THE FLOWER — FERTILIZATION AND POLLINATION 



145 



application of pollen to pistils in the same flower is close- 

 fertilization or self-fertilization. It will be seen that the 

 cross-fertilization relationship may be of many degrees — 

 between two flowers in the same cluster, between those 

 in different clusters on the same 

 branch, between those on different 

 plants. Usually fertilization takes 

 place only between plants of the 

 same species or kind. 



In many cases there is, in effect, 

 an apparent selection of pollen when 

 pollen from two or more sources is 

 applied to the stigma. Sometimes 

 the foreign pollen, if from the same 

 kind of plant, grows, and fertiliza- 

 tion results, while pollen from the 

 same flower is less promptly effec- 

 tive. If, however, no foreign pol- 

 len is present, the pollen from the 

 same flower may finally serve the 

 same purpose. 



In order that the pollen may grow, the stigma must be 

 ripe. At this stage the stigma is usually moist and some- 

 times sticky. A ripe stigma is said to be receptive. The 

 stigma may remain receptive for several hours or even 

 days, depending on the kind of plant, the weather, and how 

 soon pollen is received. Watch a certain flower every day 

 to see the anther locules open and the stigma ripen. When 

 fertilization takes place, the stigma dies. Observe, also, 

 how soon the petals wither after the stigma has received 

 pollen. 



Pollination. ^ — The transfer of the pollen from anther 

 to stigma is known as pollination. The pollen may 



Fig. 195. — Diagram to 

 represent fertiliza- 

 TION. 



i, stigma; j/, style: oz/, ovary: o, 

 ovule; p, pollen-grain; pt, 

 pollen-tube; e^ egg-cell; m, 

 micropyle. 



