THE LOWER DICOTYLEDONS. 



325 



placenta, so that the ovary becomes two-chambered, though 

 the ovules are on parietal placentas. 



When the " pod " ripens, the wall splits off 

 into two valves (right and left) from below 

 upwards, leaving a frame consisting of the 

 thickened placentas (each bearing two rows of 

 seeds) with the thin partition (" false septum ") 

 stretched between them (Fig. 128) ; the "pod" 

 opens in dry weather and the thin flat seeds 

 may be carried by the wind. This kind of 

 capsule is termed a siliqua ; it is very charac- 

 teristic of the Wallflower Family, though occur- 

 ring in a few other families besides. 



Fig. 128. Fruit 

 of Wallflower. 



335. The Wallflower Family (Cruciferae) 

 is easily recognised by the cross-like arrange- 

 ment of the four petals, the general similarity 

 of the other flower-parts to those of Wall- 

 flower, and by the fruit, which is nearly always a siliqua or a 

 silicula (short broad siliqua). 



336. Flower Mechanisms in Cruciferae. The flowers 

 are generally homogamous, though often protandrous but 

 with a relatively long overlapping period during which self- 

 pollination may occur. The flowers are visited by flies when 

 the sepals are short or spreading and the petals short- clawed, 

 and some of the small-flowered types (e.g. Shepherd's Purse, 

 whose flowers often have only 2-4 stamens when produced in 

 the colder months) are regularly self -pollinated. The larger 

 flowers, in which the sepals are erect and hold the clawed 

 petals together so as to form a sort of flower-tube, are visited 

 by bees and butterflies, the honey being partially concealed 

 and protected from rain. The large light-coloured evening- 

 scented flowers of Hesperis (Dame's Violet) and some Stocks, 

 etc., are visited by moths. 



Note the general tendency of the raceme to form a round- 

 or flat-topped inflorescence while the flowers are opening 

 the raceme is nearly always corymbose so as to make a con- 

 spicuous mass of flowers. This is especially marked in 

 Candytuft, where the flowers, especially the outer ones, have 

 a zygomorphic corolla. After flowering, the raceme-axis 



