THE ANGIOSPERMAE 1277 



ness, were actually more fertile than the others, producing a significantly 

 greater weight of seed per hundred capsules. 



Despite these arrangements for cross-pollination in Primula, Darwin 

 was aware that very few visitors can be seen on some species, notably the 

 Primrose (P. vulgaris), which he surmised was pollinated by night-flying 

 moths. There has never been a definite solution of this question, but careful 

 observations by Dallman showed that moths can be ruled out. He found 

 that in cold and cloudy weather the Primroses are neglected, but that in 

 sunny warmth there are a number of visitors, especially the large dipterans, 

 Bombylius major and B. discolor, which are probably the chief pollinators, 

 assisted by small beetles and perhaps nocturnal slugs. 



The Buck Wheat, Fagopyrum esculentum, is generally heterostylous, but 

 equal-stvled flowers may occur on plants of either type. There is seldom 

 more than one such flower on the plant and it is usually the first which 

 opens. By " equal-styled " is meant the condition where stigmas and 

 anthers are both at the same level. It is notable that on short-styled plants 

 the equal-styled flowers have both styles and stamens long, and the reverse 

 on long-styled plants. Stevens showed in this species that after legitimate 

 pollination, the embryo had already begun development after 18 hours, 

 while at this time the pollen tubes in illegitimate pollination had scarcely 

 begun to grow. Between 72 and 96 hours were required in the latter case 

 for embryo development to begin, irrespective of whether the styles were 

 long or short. These flowers are so constantly visited by insects for the sake 

 of their nectar that, in nature, illegitimate fertilization would practically 

 never occur, owing to the prepotency of the " legitimate " pollen. 



An interesting case of dimorphism is presented by the tribe Staticeae 

 of the Plumbaginaceae, especially the genera Armeria and Limonium, which 

 have been investigated by Iversen and Baker. They are dimorphic both in 

 regard to styles and to pollen. In Limonium the styles differ in length but 

 in Armeria they are all the same length and the difference is limited to the 

 stigmatic papillae. Armeria maritima exists in two types, A and B (Fig. 

 1 214). The type A has relatively smooth stigmas and large, rough-coated 

 pollen grains. The B type has markedly papillose stigmas and smaller, 

 less rough pollen grains, but there is no difference in length of style between 

 the types. They are described as para-sterile, because the A pollen will only 

 germinate on the B stigmas and vice versa. Only the species of Armeria 

 native to the European-Mediterranean region are dimorphic, those in the 

 Arctic and in America and Asia are all monomorphic. In the Arctic spe- 

 cies A. labradorica, which is monomorphic and self- fertile, both A and B 

 pollen of A. maritima will germinate on the stigmas, but its own pollen 

 will only germinate on the stigmas of A. maritima, type B. This seems to 

 indicate that the monomorphic species has originated from the A type of a 

 dimorphic ancestor. 



More complex conditions exist in some other flowers. For example, 

 Eschscholtzia califoruica possesses four styles. Certain larger flowers pro- 

 duce two long and two short styles. The longer ones stand above the anthers 



