576 Insects and Flowers 



zation in orchids, namely, the development of sensitive parts in the flower, 

 so that with a proper stimulus certain purposeful motions or movements 

 are performed by certain of the floral parts. Most of the orchids offer their 

 pollen in masses, pollinia, which adhere to the insect and are carried around 

 by it during its visits to other flowers. The stalks of these pollinia bend 

 (by contracting) after they are attached to the insect so as to bring the pollen- 

 masses into the most effective position for insuring contact with the stigmatic 

 surfaces of the flowers visited. In the remarkable orchid Catasetum, a 

 certain part of the flower is endowed with such sensitiveness and is nor- 

 mally restrained in such a tense position that when it is touched by an insect 

 (or any foreign body) it springs in such a way as to throw the pollinia at 

 and against the intruder. Darwin once irritated one of these flowers in 

 the presence of Lubbock, who was amazed to see the pollinium thrown 

 " nearly three feet, when it struck and adhered to the pane of a window." 



Some other flowers, not orchids, also possess sensitive parts; familiar 

 examples are various species of Berberis, whose stamens "when touched 

 near the base, as happens when a bee is probing for honey, will spring vio- 

 lently inward, shaking off the pollen and scattering it upon the insect visit- 

 ors." Kalmia presents a somewhat similar case "where the stamens are 

 bent over into little pockets, from which they spring out when touched, 

 throwing the pollen to some distance." 



In the examples thus far chosen the flower has been the more conspicu- 

 ous beneficiary in the partnership, and has shown the chief adaptations. 

 The advantage to the insect visitor is almost exclusively a food advantage, and 

 its adaptation has been usually simply one of the structure of its mouth-parts. 

 But there is known at least one case in which the insect pollinator does much 

 more for itself by its flower visits than find food for immediate use, and in 

 which an amazing adaptation of habit has arisen on its part. On the other 

 hand the plants concerned depend solely on the one insect kind for pollination. 

 This is the famous case of the cross-pollination of Yuccas by the small moths 

 of the genus Pronuba. There are several species of Yuccas (Spanish bayo- 

 nets) in this country, and several Pronubas, but a brief account (taken largely 

 from Stevens's Introduction to Botany) of the relations between the com- 

 mon Yucca grown in gardens (Y. filamentosa) and the moth species, Pronuba 

 yuccasela, will be typical of the interrelations of all. 



The Yucca has a lily-like flower composed of three sepals and three 

 petals, all creamy white, six stamens with fleshy outward-curving filaments 

 surrounded by small anthers, and a pistil extending much above the tops 

 of the stamens with three carpels imperfectly united at the top, and thus 

 leaving a tube entirely open at the apex. "The inner surface of this tube 

 is stigmatic. This stigmatic tube does not open directly into the cavities 

 of the ovary, but sends off three very narrow branches, each of which com- 



