FLOWERS AXD INSECTS. 131 



flowers. In these flowers self-pollination is a necessity, and 

 is found to be very effective in producing seed. 



91. Yucca and Pronuba. — There can be no doubt, also, 

 that there is a great deal of self-pollination effected in 

 flowers adapted for pollination by insects, and that the in- 

 sects themselves are often responsible for it. But in the 

 remarkable case of Yucca and Pronuba there is a definite 

 arrangement for self-pollination by means of an insect (see 

 Fig. 136). Yucca is a plant of the southwestern arid regions 

 of North America, and Pronuba is a moth. The plant and 

 the moth are very dependent upon each other. The bell- 

 shaped flowers of Yucca hang in great terminal clusters, with 

 six hanging stamens, and a central ovary ribbed lengthwise, 

 and with a funnel-shaped opening at its apex, which is the 

 stigma. The numerous ovules occur in lines beneath the 

 furrows. During the day the small female Pronuba rests 

 quietly within the flower, but at dusk becomes very active. 

 She travels down the stamens, and resting on the open 

 pollen-sac scoops out the somewhat sticky pollen with her 

 front legs. Holding the little mass of pollen she runs to 

 the ovary, stands astride one of the furrows, and pierc- 

 ing through the wall with her ovipositor, deposits an egg 

 in an ovule. After depositing several eggs she runs to the 

 apex of the ovary and begins to crowd the mass of pollen 

 she has collected into the funnel-like stigma. These actions 

 are repeated several times, until many eggs are deposited 

 and repeated pollination has been effected. As a result of 

 all this the flower is pollinated, and seeds are formed which 

 develop abundant nourishment for the moth larva?, which 

 become mature and bore their way out through the wall of 

 the capsule (Fig. 136). 



92. Securing cross-pollination. — In very many ways flow- 

 ers are adapted to the visits of suitable insects. In ob- 

 taining nectar or pollen as food, the visiting insect receives 

 pollen on some part of its body which will be likely to 

 come in contact with the stigma of the next flower visited. 



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