AN INTRODUCTION TO PLANT BIOLOGY 



79 



Pollen-Grains. (D or L) Examine some pollen-grains mounted 

 in a drop of water, using first low power and then high power of 

 microscope. 



Fertilization. When 

 pollen-grains have be.en 

 lodged on the stigma of a 

 bean flower they soon swell, 

 and each one sends out a 

 delicate tube, which grows 

 down through the tissues of 

 the style and into an ovule 

 in the ovary (Fig. 32). There 

 the pollen-tube meets a cell 

 known as the egg-cell. Then 

 a mass of protoplasm (really 

 a cell) passes from the end of 

 the pollen-tube into the egg- 

 cell. The union of the two 

 cells is fertilization. Soon 

 the fertilized egg-cell divides 

 into many cells, and these 

 form an embryo within the 

 ovule, which grows into a seed. This process of fertilization 

 in plants will be described more in detail in the section on the 

 reproduction of flowering plants in the next chapter. 



Pea Flower. (L) If material is available, make a brief examina- 

 tion of the flower of a pea, preferably of a sweet pea, in order to see 

 better some parts which are larger than in the bean flowers. 



FIG. 32. Diagram of a flower. C, calyx; 

 co, corolla; a,] anther on filament (/); 

 p, pollen-grains; st, stigma; pt, pollen- 

 tube; s, style; 0, ovary; em, egg-cell 

 in ovule; c, fertilizing cell. (From 

 Bessey.) 



The Bean Pod or Fruit 



76. Fruits. After the stigma of the bean flower has 

 been pollinated by insects, the ovary of the pistil soon begins 

 to develop into the pod containing the seeds. Botanists 

 call the pod with its seeds a fruit, and apply this name to the 



