100 THE STORY OF PLANT LIFE 



48. The Periwinkle Group. 



In the Order Apocynaceae only two species are 

 included which are found in the British Isles, the 

 Greater and the Lesser Periwinkle. 



About one thousand species of Apocynaceae which 

 are included in one hundred and thirty odd genera 

 are known. They are chiefly tropical, but a few are 

 found in the temperate zone. The members of this 

 order differ from the Gentian group in the fact that 

 the ovary is completely divided into two cells or two 

 carpels which are distinct, and the stigma is entire. 

 The affinities of the Order are with the Asclepiadeae 

 (where, however, anthers and stigma are united) and 

 Rubiaceae. ' 



Most of the plants are trees or shrubs, but a few 

 are herbaceous, and most are twining plants and 

 climbers, rarely erect. They contain a milky juice. 

 Some of the tropical plants are true lianes. The leaves 

 are simple, opposite, rarely in whorls, entire, some- 

 times evergreen as in Vinca. There are no stipules, 

 or when present they are exceedingly rudimentary, 

 and are interpetiolar and small. 



The inflorescence is a panicle at first, the later 

 branches cymose. The flowers are regular, sometimes 

 solitary, and possess both bracts and bracteoles. The 

 flowers are hermaphrodite. The parts of the flower 

 are in fours or fives. The calyx is gamosepalous, with 

 five sepals, the lobe deep, quincuncial, and the odd 

 sepal is posterior. The calyx is inferior. The corolla 



