BIOLOGY OF THE FLOWER 177 



The long-styled form is known as ' pin-eyed ', and the 

 short-styled form as ' thrum-eyed '. 



Species which thus produce two kinds of flowers are said 

 to be dimorphic. (The Loosestrife has three kinds of 

 flowers, and hence is trimorphic.) 



The pistil of the Primrose consists of five carpels, superior 

 and syncarpous, but it differs from the previous examples 

 in that the numerous ovules are borne on placentas on 

 a central column, which is free from the ovary-wall, and 

 such an arrangement of ovules in an ovary is called ' free- 

 central placentation ' (Fig. 121, 3 and 4). Round the base 

 of the ovary, honey is secreted, and an insect visiting these 

 two kinds of flowers will receive pollen on its proboscis at 

 two different points : at its base will be the larger pollen- 

 grains of the short-styled form, and in the middle of it the 

 smaller pollen-grains of the pin-eyed form. Thus pollen- 

 grains of one form are likely to be transferred to the stigma 

 of the other form. Here, as in the Heaths, Stock, and other 

 plants, self-pollination may occur by pollen-grains falling 

 on to the stigma of the same flower. 



If a Primrose plant is covered with a bag to exclude 

 insects, and its flowers compared later with uncovered ones, 

 the latter will be found to produce more ripe seeds than the 

 former. It was by similar experiments that Darwin and 

 others showed how important insects are, as pollinators 

 of flowers. 



Small flowers massed in heads. The Daisy (Fig. 122) 

 shows specialization similar to that already noticed in the 

 Chervil, where small flowers, inconspicuous in themselves, 

 are rendered attractive by being massed together in a con- 

 densed inflorescence. In the Daisy and its allies this has 

 reached the highest stage of development ; condensation 

 has occurred to such an extent that the whole inflorescence 

 has the appearance of a single flower. Such a head of 

 florets (Fig. 64) is called a capitulum. 



1296 M 



