224 REPRODUCTION IN PLANTS AND ANIMALS 



set free appear to be chemically attracted to the egg cells. If 

 a sperm (male gamete) fuses with an egg (female gamete) it is said 

 to fertilize the egg cell. From the fertilized egg a new plant will 



eventually grow. This 

 method of development, 

 which is found in all 

 higher plants and ani- 

 mals, is known as sexual 



Egg 



nucleus 



Egg nucleus 



l.-y,?-' 



Reproduction in vaucheria. 



reproduction. 



Reproduction in Flow- 

 ering Plants. — Pollen 

 grains of various flowers, when seen under the microscope, differ 

 greatly in form and appearance. Some are relatively large, some 

 small, some rough, others smooth, some spherical, and others 



Pollen grains as seen through the microscope ; note the varying shapes and sizes. 



angular. They all agree, however, in having a thick wall, with a 



thin membrane under it, inclosing a mass of protoplasm. At an 



early stage the pollen grain contains but a single cell and is a 



kind of spore. A little 



later, however, three 



nuclei may be found 



in the protoplasm. 



Hence we know that at 



least three cells exist 



there, two of which are 



sperm cells or male 



gametes. 



Under certain conditions a pollen grain will germinate. This 

 growth may be artificially produced by placing pollen grains in 

 sugar solutions of different strengths. In the life of a plant it 



Beginning of 

 pollen tube — >• 



A developing pollen grain of the pine. 



