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.
OF this sub-class we have in Britain representatives of fifteen orders, some of them very numerous and important. To it, for instance, belong many of our forest trees, such as the elm, oak, beech, birch, poplar, willow, pine, fir, etc.; and a large number of the common herbs, such as the nettles, cheno-podiums, euphorbias (spurge's), etc. The flowers, however, are generally less conspicuous (see Fig. 119) than those we have hitherto been considering, and offer fewer adaptations in relation to insects; being in many cases wind-fertilised: thus in H. Muller's work, less than ten pages are occupied by this whole sub-class, of which more than half are devoted to the Polygonaceae, and a greater part of the remainder to the Aristolochiaceae; two orders which in many respects form a marked contrast to the remainder, and have, at least in some species, conspicuous flowers. In the other orders, on the contrary, the flowers are generally minute. Thus in the Paronychiaceae, Bentham says, "Petals usually none, or represented by five small filaments;" in Santalaceae, "flowers small;" in Empetraceae, flowers "minute, axillary;" in Callitrichineae, flowers "minute;" in Urticaceae, flowers "small;" in Ulmaceae, flowers "small;" while in the Amentaceae (beech, oak, birch, &c), and Coniferae, the flowers rarely are coloured, or contain honey. Indeed, it is, I think, a strong argument in favour of Sprengel's views, that while large flowers are almost always coloured, small ones are usually greenish; thus out of thirty-nine British genera of Incompletae, by far the greater number of which have small flowers, in no less than thirty-seven genera they are also more or less greenish. In the Nettle, which is wind-fertilised, the anthers are provided with a spring which, suddenly opening, scatter the pollen.
In the Polygonaceae, the species of the genus Rumex are wind-fertilised; occasionally, however, visited by insects.
The species of Polygonum differ considerably from one another in the mode of their fertilisation. Some, as, for instance, P. avicidare (Knotweed), have small inconspicuous flowers, and very little, if any, honey. They are consequently but seldom visited by insects. Other species, on the contrary, such as P. Fagopyrum (the Buckwheat), and P. Bistorta, are much more conspicuous, contain honey, and are fertilised by insects.
These species, however, also differ considerably; P. Bistorta is proterandrous. When the flower opens the stamens are ripe, while the stigmas do not mature till the anthers have shed their pollen, and shrivelled up. P. Fagopyrum, on the contrary, is dimorphous; some plants having short stigmas and long stamens: others, on the contrary, long stigmas and short stamens. In Polygonum amphibium the stems, if growing in water, are smooth: while if on dry land they are provided with a certain number of glandular hairs. The curious arrangement by which cross-fertilisation is secured in Aristolochia, has been already described in the introductory chapter (ante, p. 31). Asarum, according to Delpino, is also proterogynous. Ruppia is an aquatic genus. At the time when the pollen is shed, the female flowers are immature, and the flower-stalk is short and submerged; when, however, the pollen has all escaped, the female flowers mature, the flower-stalk elongates and assumes a spiral form, so that notwithstanding any slight change of level, the flower rests on the surface of the water. A similar arrangement occurs in Valisneria.
Potamogeton is proterogynous (Delpino - Ult. Os-serv. Part ii. p. 22).
In the Amentaceae (oak, beech, willow, poplar, hazel, hornbeam, birch, alder, etc.) the flowers are unisexual, and generally monoecious; the males are, in some species - as, for instance, in the hazel - visited by insects for the sake of the pollen. As, however, they scarcely ever produce honey, the female flowers offer no attraction to insects, which consequently take no part in the fertilisation.