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.
There are two British genera belonging to this order; Dipsacus (the Teasel) and Scabiosa. The so-called flower is a compound flower head, as in the Composite, from which, however, this group may be at once distinguished by possessing free anthers. Each floret, moreover, is inserted in a small "involucel."
Dipsacus is a proterandrous genus. The pistil terminates in two lobes, the upper surfaces of which constitute the stigma. As, however, in consequence of the stiff spines which radiate on all sides from the flower heads of this plant, the humble bees, by which it is principally fertilised, can only touch the florets with their heads, the two lobes often get in one another's way, and according to Muller it would be a distinct advantage if one of them were absent. He points out also that in fact one of them is sometimes rudimentary, or even occasionally altogether absent. This adaptation then, it would seem, has actually commenced. The leaves form a cup round the stem in which water accumulates, and many small insects are drowned. These it has been supposed contribute to nourish the plant, and Mr. Francis Darwin has observed that protoplasmic filaments extend into the liquid.
Scabiosa arvensis is also proterandrous. About 50 florets are united in one head; they increase in size from the centre towards the circumference, while in Sc. columbaria the outer row is considerably larger than the rest, and in Sc. succisa they are nearly equal in size. The honey is at the base of the tubular florets, which, however, are more or less funnel-shaped at the mouth, thus greatly facilitating the access of insects. Not only are the florets proterandrous, but this is the case with the whole head; for, though the anthers come to maturity slowly and (as a general rule) successively from the edge to the centre, none of the stigmas emerge until the anthers have all shed their pollen, when they rapidly come to maturity. The male condition of the flower-head lasts several days; the stigmas, on the contrary, come to maturity almost simultaneously. This difference is obviously an advantage. From the length of time during which the anthers are ripening, whenever there is a sunny day, and the insects are abroad, they are almost sure to find some anthers read to dust them with pollen. On the other hand, the stigmas being mature at the same time, they are capable of being fertilized by a single visit.
Besides the flower-heads with hermaphrodite florets, there are others which contain female florets only, the stamens being more or less rudimentary. This is also an advantage, because if it were otherwise the quantity of pollen would be unnecessarily large. Scabiosa arvensis is visited by a great variety of insects belonging to several orders.
Sc. columbaria has a row of distinctly larger ray florets, while the central ones are all of equal size; the florets also are smaller than in Sc. arvensis; and consequently, in heads of the same size, more numerous; the florets appear to be all hermaphrodite; and the ripening of the anthers does not take place, successively, from the outside.