THE BREAD MOLD 57 



appear on different sides of the spore, and each grows into a 

 part of the new plant. Soon branches appear upon these 

 outgrowths (Fig. 23, C, D), and by continued growth and 

 repeated branching the condition of the mature plant is 

 reached. 



83. Asexual Reproduction. We have seen that the cells of Spiro- 

 gyra reproduce by division, but that cell division does not result, at 

 least directly, in a reproduction of the Spirogyra plant. Spore forma- 

 tion in the bread mold is brought about by cell division which, like 

 cell division in Spirogyra, increases the number of cells; but the spores 

 of the mold are a special sort of cells, each of which grows as soon as 

 possible into a new plant. Thus this particular kind of cell division 

 by which the mold spores are formed is a means of reproduction of the 

 plant as well as of the individual cells. Reproduction by means of 

 spores is commonly called asexual reproduction, to distinguish it from 

 the union of gametes, which is called sexual reproduction. We see 

 that the essential thing in asexual reproduction is a cell division; but 

 that in sexual reproduction the essential thing is a cell union. 



84. Sexual Reproduction. The bread mold has also a method 

 of sexual reproduction similar in some respects to that of Spirogyra. 



FIG. 24. The development of the gametes of the bread mold and 

 their union to form a zygote. 



The two gametes that unite are always borne on two separate plants 

 which belong to two different races or strains, spoken of as plus and 

 minus strains. As a rule, when we find Rhizopus growing on bread 

 all the plants belong to one strain. Two plants of the same strain 

 growing together will not form gametes, and this is why we do not 

 often see the sexually produced zygotes. But when two plants, one 

 of a plus and one of a minus strain, grow so close together that a branch 

 of one comes in contact with a branch of the other, on each of these 

 branches a projection appears. 



