FLOWERS AND THEIR USES 277 



led many observers to conclude that cross-pollination is of 

 greater benefit to plants than self-pollination. Darwin 

 found that in many plants, though by no means in all that 

 he studied, cross-pollination results in more vigorous and 

 more productive offspring than self-pollination. Especially 

 is this the case if the pollen and stigma are borne in flowers 

 on different plants. The results of cross-pollination between 

 flowers on the same plant are more hke those of self-pollination, 

 although even this kind of cross-pollination is sometimes 

 more favorable than self-pollination. 



The difference between the effects of cross-pollination be- 

 tween flowers on the same plant and those of cross-pollination 

 between flowers on different plants is shown by some recent 

 experiments. It is found that if the silk of Indian corn is 

 dusted with pollen from the tassels of another plant and is 

 kept free from the pollen of the same plant, the offspring is 

 as a rule more vigorous, grows to a larger size, and bears 

 more fruit, than that which results if the silk is dusted with 

 pollen from the same plant. On the other hand, self-pol- 

 lination often occurs in nature, and there are many kinds 

 of plants in which it is the regular method' and which do 

 not seem to suffer from lack of self-pollination. However, 

 in varieties of tobacco that are ordinarily self-pollinated, it 

 is found that sometimes (though not always) plants of greater 

 vigor result from cross-pollination. 



286. Arrangements that Favor Cross-Pollination. — One 

 of the simplest ways of securing cross-pollination between 

 flowers on different plants is followed when the staminate 

 and pistillate flowers are separate and borne on different 

 plants. This is the case in the willows, the poplars, and the 

 hop. The disadvantage of this arrangement is that both 

 kinds of plant (that is, those with staminate and those with 

 pistillate flowers) must grow near each other. If one sort 

 of plant is missing in a certain locality, or if plants of the 

 two kinds are too far apart, polHnation does not occur and 



