THE DICOTYLEDONES 1919 



often cultivated in gardens. The genus Calceolaria contains about 200 

 species mainly in South America. Many are cultivated and a large number 

 of ornamental hybrids have been produced. The genus Antirrhinum which 

 contains thirty-three species is widely distributed in the northern hemi- 

 sphere. One species, A. majus (Fig. 1844), the Snapdragon, is found in 

 Britain but is probably an escape. Many hybrid forms have been produced 

 which are generally used in summer bedding. The recent spread of a Rust 



Fig. 1844. — Antinliinniii majua. Snapdragon. Flower in 

 profile, showing " personate " form. 



[Puccinia antirrhini) has caused great losses among these bedding plants 

 and somewhat reduced their popularity. Rust-resistant varieties have 

 recently been produced. 



The pollination mechanism in the Snapdragon (Fig. 1845) is worthy of 

 special mention. These flowers can only be pollinated by humble bees, 

 for the entrance is completely closed by the upper and lower lips of the 

 corolla. The lower lip possesses two swellings which serve as an alighting 

 platform and fit into two depressions in the upper lip. The anthers are 

 enclosed within the corolla and lie closely against the upper lip. When 

 ripe, the pollen is liberated in two rounded balls which adhere to the back 

 of a bee which visits the flower to reach the nectar secreted at the base of 

 the ovary. Visits by small insects, including small bees which would not 

 effect pollination, is prevented by the closed flower, for only a humble bee 

 is heavy enough to depress the lower lip and thus gain access to the nectar. 

 Small bees may sometimes visit the older flowers, which are not entirely 

 closed, in the hope of finding a little residual nectar. Humble bees some- 

 times rob the flowers by biting through the corolla tube and sipping the 

 nectar from the outside. Should cross-pollination fail self-pollination is 

 possible in some species by pollen adhering to the hairy lining of the lower 

 lip, which is brought into contact with the style. 



Both in species of Antirrhinum and also in Linaria, nectar guides are 



