Seeds for the Garden 



117 



must be carried by the wind or 

 by insects from the flowers with 

 only stamens to the flowers with 

 only pistils. 



In other garden plants (bean, 

 pea, salsify, and tomato are good 

 examples) the stamens and pis- 

 tils are both present in each 

 flower. But even in these the 

 wind and the insects very often 

 carry pollen from one flower to 

 the pistils of another. 



The carrying of pollen from 

 the stamens to the pistils is 

 called pollination. If the pollen 

 which reaches *a pistil is from 

 the same flower or another 

 flower of the same plant, it is 



self-pollination. If it is from a different plant, it is cross- 

 pollination. 



Fertilization. After the pollen grains have been 

 placed on the stigma, a tiny, thread-like tube sprouts out 

 from each pollen grain. These tubes grow downward, 

 making their way among the cells of the pistil, until they 

 reach the sac-like structures (ovules) . The pollen tube 

 enters the ovule through a tiny opening that is present 

 in its wall and continues growing until it reaches the 

 egg. There it bursts open at the tip, and a little cell 

 called the sperm cell, which was within the tube, unites 

 with the egg cell. This uniting of a sperm and an egg 

 cell is called fertilization. 



FIG. 6g. a is a pollen grain, and 

 b and c show pollen tubes which 

 have developed from grains ger- 

 minated on sugar-agar. The 

 nucleus of the vegetative cell 

 of the pollen tube is shown near 

 the end of the longer tube ; the 

 two male nuclei are shown far- 

 ther back in the same tube. 

 The grains are here shown 220 

 times natural size. 



