156 ORGANIC EVOLUTION 



zation, because of its more prompt sprouting and the more 

 rapid growth of its pollen tube. 



Let us now observe a few illustrations of special adap- 

 tions in the form of blossoms, by which plants secure cross- 

 fertilization. 



The flowers of Mitchella, the beautiful little partridge 

 berry of our woods, are adapted to secure cross-fertilization 

 by insects through having their stamens and pistils of dif- 

 ferent lengths (Plate 88). In all the blossoms of one plant 

 the stamens will be long and the pistils short, while in all the 

 blossoms of another plant these relations will be reversed, the 

 pistils being long and the stamens short. An insect visiting 

 these blossoms will have one part of its body dusted with 

 pollen from the short stamens and another part with pollen 

 from the long stamens. As it passes from blossom to blos- 

 som it will carry pollen from the short stamens of one flower 

 to the short pistils of other flowers, and the pollen from the 

 long stamens will be carried to the long pistils. In this way 

 cross-fertilization will be secured, since long stamens and 

 long pistils do not occur on the same plant, nor are short 

 stamens and pistils found on the same plant. 



Many orchids show an interesting method of using insects 

 to secure cross-fertilization. In these species the stigma is 

 in the centre of the flower, while the anther with its two 

 pollen masses lies above the stigma (Plate 89, A, 2 and j). 

 The two pollen masses protrude a little, and at their protu- 

 berant ends are attached to a sticky "rostellum" (Plate 89, 

 A, 4). The corolla of the flower is so developed as to form 

 a flat landing-stage for the visiting bee or other insect (com- 

 pare Plate 90, C), and the rostellum protrudes into the 

 centre of the flower, above this landing-place, in such a way 



