4

RELATION OF PLANTS AND INSECTS. [CHAP

In by far the majority of cases, the relation between flowers and insects is one of mutual advantage. In some plants, however, as for instance in our common Drosera, we find a very different state of things, and the plant catches and devours the insects. The first observation on insect-eating flowers was made about the year 1768, by our countryman Ellis. He observed that in Dionæa, a North American plant, the leaves

have a joint in the middle, and thus close over, kill, and actually digest any insect which may alight on them.

In our common Sundew (Drosera rotundifolia, Fig. 4) the rounded leaves are covered with glutinous glandular hairs or tentacles-on an average about 200 on a full-sized leaf. The glands are each surrounded

1 See Darwin's Insectivorous Plants.

1.]

INSECTIVOROUS PLANTS.

5

by a drop of an exceedingly viscid solution, which, glittering in the sun, has given rise to the name of the plant. If any object be placed on the leaf, these glandular hairs slowly fold over it, though if it be inorganic they soon unfold again. On the other hand, if any small insect alights on the leaf it becomes entangled in the glutinous secretion, the glands close over it, their secretion is increased, and they literally digest their prey. Mr. Frank Darwin has recently shown that plants supplied with insects grow more vigorously than those not so fed. It is very curious that while the glands are so sensitive that even an object weighing only of a grain placed on them is sufficient to cause motion, yet they are "insensible to the weight and repeated blows of drops" of even heavy rain.

Drosera, however, is not our only English insectivorous plant. In the genus Pinguicula, which frequents moist places, the leaves are concave with incurved margins, and the upper surfaces are covered with two sets of glandular hairs. In this case the naturally incurved edges curve over still more if a fly or other insect be placed on the leaf.

Another case is that of Utricularia, an aquatic species, which bears a number of utricles or sacs, which have been supposed to act as floats. Branches, however, which bear no bladder float just as well as the others, and there seems no doubt that their real use is to capture small aquatic animals, which they do in considerable numbers. The bladders in fact act on the principle of an eel-trap, having an orifice closed with a flap which permits. an easy entrance, but

6

FLOWERS FERTILISED BY INSECTS. [CHAP.

effectually prevents the unfortunate victim from getting out again.

I will only allude to one foreign case, that of the Sarracenia.1 In this genus some of the leaves are in the form of a pitcher. They secrete a fluid, and are lined internally with hairs pointing downwards. Up the outside of the pitcher there is a line of honey glands which lure the insects to their destruction. Flies and other insects which fall into this pitcher cannot get out again, and are actually digested by the plant. Bees, however, are said to be scarcely ever caught.

Every one knows how important flowers are to insects; every one knows that bees, butterflies, etc., derive the main part of their nourishment from the honey or pollen of flowers, but comparatively few are aware, on the other hand, how much the flowers themselves are dependent on insects. Yet it has, I think, been clearly shown that if insects have been in some respects modified and adapted with a view to the acquirement of honey and pollen, flowers, on the other hand, owe their scent and honey, their form and colour, to the agency of insects. Thus the lines and bands by which so many flowers are ornamented have reference to the position of the honey; and it may be observed that these honey-guides are absent in night flowers, where they of course would not show, and would therefore be useless, as for instance in Lychnis vespertina or Silene nutans. Night flowers, moreover, are generally pale; for instance, Lychnis vespertina is white, while Lychnis diurna, which flowers by day, is red.

1 See Hooker, British Association Journal, 1874.

1.]

FLOWERS FERTILISED BY WIND.

7

Indeed, it may be laid down as a general rule that those flowers which are not fertilised by insects, as for instance those of the Beech and most other forest trees, are small in size, and do not possess either colour, scent, or honey.

Before proceeding further let me briefly mention the terms used in describing the different parts of a flower.

If we examine a common flower, such for instance as a Geranium, we shall find that it consists, firstly, of an outer envelope or calyx, sometimes tubular, sometimes consisting of separate leaves called sepals; secondly, an inner envelope or corolla, which is generally more or less coloured, and which, like the calyx, is sometimes tubular, sometimes composed of separate leaves called petals; thirdly, of one or more stamens, consisting of a stalk or filament, and a head or anther, in which the pollen is produced; and fourthly, a pistil, which is situated in the centre of the flower, and consists generally of three principal parts; one or more compartments at the base, each containing one or more seeds; the stalk or style; and the stigma, which in many familiar instances forms a small head at the top of the style or ovary, and to which the pollen must find its way in order to fertilise the flower.

But though the pistil is thus surrounded by one or more rows of stamens, there are comparatively few cases in which the pollen of the latter falls directly on the former. On the contrary this transference is in most cases effected in other ways—generally by means of the wind, of insects, or, in some cases, of

8

WASTE OF POLLEN.

[CHAP.

birds. In the former case, however, by far the greater part of the pollen is wasted; and much more. must therefore be produced than in those cases where the transference is effected by insects.

One advantage, of course, is the great economy of pollen. We have not much information on the subject, but it would seem, from the few observations that have been made, that half a dozen pollen grains are sufficient to fertilise a seed. But in plants in

a

FIG. 5.-Geranium pratense (young flower). Five of the stamens are

erect.

FIG. 6.-Geranium pratense (older flower). The stamens have retired, and the stigmas are expanded.

which the pollen is carried by the wind, the chances against any given grain reaching the pistil of another flower are immense. Consequently by far the greater part of the pollen is lost. Every one, for instance, must have observed the clouds of pollen produced by the Scotch fir. In such flowers as the Pæony the pollen is carried by insects, and far less therefore is required; yet even here the quantity produced is still large; it has been estimated that each flower produces