616 IMPROVEMENT OF LIVING THINGS BY MAN 



A part of a filament of Vaucheria. 

 how a new plant is formed. 



Explain 



poller? tub 



or undeveloped seeds. At an early stage the pollen grain contains 

 but a single cell. A little later, however, three nuclei may be found 

 Sperms in ^ e P r °toplasm. Hence we 



know that at least three cells 

 exist there, two of which are 

 sperm cells or male gametes. 

 We have learned that within 

 the ovary of the flower is an egg 

 cell. It is to this cell that one 

 of the sperm nuclei of the pollen 

 tube grows, ultimately uniting 

 with it. The union of the 

 sperm nucleus with the nucleus of the egg to form a single cell is 

 known as fertilization. This single cell formed by the union of the 

 pollen-tube sperm cell and the egg cell is now called a fertilized egg 

 or zygote and is the beginning of a baby plant, or embryo. The 

 second sperm nucleus 

 unites with another nu- 

 cleus in the embryo sac 

 and grows into food sub- 

 stances called the en- 

 dosperm, which in some 

 plants is a very impor- 

 tant part of the seed 

 since it supplies the grow- 

 ing embryo with nourish- 

 ment. 



Practical Exercise 7. 

 Make a series of labeled dia- 

 grams to illustrate different 

 stages in the development of 

 sexual reproduction in 

 plants. Make a distinction 

 between asexual and sexual 

 reproduction without using 

 the words "sex" or "sex 

 cells." 



Field Exercise. During 

 the first warm days in 

 March or April, look for 



embryo 



acre 



Sperm* 



^perm* 



egg" nucleus 

 -nucleus* 



\ 



fertile ed£ 

 Will forSx 

 embryo 

 fertilUecC 



prodU 



'cells 



aperm-i 

 fuses 

 Vtth e& 

 nuclei 



j^Sperra* 

 j fuses "with 

 i nuc\eix$ z 



Explain how fertilization takes place in plants. 



