STRUCTURE AND ARRANGEMENT OF FLOWERS. 19 



self-fertilized seed are ahvays stronger and more vigorous than those 

 from cross-pollinated seed when the crossing is within the variety. 

 In SprengeFs discourse on the cross and self pollination of plants 

 the statement is found that '* nature seems to have wished that no 

 flower should be fertilized by its own pollen." Later. Darwin stated 

 that " nature abhors perpetual self-fertilization," but, unlike Spren- 

 gel, recorded a number of exceptions to this rule, and tobacco was 

 among them. The experiments of the writers conclusively substan- 

 tiate the findings of Darwin in this connection. They have found 

 self-pollination in the case of tobacco to be most desirable in all cases. 



A very interesting phenomenon of growth takes place in the fila- 

 ments of the stamens immediately after the opening of the flowers, 

 which can be taken* as an evidence of the natural self-fertilizing 

 habits of the plants. An examination of the flower just previous to 

 the time of opening will reveal the fact that the pistil is longer and 

 extends up beyond the stamens, but when the anthers open and the 

 stigma becomes receptive a very rapid growth of the filaments takes 

 place, which causes the open pollen sacks to be pushed up past the 

 stisma, and in almost all cases thev come in direct contact with the 

 stigma in passing upward. This gives an opportunity for at least 

 a portion of the pollen grains to adhere to the viscous surface of the 

 stigma and for self-fertilization to take place, as shown in figure 3. 

 It is just before this process occurs or while it is in progress that 

 there is danger of, or opportunity for, cross-pollination. The open 

 flower contains a small drop of nectar at the base of the corolla tube, 

 which is sought by honeybees, bumblebees, and humming birds, as 

 well as by many species of minute insects, all of which carry pollen 

 from flower to flower and from plant to plant in their constant search 

 for the honey-like substance secreted in the corolla tube. In passing 

 in and out of the flowers the bodies of the bees and other insects and 

 the beaks of the humming birds become dusted with pollen, which is 

 transported by them to the pistils of the next flower visited. The 

 ovules are as readily fertilized by pollen from the surrounding- 

 plants as by the pollen from the flower in which they are produced. 

 This continuous crossing necessarily results in the introduction and 

 intermixture of poor and undesirable varieties in our best strains of 

 tobacco. 



The observations of the writers plainly show the absolute necessity 

 for protecting the flowers of the seed plants from cross-pollination. 

 Careful experiments have demonstrated that in many instances the 

 stigma of the tobacco flower remains in a receptive condition for 

 three or four days. This condition results in a twofold disadvantage 

 when no protection is used against cross-pollination. In the first 

 place, it affords abundant time and opportunity for complicated 



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