THE DICOTYLEDONES 1883 



few, as for example Vinca (Periwinkle) (Fig. 1794), are commonly natura- 

 lized in this country. Most of the genera are shrubs or twiners, and in the 

 tropics they form large lianas. The vascular bundles are bicollateral and 

 latex is always present. I'he flowers are usually produced in panicles, but 

 are rarely solitary. 



Fig. 1794. — Vinca minor. Lesser Peri- 

 winkle. Flower. 



The Asclepiadaceae are also a fairly large family containing nearly 

 2,000 species arranged in 280 genera. They are mostly erect herbs or woody 

 climbers but some are succulent. They are mainly tropical in distribution 

 occurring especially in the drier parts of Central and South America. 

 Anatomically they resemble the Apocynaceae in having bicollateral bundles 

 and latex. They also frequently possess a mycorrhizal fungus in their 

 roots. The flowers are remarkably uniform, with three whorls ot five 

 alternating sepals, petals and stamens. The ovary is bicarpellary and the 

 ovaries remain free, though the two styles are united into a common stigma. 

 The fruit consists of a pair of follicles. Few of the genera are commonly 

 grown in this country, but we may mention Stape/ia, which is sometimes 

 grown as a succulent, and Ceropegia, which has already been referred to on 

 account of the swollen, aerial tubers which it produces at intervals along 

 its stem. These serve as a means of vegetative propagation. Species of 

 Asclepias (Fig. 1795) are woody plants sometimes cultivated in greenhouses 

 on account of their bright red flowers which last for several weeks. 



The pollination mechanism of Asclepiadaceae calls for special comment, 

 for it is unique and is one of the most remarkable among flowering plants. 

 In Vincetoxicii/n officinale the ovary is surrounded by a fleshy column, 

 formed by the fusion of the stamens, and is covered by a fleshy disc, under 

 which are the entrances to the five stigmas. On the staminal column are 



