114 BIOLOGY FOR BEGINNERS 



Pollen Structure. The pollen grain is at first a single cell but 

 if transferred to the stigma of a flower of its own kind, it begins to 

 grow, forming three cells, one of which develops into a very long 

 tube which reaches from stigma to ovary, no matter how long 

 that may be. The other two cells, containing the most active 

 kind of protoplasm, are called the sperm or male cells, and their 

 union with the ovule is called fertilization and produces the 

 embryo in the seed. 



Ovule Structure. The ovules (undeveloped seeds) are protected 

 inside the ovary and can be reached only by way of the pollen tube 

 from pollen grains on the stigma. They are much larger and more 

 complicated than the pollen grains. Each ovule in the ovary has 

 a protective covering which later becomes the testa of the seed. 

 Within this is the nucleus of the ovule cell which divides into eight 

 cells, two of which form the endosperm and one, the most im- 

 portant, becomes the egg or female cell. As has been said, the 

 pollen tube grows downward through the style till it reaches the 

 place where an ovule is attached to the ovary wall ; near this point 

 of attachment is an opening through the ovule coats, called the 

 micropyle, and through this the pollen tube makes its way till it 

 reaches the egg cell within. 



Fertilization. The sperm cell then passes down the pollen tube 

 and unites with the protoplasm of the egg nucleus. This union of 

 the sperm nucleus of the pollen with the egg nucleus of the ovule 

 is called fertilization. The fertilized egg now has the very re- 

 markable power to grow, and from its one cell, to develop the 

 countless numbers which go to make up the embryo within the 

 seed and finally the whole new plant. Notice that in this wonder- 

 ful process each plant is reduced to a single cell, the sperm or 

 the egg, that they unite and again form a single cell, and that 

 from this develop the embryo and the whole organism. 



Fertilization is essentially the same in both plant and animal so 

 you must try to think of all living things as having developed from 

 a single fertilized egg cell. 



Origin of Seed Parts. Look back at Chapter VI and notice 

 that we have just been studying the origin of all parts mentioned 



