1944 



A TEXTBOOK OF THEORETICAL BOTANY 



pollination is ensured by dioecism, there being four different kinds of 

 flowers: firstly, male flowers with no stigma and with a large corolla; 

 secondly, male flowers with a rudimentary stigma and a small corolla; 

 thirdly, female flowers with vestiges of stamens and small corollas; and 

 finally female flowers with no stamens and even smaller corollas. In V. 

 officinalis the anthers develop first and the trifid style later, thus ensuring 

 cross-pollination. 



In Centranthtis ruber (Fig. 1873) the corolla tube is divided longitu- 

 dinally by a septum into two parts, in one of which is the style. Only one 



Fig. 1873. — Centranthtis ruber. A, Group of 

 flowers showing spurs and exsertcd stamens. 

 B, Flower in first, male stage. C, Flower in 

 later, female stage with stamen recurved and 

 stigmas expanded. 





stamen develops. In the first stage this stamen lies in the mouth of the 

 flower covering over the immature style. Later after the pollen has shed, it 

 bends backwards over the corolla and the bifid stigma now occupies the 

 centre of the flower. Thus an insect visiting a young flower in search of the 

 enclosed nectar will touch the anther with its head but when visiting an 

 older flower will encounter the stigma and deposit pollen on it. 



The Dipsacaceae are mainly centred in the Mediterranean region, but 

 spread thence over a wide area and even reach South Africa. Several 

 species are found in Britain. The family contains ten genera and about 150 

 species. They are mostly herbs, with opposite leaves, but a few are shrubs. 

 The flowers are hermaphrodite and zygomorphic. The calyx is epigynous 

 or divided into pappus-like segments, and each flower has a cup-shaped 

 involucre made up of a pair of bracteoles. The corolla is divided into four or 

 five lobes of varying sizes. The stamens are basically four but the number 

 may be reduced by abortion. These alternate with the corolla lobes, and are 

 inserted at the bottom of the tube. The filaments may be free or may be 

 united in pairs. The ovary is inferior and unilocular but derived from two 

 carpels. There is a single pendulous ovule. The fruit is dry and one-seeded, 



