LABORATORY PRACTICE 



moved. Consequently, first, the pollen grain must get from 

 its home in the anther cell to the stigma. This is the first 

 step in fertilization, and is called pollination. Pollination 

 deals with the transfer of the pollen from the anther to the 

 stigma. 



Then the pollen grain must "grow down" through the 

 stigma and style into the ovary cell, and penetrate the 

 ovule, so that the two nuclei may unite. When the pollen 

 grain falls upon the stigma, it is caught and held there by 

 the roughness of the stigma and by a sticky sugary fluid 

 upon it. Under the influence of this sticky fluid, the pollen 

 grain grows out into a tube which bores its way down into 

 the stigma and through the style (if there is any) until it 

 comes to the ovule. The end of the tube finds its way into 

 the ovule [through the small opening which persists in the 

 ripened seed and is called the micropyle (compare Chapter 

 I, II, 6, etc.)], the nucleus which it carries comes into 

 contact with the nucleus in the ovule, and they unite. 

 Fertilization is then complete, and the embryo begins to 

 form. 



This second step in fertilization is called the descent of the 

 pollen tube. It is very similar in all plants, and can be stud- 

 ied only by the aid of the compound microscope and careful 

 and complicated preparations. But the first step, that of 

 pollination, is very different in different plants, and can be 

 much more easily made out. 



We distinguish first as to whether the pollen which acts 

 upon the stigma of a particular flower comes from the sta- 

 mens of the same flower or from those of a different flower. 

 If the former, it is called close-fertilization; if the latter, 

 cross-fertiliza tio n . 



Cross- fertilization is more common, and we shall study 

 that first. Of course the pollen grains, having no motion 



