How the wallflower advertises.—Botanists have proved that a flower produces more, and also better, seeds when it is fertilised by pollen from another flower of the same species. This is called cross fertilisation. The wallflower relies upon bees for the transference of the pollen from one flower to another; and it is solely to attract them that the petals are so delicately scented and brilliantly coloured, and that sweet nectar collects in the sepal-pouches. The gaily coloured petals are therefore advertisement placards which are [Pg 93] hung out to attract the attention of bees. A bee comes to a wallflower for the sake of both nectar and pollen—the “bee-bread.” As the bee thrusts its proboscis down between stamens and pistil in search of the sweet liquid in the pouches, its head is pretty certain to come in contact with, and to brush off, some of the pollen dust hanging loose on the inner faces of the anthers. When the bee flies off to another wallflower and continues its search for nectar, it almost invariably leaves some of the pollen, from the first flower, on the hairy and sticky stigma of the second. In almost all cases when a flower is brightly coloured it depends upon the help of insects for cross fertilisation. 16. THE WALLFLOWER FAMILY. Fig. 60.—Shepherd’s Purse. (× ½.) 1. Shepherd’s purse.—Compare the shepherd’s purse (Fig. 60) with the wallflower. The flower is very much smaller, and white, but the parts have the same arrangement as in the wallflower, viz., four sepals, four petals arranged in the form of a cross, six stamens (two short and four long), and a central pistil, all arranged separately on the receptacle. Look down the plant, and notice that in the oldest (lowest) flowers, everything but the pistil has dropped off, and that this has become greatly enlarged to form a fruit. Cut some fruits open, both lengthwise and crosswise, and observe that each consists of two pocket-like chambers, separated by a thin partition on which the seeds are borne. Notice the manner in which the oldest fruits have opened naturally. [Pg 94] 2. Other relatives of the wallflower.—Compare also the flowers and fruits of the stock and candytuft, and also cress, mustard, radish (Fig. 61), cabbage, and turnip, which have been allowed to “run to seed.” Examine the roots of the turnip and radish. Make a note of the earliest dates on which you see the above plants in flower.