THE DICOTYLEDONES 1935 



of the visitor. When the insect withdraws its head the pressure on the lever 

 is released and the anthers return to their former position. 



In an older flower in which the bifid stigma is mature we find that the 

 style has elongated and now occupies a position in the front of the flowers 

 where it must inevitably be brushed by an insect visiting the flower. Varia- 

 tions in the form of the flowers are known, large-flowered and small- 

 flowered hermaphrodite types occurring as well as large and small female 

 forms in which the lever mechanism is greatly reduced. 



In the genus Lamiiim the mechanism is simple. Here the flo.wers 

 are only visited by bees and the shape of the corolla exactly fits the 

 body of the insect. Nectar is secreted at the bottom of the corolla tube 

 and is usually protected by a circle of hairs above the secretion. The 

 upper lip shelters the anthers while the lower lip serves as a platform for 

 visitors. The stigma is bifid, one lobe lying directed horizontally outwards 

 and the other turned vertically downwards. When a bee enters the flower 

 it first touches the lower lobe of the bifid stigma and afterwards the anthers, 

 which mature simultaneously. The second lobe of the stigma is directed 

 out of the flower and therefore lies between the anthers. Should cross- 

 pollination fail, the flower is almost certain to be self-pollinated owing 

 to the simultaneous maturation and close proximity of the stigma and 

 anthers. Insect visits are however frequent and few flowers are ever missed. 

 Short-tongued bees unable to reach the nectar by the orthodox method 

 sometimes perforate the corolla tube. 



RUBIALES 



The Rubiales are Metachlamydeae in which the flowers are herma- 

 phrodite and either actinomorphic or zygomorphic, and the parts arranged 

 either in fours or fives. The stamens are usually equal in number to the 

 corolla lobes, occasionally fewer. There may be five carpels but that 

 number is usually reduced, often to one. The ovary is inferior, with one or 

 more chambers containing from one to many anatropous ovules with axile 

 placentation. The seeds mostly contain endosperm. 



The plants are trees, shrubs or rarely herbs with opposite entire leaves 

 and occur mainly in the tropics. 



The group is a natural one and the families included in it show a 

 gradual reduction in the number of floral parts and an increasing tendency 

 to zygomorphy as a result. There is some difference in opinion as to the 

 precise line to be drawn between the Rubiales and the following order, the 

 Asterales. Rendle, following the older treatment of Engler, recognizes 

 five families in the order, while Hutchinson includes only the first two, the 

 three others being transferred to the Asterales. Here we shall follow 

 Hutchinson and retain only the families Rubiaceae and Caprifoliaceae in 

 the Rubiales. 



The Rubiaceae are a large and cosmopolitan family containing nearly 

 400 genera and 4,500 species. Many are found in the tropics but some 

 2F 



