



'mature egg 



CHAPTER XXVIII 



SEXUAL 

 REPRODUCTION 



sperm cells 



Maturation of an egg. Maturation of a sperm. 



What is sex? What are some different types of sexual reproduction f 



Sexual reproduction in mold. In many organisms, instead of a 

 single organism developing directly from a half or a part of another 

 one, two special cells from different organisms unite to form one 

 cell. The cell resulting from this union develops into a new 

 organism. The two cells that unite are called gametes, and the 

 cell formed as a result of the union is called a zygote. The forma- 

 tion of a zygote is called sexual reproduction. Certain lower plants 

 reproduce both asexually and sexually. Normally, the bread mold 

 reproduces by asexual spores. When growth conditions are 

 unfavorable, a sexual method of reproduction may occur in this 

 organism. Two threads or hyphae of mold plants will grow 

 toward each other until the tips meet. A cell wall forms near the 

 end of each tip, cutting off a part of the protoplasm at the end of 

 each hypha. These are similar in size and appearance, and are 

 known as the gametes. The intervening walls between the gametes 

 are dissolved and the contents of the two cells intermingle. A 

 thick wall develops around the fused material, the zygote. As the 

 zygote grows the outer wall becomes black. The structure in this 

 stage is called a zygospore. Under favorable conditions the zygote 

 germinates, and develops into a new plant. 



When the uniting gametes of an organism are. very similar in 

 size and activity, their union is called conjugation. Therefore, a 



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