FAMILIES OF FLOWEKING PLANTS 223 



Several very poisonous trees of the Apocynaceae are of interest as 

 being " ordeal trees." The most familiar of these is the tanghin, Tan- 

 ghinia venenifera, confined to Madagascar. The natives use the seeds 

 in the trials of evil-doers, forcing each culprit to swallow a portion. In 

 case the seed is retained in the stomach it proves immediately fatal, and 

 guilt is then considered estabhshed. If on the other hand it is vomited, 



Fig. 193. The Swamp Milkweed {Asdepias incarnatd). After Britton 

 and Brown, 111. Fl., Northeast, U. S. 



recovery usually ensues, and the prisoner is released as innocent. In 

 Polj^nesia Cerhera is used in a similar manner. 



Family Asclepiadaceae. Milkweed Family. Contains about 220 

 genera and 1,900 species, of wide distribution. They are herbs, vines 

 or slirabs, with milky juice, and flowers borne usually in umbels. The 

 corolla and calyx are 5-parted ; between the corolla and stamens is a 

 5-parted body, known as a crown or corona. The pollen is coherent in 

 masses, as in the orchids, and exhibits many interesting adaptations to 

 cross-fertilization through insect agency. The ovary consists of two 

 carpels, which become follicles in fruit ; the seeds are usually appen- 

 daged with a long tuft of hair or coma. The milkweeds proper {As- 

 clejjias) are among our most showy wild flowers ; the different species 

 exhibit orange, yellow, pink, purple and white flowers (Figs. 192 and 193). 

 In the desert regions of northern Africa there are a number of leafless 

 succulent plants belonging to tliis family ; these, with Euphorbias of 

 similar habit, replace the cacti of our western plains. The juice is often 

 poisonous, like that of the dogbanes, but in some cases it possesses val- 

 uable emetic and purgative properties. The family is very widely dis- 

 tributed, but is particularly abundant in South Africa. 



