LECTURES AND ESSAYS. 423 



of plants, however, especially if the work is done by insects, are fertilized 

 by pollen from other plants. This desirable end is accomplished by various 

 modifications of the liowers. In some plants one set of organs matures 

 before the other, so that if the pistil is fertilized it must be done by pollen 

 from other flowers. 



Sometimes plants will bear long stamens and short styles, while other 

 plants of the same species will have short stamens and long styles, an 

 arrangement which renders the pistils likely to be fertilized by pollen from 

 other plants. In other cases the flowers have various appendages which 

 cause insects entering the flowers to come in contact with the pistil, thus 

 depositing the pollen, but on leaving, laden with pollen from the anthers, 

 the same, or other appendages shield the stigma from its touch. Nature is 

 very economical, and it is evident that all this pains would not have been 

 taken unless cross-fertilization is of some value to the plant. Experiments 

 by careful and painstaking observers have shown a marked difference, both 

 in the growth of the plants, and in the production of seed ; it is noticeable 

 in the first generation and a continuance causes the difference between close 

 and cross-bred plants to be still more marked. 



In a series of experiments by Darwin, 1 with plants of morning-glory, it 

 was found that in the first generation the crossed plants made a growth one- 

 third greater than those grown from self-fertilized ovules, and the number 

 of seeds produced was one-half more. The experiment was continued, one 

 set of seeds being cross, and the other self-fertilized, and the tenth genera- 

 tion produced nearly twice as much growth in length, from cross-fertilized 

 seeds, as was obtained from those self-fertilized, with a weight two and one-half 

 times as great. In the production of the crossed seed the pollen was taken 

 from different plants of the same generation, and as this was kept up, the 

 plants became quite closely related, and it came very near being in-breeding. 

 To see if the introduction of fresh pollen of a different stock, but of the 

 same variety, would have any effect, plants of the ninth generation were fer- 

 tilized, some with distinct pollen, and others intercrossed as usual. Tne 

 result was, that the plants fertilized with fresh pollen made a third more 

 growth, and produced twice as much seed as those intercrossed. 



The introduction of fresh pollen in a similar experiment with musk-plant 

 gave still more marked results, as the growth was as 100 to 56, and the seeds 

 produced were as 100 to 4, compared with plants grown from intercrossed 

 seeds. 



Another interesting experiment indicates that plants produced from ovules 

 fertilized with pollen from the same flower are as vigorous as those fertilized 

 with pollen obtained from different flowers on the same plant. 



While some kinds of pollen must be used at once, that of other plants will 

 remain fresh for a number of days, and in some cases has been found active 

 at the end of a year. 



The pollen of some kinds of plants will begin the development of pollen 

 tubes within a half minute after reaching the stigmas, but other species 

 require from twenty-four to thirty-six hours. The time necessary for the 

 tubes to reach the ovules also varies greatly, generally being only a few 

 hours, but sometimes days and even weeks elapse before the journey is 

 completed. 



1 Cross and self-fertilization in the vegetable kingdon, p. 28-41. 





