.

But though ants are in some respects very useful to plants, they are not wanted in the flowers. The great object is to secure cross-fertilisation; but for this purpose winged insects are almost necessary, because they fly readily from one plant to another, and generally, as already mentioned, confine themselves for a certain time to the same species. Creeping insects, on the other hand, naturally would pass from each floret to the next; and, as Mr. Darwin has shown in his last work, it is of little use to bring pollen from a different flower of the same stock; it must be from a different plant altogether. Moreover, creeping insects, in quitting a plant, would generally go up another close by, without any regard to species. Hence, even to small flowers (such as many cruciferæ, compositæ, saxifrages, &c.), which, as far as size is concerned might well be fertilised by ants, the visits of flying insects are much more advantageous. Moreover, if larger flowers were visited by ants, not only would these deprive the flowers of their honey, without fulfilling any useful function in return, but they would probably prevent the really useful visits of bees.

If

11.]

CHEVAUX DE FRISE.

37

you touch an ant with a needle or a bristle, she is almost sure to seize it in her jaws; and if bees, when visiting any particular species, were liable to have the delicate tip of their proboscis seized on by the horny jaws of an ant, we may be sure that such a plant would soon be deserted.

On the other hand, we know how fond ants are of honey, and how zealously and unremittingly they search for food. How is it, then, that they do not anticipate the bees, and secure the honey for themselves? Kerner has recently published a most interesting mernoir on this subject, and has pointed out a number of ingenious contrivances by which flowers protect themselves from the unwelcome visits of such intruders. The most frequent are the interposition of chevaux de frise, which ants cannot penetrate, glutinous surfaces which they cannot traverse, slippery slopes which they cannot climb, or barriers which close the way.

Firstly, then, as regards chevaux de frise. In some respects these are the most effectual protection, since they exclude not only creeping insects, but also other creatures, such as slugs. With this object, it will be observed that the hairs which cover the stalks of so many herbs usually point downwards. A good example of this is afforded, for instance, by a plant, Knautia dipsacifolia (Fig. 31), allied to our common blue scabious. The heads of the common carline, Carlina vulgaris (Fig. 33), again, present a sort of thicket, which must offer an almost impenetrable barrier to ants. Some species of plants are quite smooth, excepting just below the flowers.

The

38

GLANDULAR HAIRS.

[CHAP.

common but beautiful Cornflower (Centaurea cyanus) is quite smooth, but the involucres forming the flower-head are bordered with recurved teeth. In this case, neither the stem nor the leaves show a trace of such prickles.

The same consideration throws light on the large number of plants which are more or less glutinous, a condition generally produced, as, for instance, in the

FIG. 31.-Knautia dipsacifolia.

flowers of the Gooseberry and of Linnæa borealis (Fig. 32), by the presence of glandular hairs. Kerner has called attention to a very interesting illustration afforded by Polygonum amphibium. In this species the stigma projects about one-fifth of an inch above the flower, so that if ants could obtain access, they would steal the honey without fertilising the flower; a flying insect, on the contrary, alighting on the flower, could scarcely fail to touch the stigma.