496 Introduction to the Study of Science 



nectar of many flowers located at the base of the stamens? 26. What 

 is the significance of the structure of the snapdragon? 



27. (a) How is close-pollination prevented in such blossoms as 

 the apple and pear? (6) As the bean, daisy, and sunflower? (c) As 

 the primrose and cowslip? 28. How may offspring differ from their 

 parent plants ? 29. What use does man make of the normal variation 

 of plants? ' 30. How may the normal variation of any species be 

 demonstrated ? 



IV. THE IMPROVEMENT OF PLANTS 



238. Three ways of improving plants. The facts concern- 

 ing cross-pollination and the character of the varieties grown 

 from selected parents have been carefully studied and applied 

 in the improvement of all plants of economic importance. Im- 

 provements are secured in these ways : (1) by the selection of 

 variations that show desired characters (discussed pages 481 ff.) ; 

 (2) by controlled cross-pollination; (3) by the selection and 

 propagation of sports. 



239. Controlled cross-pollination. Cross-pollination of se- 

 lected plants presents no serious difficulties. There are two 

 practical steps to be taken : (1) the protection of the stigma 

 of the plant to be pollenized from possible pollination by any 

 plant other than that selected ; (2) the transfer of the pollen 

 from the anther of one plant to the stigma of the other when 

 this is mature and ready to receive it. 



In the first, if the flower is one in which close-pollination 

 may occur, the stamens are carefully removed before their 

 pollen is ripe. The flower is covered with a paper bag so tied 

 about the stem that no insects can enter, and the pistil will be 

 kept from injury. Since the petals have no function to per- 

 form, they may be carefully cut away. 



The second step in pollinating plants may be effected in 

 several ways. One is to shake the blossom with the ripe pollen 

 against the receptive stigma. Another is to gather the pollen 

 in a dish and apply it to the stigma with a camel's-hair brush. 

 The pollinated flower must be protected with a bag as previously. 



