414 



How Animals and Plants Reproduce unit \ui 



end of the cell and divides into two equal 

 parts. The two new nuclei are therefore 

 at one end of the cell. A new cell wall 

 forms between them, thus two new cells 

 are formed. This is not binary fission be- 

 cause the two cells are very unequal in 

 size. The larger cell is called the mother 

 cell and the smaller one is the daughter 

 or bud. This kind of reproduction, in 

 which a small piece is formed that will 

 later grow into a full-sized organism, is 

 called btiddhjg. In yeast, the bud may re- 

 main attached to the mother cell for a 

 time; in fact, sometimes it remains at- 

 tached until it has itself produced a bud. 

 Thus a small chain of yeast plants may 

 be formed. If the bud is shaken loose, it 

 can also grow and bud. Now do Exer- 

 cise 3 to see budding in yeast. 



A process called budding occurs in a 

 few many-celled animals. One of them is 

 Hydra. See Figure 85, page 59. In these 

 animals the bud is composed of several 

 to many cells. 



Reproduction by spore formation. 

 Sometimes in reproduction a cell di- 

 vides into many new cells instead of only 

 two. The nucleus begins the process by 

 dividing in two; then each new nucleus 

 divides, continuing until there are eight 

 or sixteen or a much larger number of 

 nuclei. A little cytoplasm gathers around 

 each new nucleus, and from the original 

 cell there are soon formed a larire num- 

 ber of tiny cells. Each cell, as a rule, de- 

 velops a thick, protective wall. A repro- 

 ductive cell formed in this way is called 

 a spore, and the method of reproduction 

 is called spore formation or spondation. 

 Study sporularion by doing Exercise 4. 

 If you have a microscope, you can also 

 do Exercise 5. 



Spore case 



Hyphae ^^^ ^ 



Spores 



Fig. ^63 Spores of the bread mold (Rhizopus) 

 f^row into hyphae. ]Vhat cJ^anges occur in the 

 hyphae?" Where are the spores produced? 



Sporulation occurs in molds. Molds 

 grow on a variety of foods under the 

 right conditions, even on damp leather or 

 wood. One of the very common species 

 is the bread mold (Rhizopus), a fuzzy 

 white plant that appears to turn gray and 

 finally black when the spores are ripe. 

 A bread mold plant starts from a spore, 

 a single cell which grows into a small 

 white thread. This thread g^rows and 

 branches in all directions. Some of the 

 threads {hyphae — \\\^W{&&) push down 

 into the substance on which the mold is 

 growing and extract food material. Some 

 grow upward into the air. 1 he threads 

 that grow upward are used in reproduc- 

 tion. They swell at the tip, and it is in 

 this tip that sporulation takes place. The 

 several nuclei in the swollen tip are sur- 

 rounded by bits of cytoplasm, so that 

 fin:ill\ many small cells are formed. Each 

 tiny cell becomes surrounded with a 

 tough wall; it becomes a spore. While 



