THE DICOTYLEDONES 



1769 



and may be either tetramerous or pentamerous. The perianth usually 

 consists only of petaloid sepals, but a corolla is present in some genera. The 

 flowers are strongly perigynous, the apparent perianth tube being due to the 

 cup-shaped receptacle which often remains embedded in the fruit. The 

 stamens may be equal in number to the sepals, 

 or may be half or twice as many. The ovary 

 is generally unilocular with one pendulous, 

 anatropous ovule. The flowers are entomo- 

 philous, nectaries usually occurring at the base 

 of the ovary. 



Many of the species are found in dry situ- 

 ations, appearing frequently where the vegeta- 

 tion is periodically destroyed by fire. Such 

 species often possess stout woody rhizomes 

 which give off clumps of erect branches and 

 terminate in brightly coloured inflorescences. 



In Britain the family is represented by only 

 two species. Daphne mezereum which, though 

 very rare wild, is often planted in gardens, 

 and D. laiireola, the Spurge Laurel, which 

 is a small, evergreen woodland shrub. The 

 family is a natural one but shows no very close 

 afiinities with other groups. 



The Lythraceae are a cosmopolitan family 

 of some twenty-one genera and 500 species and 

 are represented in Britain only by Lythrum 

 salicaria, the Purple Loosestrife (Fig. 1655), 

 L. hyssopifolia and PepUs portula, the Water 

 Purslane. 



Lythrum salicaria deserves some notice because of the complex pollina- 

 tion mechanism which it exhibits (Fig. 1656). Three forms of the 

 flower appear, each on a separate plant. The flowers all possess six sepals 

 and six petals, and are developed in verticillasters similar to those in the 

 Labiatae. It is in the arrangement of the twelve stamens and in the length 

 of the single style that the differences occur. (See also p. 1278.) The top 

 figure represents a longitudinal section of the long-styled flower, the centre 

 figure represents the mid-styled flower, while the lowest figure shows the 

 short-styled flower. If S denotes the style and A the anthers we may 

 represent the arrangement of these flowers in the following way. The 

 numbers refer to the relative lengths of the organs. 



Long-stvled flower Mid-styled flower Short-styled flower 



S.3 A.3 A.3 



A.2 S.2 A.2 



A.I A.I S.I 



When the flowers are visited by insects there will be a tendency for pollen 



Fic;. 1655. — Lyt/iruiu Siilicarici. 

 Purple Loosestrife. Flower- 

 ing shoots. 



