This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
Figs. 1 to 6, taken from Axell's work, illustrate this difference. In the alder (Fig. 1), the hop (Fig. 2), and wheat (Fig. 3), the pollen is wind-borne, whence they have been termed by Delpino "anemophilous;" while in the willow (Fig. 4), the flax (Fig. 5), and nuphar, (the yellow water lily) (Fig. 6), it is carried by insects, whence such plants have been termed "entomophilous"
Fig. 1. Stigma of the Alder. Fig. 2. - Of the Hop. Fig 3. - Of the Wheat; which are anemophilous. Fig. 4. - Of the Willow. Fig. 5. - Of the Flax. Fig. 6. - Of Nuphar; which are entomophilous.
Even in nearly allied plants this difference is well marked, in illustration of which Axell gives the following figures taken from Maout and Decaisne's "Traite generate de Botanique": - Fig. 7 represents a section of a flower of Plantago major, which is wind-fertilised; Fig. 8 of an allied species, Plumbago Europea, which is insect-fertilised. Again, Fig. 9 represents a section of Poterium sanguisorba, which is wind-fertilised; Fig. 10 of the nearly allied Sanguisorba officinalis, which is fertilised by insects.
Fig. 7 - Section of Plantago Major. Fig. 8. - Of Plumbago Europea.
Fig 9._Flower of Poterium sanguisorba. Fig. 10. - Of Sanguisorba officinalis
It is an almost invariable rule that wind-fertilised flowers are inconspicuous; but the reverse does not hold good, and there are many flowers which, though habitually visited by insects, are not brightly coloured. In some cases, flowers make up by their numbers for the want of individual conspicuousness. In others, the insects are attracted by scent; indeed, as has already been mentioned, not only the colour1 of flowers, but the scent also, has no doubt been greatly developed through natural selection, as an attraction to insects. But though bright colours and strong odours are sufficient to attract the attention of insects, something more is required. Flowers, however sweet-smelling or beautiful, would not be visited by insects unless they had some inducements more substantial to offer. These advantages are the pollen and the honey; although it has been suggested that some flowers beguile insects by holding out the expectation of honey which does not really exist, just as some animals repel their enemies by resembling other species which are either dangerous or disagreeable.
Night flowers are generally white or pale yellow, these being the tints which render them most conspicuous in the dusk of evening. Thus Lychnis dhirna, which opens by day, is red; while L. vespertina, which opens in the evening, is white.
It will scarcely, I think, be doubted by any one that scent is an advantage to flowers by attracting insects. No wind-fertilised flowers are scented. On the other hand, while colour is as useful as scent by-day, at night it is of course less easily perceived. Hence night flowers are specially odoriferous, and there are some - such as Hesperis matronalis, Silene nutans, etc - which are very sweet in the evening and yet emit little or no odour by day.
1 In confirmation of this it is stated that when insects are excluded, the blossoms last longer than is otherwise the case; that when flowers are once fertilised, the corolla soon drops off, its function being performed.
The honey is secreted, sometimes by one part of the flower, sometimes by another; and great variations may be found in this respect even within the limits of a single order. Thus in the Ranunculaceae the honey glands are situated on the calyx, in certain Paeonies; on the petals, in buttercups and hellebore; on the stamens, according to Muller, in Pulsatilla; and on the ovary, in Caltha.
The real use of the honey in flowers, indeed, now seems so obvious that it is remarkable to see the various theories which were entertained on the subject. Patrick Blair thought it absorbed the pollen, and thus fertilised the ovary. Linnaeus confessed his inability to solve the question. Other botanists considered it was useless material thrown off in the process of growth. Krunitz even thought he had 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, that the bees were of use in carrying it off. Sprengel was the first to show that the real office of the honey is to attract insects, but his view was far from meeting with general consent, and even so 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 is right in considering that the object is to attract insects, and thus to secure cross-fertilisation. Thus most of the Rosacese 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. As also the Maples are almost all fertilised by insects and produce honey, but Acer negundo is anemophilous and honeyless. So also among the Polygonaceae, 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 that some plants secrete honey on other parts than the flowers.