METHODS OF POLLINA TION 



189 



are visitors to many of these flowers. Examples are seen in the 

 columbine, nasturtium, etc. The outer parts of some flowers 

 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 



Fig. 14 



Epipactis with portion of perianth removed to show details. /, labellum; si, stigma; r, 

 istcllum; p, pollinium. When the insect approach 



rostellum; p, pollinium. When the insect approaches the flower, its head strikes the disk of 

 the pollinium and pulls the pollinium put. At this time the pollinium stands up out of the 

 way of the stigma. By the time the insect moves to another flower the pollinia have moved 

 downward so that they are in position to strike the stigma and leave the pollen. At the right 

 is the head of a bee, with two pollinia (a) attached. 



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 



