1840 A TEXTBOOK OF THEORETICAL BOTANY 



contains. Euphorbia pidcherrima is the Poinsettia so frequently grown in 

 greenhouses for the sake of its large scarlet bracts. 



Fig. 1742. — Euphorbia splemiens. Apex of one 

 of the succulent stems with an inflores- 

 cence. 



The pollination mechanism employed in the genus is worthy of note. 

 The inflorescence, which behaves in pollination like a single flower, is 

 strongly protogynous. The three bilobed stigmas emerge first from the 

 involucre and may be dusted with pollen if the flower is visited by an insect. 

 Later, when the ovary on its long, curved pedicel has grown out and hangs 

 down below the involucre, the stamens gradually elongate one after another, 

 and come to occupy the place where the ovary formerly was situated. 

 Pollination is generally eflfected by flies, though wasps and beetles visit the 

 flowers occasionally. They are attracted by glands situated on the involucre 

 which secrete nectar in a completely exposed layer. Apparently the incon- 

 spicuous nature of the flowers keeps away bees, but where large masses of 

 flowers are found together, bee visits do sometimes occur. 



Among the genera not found wild in Britain the most familiar is 

 Ricinus. The genus is monotypic. R. communis is a shrub in the tropics 

 but when found in Europe it is herbaceous. The plants (Fig. 1743) are 

 monoecious and the androecium is peculiar, for the stamens are branched, 

 treelike structures, with the anthers borne on short terminal branchlets. 

 The fruit explodes, when ripe, into the separate carpels, which at the same 

 time open and drop the seeds. These seeds possess a caruncle and contain 

 a large quantity of oily endosperm which is used medicinally as the source 

 of Castor Oil. It is also employed as a lubricant. The plant is a native of 

 Africa but on account of its economic use it is widely cultivated. It is 

 grown under glass in this country on account of its highly decorative foliage. 

 The genus Acalypha is notable for having stamens with long appendages. 

 It is a large tropical and subtropical genus with some 400 species. 



