SEXUAL SPORES 



223 



process known as budding. In this process a smaller daughter cell 

 is cut off from the parent cell. But if unfavorable conditions come, 

 each yeast plant divides into four cells, which have thick resistant 

 walls. These cells or spores are able to withstand unfavorable con- 

 ditions. When favorable conditions of moisture, temperature, 

 and food come again, the 

 spores will grow and form 



Yeast : A, reproduction by budding ; 

 B, formation of spores. 



yeast plants as before. 



Sexual Spores. — In most 

 plants which produce spores, 

 however, the spores are formed 

 in a different way. The com- 

 mon pond scum or spirogy'ra, found in freshwater ponds or quiet 

 brooks, is an example. The body of this plant is a filament made 

 of cylindrical cells. This body grows by ceU division, each cell 

 dividing into two new ceUs that look like the original ceU. Thus 

 the plant body grows by asexual reproduction. But another 

 method of reproduction occurs. Two filaments lie side by side, 

 little projections grow out from adjoining cells, these meet, and 

 the contents of the cells in one filament 

 pass over and mix with the cell contents 

 of the other filament. The cells thus 

 formed become resting spores (zy'gospores. 

 or zy'gotes) . The cells which came together 

 to form these spores are called gametes 

 (gam'ets; Gr. gamete, wife). This is a 

 very simple tj^e of sexual reproduction 

 called conjugation. Under favorable con- 

 ditions each zygospore can develop into 

 a new^ plant. 



• |i» Reproduction in Some Low Plants. — 



^^ Zygospore ^^ other algae we have a step higher in 

 ^^^^ the development of gametes. In the 



vauche'ria, a branched alga, two struc- 

 tures may be produced from the filament. 

 One is a sac and contains a large gamete 

 The other is tubelike and contains small 

 The sperms when 



Two cells 

 conjugating 



Conjugation in spirogyra. 



which is called an egg. 



gametes, called sperms, each with two cilia. 



