56 TEXTBOOK OF BOTANY 



branches we have already studied ; they start from the hori- 

 zontal branches at about the same places at which the groups of 

 downward branches start, but they grow upward into the air. 

 After one of these upright branches has grown for a time, its 

 upper end begins to swell, forming a globular body that will 

 become a spore sac (Fig. 22, c). When the spore sac has 

 reached nearly its full size, a dome-shaped wall is formed in 

 it, dividing it into a central portion and an outer portion. 

 The protoplasm (including cytoplasm and nuclei) which fills 

 the outer portion of the spore sac now cuts itself up into a 

 large number of small cells (Fig. 22, d). When the spores 

 are ripe, the outer wall of the spore sac breaks (Fig. 22, e, /), 

 and the spores are scattered. If. a spore falls upon moist 

 bread or some other substance on which the mold can live, 

 it begins to grow into a new plant. The spores are so light 

 that they are easily carried 

 about by currents of air. Like 

 the spores of bacteria and of 

 yeasts, they are present in the 

 air practically everywhere that 

 human beings live, and this is 

 why Rhizopus nearly always 

 appears on moist bread that is 

 exposed to the air. 



82. Germination of the 

 Spores. A mold spore can 



7)P! 



live for some time in a dry 



place without germinating. 



FIG. 23. A, a ripe spore of But if it falls in a place 



the bread mold, much enlarged, favorable for growth, it SOOn 



B, C, D, stages in the development , , , , 



of spores into young plants. develops into a new^ plant. 



Germination begins with the 



appearance of a swelling at one side of the spore (Fig. 23, B) ; 

 this swelling grows longer and begins to look like a branch 

 of a mature plant. Sometimes two or more outgrowths 



