THE ANGIOSPERMAE 1275 



both these groups there is a continuous variation in the length of the style, 

 between the middle level of the corolla tube and exsertion of the stigma 

 above the flower. The first group rarely produces pollen and rarely has more 

 than sterile stamen filaments. The second group has good pollen and 

 stamens which vary in length like the styles. The suggestion of heterostyly 

 is negatived by the continuous nature of the variation in length and the fact 

 that any length of style may be combined with any length of stamen. The 

 flowers with dry stigmas never set seed, although the ovary and ovules 

 appear perfect and the species is therefore functionally dioecious though 

 structurally hermaphrodite. 



Dioecism is usually associated with some degree of protogyny and the 

 female plant not only flowers sooner but often lives {Cannabis, Trinia) 

 longer than the smaller and slighter male. There may be quite a marked 

 difference in the growth-habit and appearance of the two sexes. Cannabis 

 indica is a case in point (see Fig. 1630). Other examples are Osyris (Santa- 

 laceae) and Myzodendron (Myzodendraceae) where foliage and habit are 

 quite distinct. 



Flowers which are dimorphic or heterostylous are sometimes regarded 

 as being on the road towards dioecism. True heterostyly, however, as 

 Darwin showed, is not definable purely on the morphological differences 

 between flowers in respect of the length of style, but includes the physio- 

 logical difference that neither type is fully fertile unless pollinated by the 

 other. The condition is remarkably widespread and has been recorded 

 from seventeen families, including Primulaceae, Oxalidaceae, Polygona- 

 ceae, Lythraceae, Plumbaginaceae, Gentianaceae, Boraginaceae and Rub- 

 iceae. In only a relatively small number of cases has the effectiveness of 

 pollination been tested. 



The classic case of Pritnula (P. veris) (Fig. 12 13) was the first to be 

 described in detail, by Darwin, who sums up the differences thus : The 

 long-stvled flowers have globular and much rougher stigmas, covered with 

 papillae which are twice to three times as long as in the short-styled flowers, 

 and standing high above the anthers. The stamens are short, the grains of 

 pollen smaller and oblong in shape, though the anthers are the same size 

 in both types of flower. The upper half of the corolla tube is more expanded. 

 The number of seeds produced is smafler and the ovules larger. The plants 

 tend to flower first. 



The short-styled flowers bear the stigma at about half the height of the 

 corolla tube, with a smooth, depressed stigma standing well below the 

 anthers. The stamens are long, the grains of pollen are spherical and larger. 

 The tube of the corolla is of uniform diameter except close to the upper end. 

 The number of seeds produced is larger. 



There are no intermediate forms, the two types are always borne on 

 different plants, and plants of one type never change into the other. The 

 two types exist in nature in approximately equal numbers. Most species 

 of Primula have analogous differences between their flowers, but a few are 

 homostylous and self-fertile, including the British species P. scotica. Dar- 



