GROWTH AND WORK OF PLANTS 



are covered with a sticky substance, which probably prevents 

 crawling insects like ants, which could not aid in cross-pollination, 

 from reaching the nectar. This is shown in the catchflies (Silene), 

 where the peduncle of the flower is sticky; in the mullein, where 

 the hairs on the calyx are sticky; and in the rhododendrons, where 

 the calyx and corolla are sticky. In many flowers of the pea 

 family the stamens are protected in the keel formed of the two 

 lower petals. Many flowers, as the dandelion, hawkweed, chic- 

 ory, pond lily, crocus, close on cloudy and rainy days and thus 

 are protected from rain. As stated above, self or close pollination 

 is prevented in many flowers: first, in imperfect flowers (monoe- 

 cious and dioecious flowers); second, flowers in which the stamens 

 and pistils are of different lengths; third, flowers in which the 

 stamens and pistils open at different times; fourth, the special 

 and peculiar structures found in many orchids, in the canna, etc., 

 where movements of certain of the floral organs, in connection 

 with insect visits, assist in cross-pollination, or the insect is led 

 into one part of the flower and out at the other in such a way as 

 to bring about cross-pollination. 



291. Imperfect flowers. In those flowers where the stamens 

 and pistils are in different flowers, often in different flower clusters 

 or on different plants, cross-pollination is necessitated. Many 

 of these are wind-pollinated as stated above in many of the trees 

 with catkins. Others of this group are insect-pollinated, as in 

 the willow and chestnut. Here the stamens and pistils are the 

 showy parts, and the nectar also attracts the insects. 



292. Flowers with stamens and pistils of unequal length. 

 In flowers where the stamens and pistils are of unequal length, 

 so that when the insect visits one flower one part of its body 

 comes in contact with the pistil and another part comes in contact 

 with the anthers, this part brushes off and carries some of the 

 pollen. In the next flower, or one of the succeeding ones which 

 the insect visits, the length and position of the stamens and 

 pistils will be reversed, so that the pollen from one of the pre- 

 viously visited flowers will be brought in contact with the stigma 

 and its viscid surface will pull off some of the pollen. The common 



