PROBLEM 3, 



Embryo sac 



H01U Complex Plants Reproduce aai 



Ovule coats double nucleus Egg nucleus 



Fig. 389 Diagrmmnatic sections showing a young ovule growing into a larger ovule. 

 Note what happens to the nucleus of the large cell. 



Fig. 390 This bee is gathering nectar jroni a 

 blossom. How is the bee of help to the plant'/ 



(U. S. DEPARTMENT OF AGRICULTURE) 



you examine an ovule under the micro- 

 scope, you will discover that it consists 

 of many cells. Some of these cells form 

 two protective coats around one very 

 much larger cell. Study Figure 389 and 

 you will see that the nucleus of this large 

 cell divides until finally it is replaced 

 by eight cells. Often these cells are not 

 separated by membranes, so they look 

 like one mass of cytoplasm with eight 

 nuclei. One of these nuclei with its sur- 

 rounding cytoplasm is the egg cell, or 

 female gamete. Two of the other nuclei 

 usually fuse and move to the middle of 

 the large "cell" which is now called the 

 embryo sac. The egg cell, or female 

 gamete is the most important cell in the 

 ovule. We shall see what happens to it 

 and to the fused, or double, nucleus. 



The stamen and pollination. The sta- 

 men is composed of a more or less slen- 

 der stalk {filament) with a bulky pollen 

 case {anther) on top. If you shake a ripe 

 stamen, large quantities of powdery pol- 

 len will fall from the pollen case. In na- 

 ture pollen often falls on a pistil. The top 

 of a pistil, the stigma, is usually hairy or 

 sticky. The pollen may be blown by the 

 wind or may be accidentally carried by 

 insects flying from one flower to an- 

 other as they gather nectar. Thus, by 

 wind, insects, birds, or other agents the 

 transfer of pollen from anther to stigma 

 is made. This process is called pollina- 

 tion. Transfer of pollen from an anther 

 to any stigma on the same plant is called 

 self-pollination. If pollen is transferred 

 to the stigma on a different plant the 

 process is called cross-pollinatiofi. If the 

 pollen is carried to flowers of another 

 species the pollen dies. Much of the pol- 

 len never reaches another flower. Sailors 

 at sea have reported "sulfur showers," 

 which are clouds of pollen grains blown 

 off the shore. Self-pollination occurs 



