DIOECIOUS PLANTS 81 



ation. If we examine flowers carefully we will find some in 

 which the stamens and pistils mature at different times and, 

 therefore, it is impossible for the flower to be pollinated by its 

 own pollen. Of course, the flowers do not open at the same 

 time and pollen of some flowers will mature at just the right 

 time to be transferred to, and grown on, the stigma of others. 

 Sometimes the flowers will be pollinated from other flowers of 

 the same plant and sometimes by flowers of different plants. 

 In some plants we can see the effects of this cross pollination 

 in the seeds, as in the case where corn of one color has been 

 pollinated by corn of another, resulting in the speckled or mixed 

 ears. But in most plants we do not see the results of cross 

 pollination until we grow the new plants. Cross pollination is 

 accomplished in various ways. Some plants have two kinds of 

 tubular flowers, some with short stamens and long pistils, and 

 others long stamens and short pistils. The insect visiting the 

 first flower gets pollen on the front part of its body; visiting 

 the latter it leaves some of the pollen on the short pistil and gets 

 a load of pollen on the back part of its body. Therefore, it is 

 practically impossible for the flower to be pollinated with its 

 own pollen. Some plants, especially cultivated varieties, have 

 impotent pollen and, therefore, cannot be pollinated with their 

 own pollen or with pollen from plants of the same variety. 

 Many apple, pear and plum growers mix their varieties in the 

 orchard so as to insure pollination and thus get the stimulating 

 effect of fertilization and secure a richer harvest. In some cases, 

 growers may even set a few trees of an inferior variety in order 

 to secure satisfactory cross pollination of the desirable varieties. 

 In some plants, such as the common banana, the pollen is useless 

 and the plants no longer produce seeds. 



Dioecious plants (Chapter V) are necessarily cross pollin- 

 ated, and many savage and half-civilized races learned, many 

 centuries ago, the necessity of growing the staminate as well 

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