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.
The Wallflower, Stock, Cabbage, Shepherd's Purse, Watercress, etc, belong to this group.
The Cruciferae are easily distinguished from other orders by their four sepals and petals, and six stamens; but the genera into which they are divided are by no means so well marked, and are to a great extent distinguished by differences in the pods and seeds. The general structure of the flower is more or less similar throughout the order, but the number and position of the honey-glands differ in almost every species. Hesperis matronalis is one of those plants which are specially odoriferous in the evenings, and is therefore probably in most cases fertilised by moths, though it is also visited by day-insects, as for instance, by the hive bee, the white butterflies (Pieris brassicae, P. rapiy and P. napi), Halictus teitcopus, Andrena albicans, Volucella pellucens, Rhingia rostrata, etc.
But though the colour, honey, and scent of the Cruciferae have evident reference to the visits of insects, this order does not offer so many special and specific adaptations as we shall meet with in other groups; and the majority of species, at any rate, appear to have retained the power of self-fertilisation.
Flowers bisexual, small, greenish, sometimes scented irregular. Sepals and petals 4-7. Stamens many inserted on a broad disk. Pistil one, with 2-3 stigmas.
This order is represented in Britain by one genus Reseda (the Mignonette), containing three species. In the common garden mignonette the upper half of the base of the flower raises itself between the stamens and the sepals into a quadrangular, perpendicular plate, which is first yellowish, and after the flower has faded, brown. It is enclosed in a sort of box, the three upper petals forming the lid. Its hinder surface secretes honey. The mignonette is said to be specially frequented by bees of the genus Prosopis.
This order contains only a single British genus, Helianthemum, with four species. The flowers do not secrete honey. The stamens are numerous. As the pistil projects above them, insects, in alighting on the flower, generally touch the pistil before the stamens; and cross-fertilisation must therefore often take place. At the same time, if from any cause insect-visits are deferred, the flower is almost sure to fertilise itself.