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THE CONTINUITY OF THE RACE 



We have seen that in the Metazoa bisexual reproduction is dependent 

 upon some appropriate behavior of the parent organisms that brings the 

 motile sperms within a short distance of the egg. Although the lower 

 plants have male gametes that are motile and are capable of swimming 

 short distances to meet the egg, in the flowering plants the pollen grains 

 lack any means of locomotion and must be passively carried to the stigma. 



Self-pollination. In contrast to the rarity of self-fertilization among 

 the Metazoa, self-pollination is, for a number of important groups of 

 plants, a normal and almost invariable process. In the bean or pea for 

 example, the anthers and stigma are enclosed within the other floral 

 parts, and pollination is accomplished by the pollen falling directly from 



Fig. 17.17. The flower of a beardtongue (Penstemon digitalis, family Scrophulariaceae), 

 showing adaptations to insect pollination. The outer surface of the flower (left) has sticky 

 glandular hairs that discourage robber bees from puncturing the base of the flower for 

 nectar instead of entering it. Shortly after the flower matures (center), the ripe anthers 

 take a position in which pollen is shed on the backs of insect visitors. Later (right) the 

 stigma pushes down into the position previously occupied by the anthers, and is thus 

 ready to receive pollen brought by insects from other beardtongues. Ripening of pollen 

 before the stigma becomes receptive, accompanied by mechanical devices such as this, is 

 a common means of ensuring cross-pollination. (Photos by Prof. E. B. Mains.) 



the ripe anthers to the stigma. In other self-pollinating flowers a properly 

 timed growth of the pistil thrusts the ripe stigma against the anthers as 

 they open to expose the pollen grains. 



Cross-pollination. In most of the flowering plants cross-pollination 

 is the rule, and here some other agent than gravity or the growth of the 

 pistil must be utilized. In most flowers this is effected by animal visitors. 

 The flowers are specially adapted to attract such visitors by having 

 nectaries that secrete a sweet fluid called nectar and by producing quan- 

 tities of pollen in excess of their own needs. Bees are the commonest 

 pollinating agents, but other animals such as hummingbirds, butterflies, 

 moths, beetles, and certain other insects also play a part. Almost all 

 flowers that are not insect-pollinated are wind-pollinated; they produce 

 vast quantities of pollen, which is broadcast in the air and reaches its 



