NEW ORGANISMS 



293 



340. Spores. The 



cells of yeast change 

 their behavior when 

 the yeast is growing 

 in a solution that is 

 gradually evaporat- 

 ing, or when the 

 food in the solution 

 is gradually being 

 used up, or when it 

 is exposed to an ex- 

 treme change in tem- 

 perature. It is often 

 found under such un- 

 favorable conditions 

 that the yeast plant 

 will produce peculiar 

 kinds of cells (see 

 Fig. 124). 



A special cell like 

 those described in the 

 yeast (that is, a cell 

 capable of continu- 

 ing the growth of the 

 plant from which it 

 is derived) is called 

 a spore. Spores are 

 produced by nearly 

 all plants and by a 

 number of animals. 



FIG. 126. Spore capsule of moss 



a, spore case with stalk, on top of leafy plant ; b, enlarged 

 view of spore case, with cap removed ; c, closed surface 

 of capsule tip after removal of cap ; d, capsule bursting 

 open and discharging spores ; e, spores, greatly magni- 

 fied ; /, spore beginning to germinate by sending out 

 a fine thread of protoplasm, a, Thutdium virginianum ; 

 6, c, Funaria americana ; d, Orthotrichum schimperi; 

 e, Bartramia pomiformis ; f, Funaria hygrometrica 



The bacteria often 

 produce spores by the 

 formation of a rather thick cell wall when the conditions for growth 

 are not favorable. It is the resisting power of the spores that makes 

 it so difficult to kill certain species of bacteria by boiling. 



