Sexual Reproduction in Flowering Plants 245 



Fig. 152. 



Pollen grains and pollen tubes. 5 is the two sperms or male cells, 

 and T the tube nucleus. 



Cross-pollination. When a flower is pollinated with its own 

 pollen or with that from another flower on the same plant, it is 

 said to be self- pollinated. If the pollen comes from another plant, 

 a flower is said to be cross-pollinated. In many plants it makes 

 no difference whether the pollen comes from the stamens of the 

 same plant or from those of another plant. In the common 

 tobacco plant the pollen may be transferred to the stigma of the 

 same flower, and seeds will be produced. In some plants, how- 

 ever, it is only when the flowers are cross-pollinated that seeds 

 are formed. The sunflower is a good example of this kind 

 of plant. In still other plants, seeds that are formed after 

 self-polKnation are less vigorous than those formed after cross- 

 pollination. 



In some species of plants that are self-sterile, the pollen from 

 plants started by cuttings will not fertilize the egg ceUs on other 

 plants derived from cuttings of the same plant. This is a matter 

 of practical importance in cultivating blueberries, which are 

 propagated by cuttings. Unless fertilization takes place, perfect 

 fruits are not formed. Hence, cuttings from different sources 



