during the day by humble bees, this is not the case with the Primrose, which, in Mr. Darwin's opinion, is fertilized almost exclusively by moths.

The genus Lythrum (Fig. 33) affords a still more complex case, for here we have three sets of flowers. The stamens are in two groups; in some plants, the pistil projects beyond them; in the second form it is shorter than any of the stamens, and in the third it is intermediate in length, so that the stigma lies between the two sets of anthers. These three positions appear to correspond respectively to the head, thorax, and abdomen of the bee.

FIG. 33.-Lythrum salicaria.

Although flowers present us with these beautiful and complex contrivances, whereby the transfer of pollen from flower to flower is provided for, and waste is prevented, yet they appear to be imperfect, or at least not yet perfect in their adaptations. Many small insects obtain access to flowers and rob them of their contents. Malva rotundifolia can be, and often is, sucked by bees from the outside, in which case the flower derives no advantage from the visit of the insect. In Medicago sativa, also, insects can suck the honey without effecting fertilization, and the same flower continues to secrete honey after fertilization has taken place, and when, apparently, it can no longer be of any use. Fritz Müller has observed that, though Posoqueria fragrans is exclusively fer

tilized by night-flying insects, many of the flowers open in the day, and consequently remain sterile. It is of course possible that these cases may be explained away; nevertheless, as both insects and flowers are continually altering in their structure, and in their geographical distribution, we should naturally expect to find such instances. Water continually tends to find its own level; animals and plants as constantly tend to adapt themselves to their conditions. For it is obvious that any blossom which differed from the form and size best adapted to secure the due transference of the pollen would be less likely to be fertilized than others; while on the other hand, those richest in honey, sweetest, and most conspicuous, would most surely attract the attention and secure the visits of insects; and thus, just as our gardeners, by selecting seed from the most beautiful varieties, have done so much to adorn our gardens, so have insects, by fertilizing the largest and most brilliant flowers, contributed unconsciously, but not less effectually, to the beauty of our woods and fields.1

1 I have treated the subject of this lecture at greater length in a little book on Flowers and Insects, forming one of the "Nature Series."

ON PLANTS AND INSECTS.

LECTURE II.

In the last lecture I endeavoured to show in a variety of cases how beautifully flowers are constructed, so as to secure their fertilization by insects. Neither plants nor insects would be what they are, but for the influence which each has exercised on the other. Some plants, indeed, are altogether dependent on insects for their very existence. We know now, for instance, that certain plants produce no seeds at all, unless visited by insects. Thus, in some of our colonies, the common Red Clover sets no seeds, on account of the absence of humble bees; for the proboscis of the hive bee is not long enough to effect the object. According to Mr. Belt, the same is the case, and for the same reason, in Nicaragua, with the scarlet-runner. But even in those instances in which it is not absolutely necessary, it is an advantage that the flowers should be fertilized by pollen brought from a different stock, and with this object in view, insects are tempted to visit flowers for the sake of the honey and pollen; while the colours and scents are useful in making the flowers more easy to find.

Fortunately for us, bees like the same odours as we do; and as the great majority of flowers are adapted for bees,

they are consequently sweet; but it might have been otherwise, for flies prefer unpleasant smells, such as those of decaying meat, and other animal substances on which they live as larvæ, and some flowers, consequently, which are fertilized by them, are characterized by very evil odours. Colours also are affected in the same manner, for while bee-flowers (if I may coin such an expression) have generally bright, clear colours, flyflowers are usually reddish or yellowish brown.

The real use of honey now seems so obvious that it is curious to see the various theories which were once entertained on the subject. Patrick Blair thought that the honey absorbed the pollen, and then fertilized the ovary; Pontedera that it kept the ovary in a moist condition. Linnæus confessed his inability to solve the question. Other botanists considered that it was useless material thrown off in the process of growth. Krünitz thought he observed that in meadows much visited by bees the plants were more healthy, but the inference he drew was, that the honey, unless removed, was very injurious, and that the bees were of use in carrying it off. Kurr observed that the formation of honey in flowers is intimately associated with the maturity of the stamens and pistil. He lays it down, as a general rule, that it very seldom commences before the opening of the anthers, is generally most copious during their maturity, and ceases so soon as the stamens begin to wither and the development of the fruit commences. Rothe's observations also led him to a similar conclusion, and yet neither of these botanists perceived the intimate association which exists between the presence of honey

D

and the period at which the visits of insects are of importance to the plant. Sprengel was the first to point out the real office of honey, but his views were far from meeting with general assent, and, even as lately as 1833, were altogether rejected by Kurr, who came to the 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 crossfertilization. Thus, most of the Rosacea are fertilized by insects, and possess nectaries; but, as Delpino has pointed out, the genus Poterium is anemophilous, or wind-fertilized, and possesses no honey. So also the Maples are almost all fertilized by insects, and produce honey; but Acer negundo is anemophilous, and honeyless. Again, among the Polygonacea, some species are insect-fertilized and melliferous, while, on the other hand, certain genera, Rumex and Oxyria, have no honey, and are fertilized 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

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.