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A TEXTBOOK OF THEORETICAL BOTANY 



A B 



Fig. 12 1 3. — Primula veris. Heterostyly. A, Long-styled flower. B, Short- 

 styled flower. The two forms are known traditionally as " pin-eyed " and 

 " thrum-eyed " respectively. {After Dandn.) 



win showed in Primula veris that polhnation of short-styled by long-styled 

 flowers, or vice versa, gave more than twice the number of good capsules 

 per hundred, and more than three times the weight of good seed, in com- 

 parison with the pollination of each type of flower with pollen from the same 

 type. Here there is a specialized kind of sexual segregation. Pollination, 

 even from another individual, is relatively ineffective if the flowers are of 

 the same type. Not cross-pollination with any other individual, but only 

 with a limited class of individuals carries with it the benefits of crossing. 

 We shall see presently that a similar limitation obtains in some other 

 plants where no difference in flower type is present. 



The two types of flower are determined by a single Mendelian character. 

 The long-styled type is homozygous and recessive, the short-styled type 

 is heterozygous and dominant. Short-styled plants if self-pollinated give 

 short and long-styled offspring in the 3 : i ratio. Long-styled plants if 

 selfed give only long-styled plants. 



Darwin called the fully fertile pollination " legitimate " and the less 

 fertile one " illegitimate ", and he showed that the disproportion in the 

 fertility of the two kinds of union was even greater in some other species of 

 Primula than in P. veris. The Oxlip, P. elatior, is exceptional in that it 

 produces equal-styled as well as heterostyled flowers and that both long- and 

 short-styled flowers may occur on the same plant. The same kind of rela- 

 tionships in regard to pollination hold good, however, as in other species. 



The impression that heterostyly was an approach towards morphological 

 dioecism, Darwin showed was incompatible with the observation that the 

 short-styled flowers, which might be supposed to be tending towards male- 



