36 TEXTBOOK OF BOTANY 



changes both inside and outside the cell. Since the changes 

 produced by yeasts are of such great practical importance, 

 their enzyms have been much studied. The fermentation of 

 sugar is due to one of these enzyms. This is shown by the 

 fact that the yeast cells can be killed and the enzym ex- 

 tracted and filtered, and the substance so obtained will pro- 

 duce exactly the same change in sugar that ^oes on in the pres- 

 ence of living yeast. This particular enzym will not fer- 

 ment all sugars, but only certain comparatively simple ones, 

 and especially those of the class to which glucose belongs. 

 If the yeast is placed in a solution containing cane sugar (or 

 beet sugar, which is chemically the same thing) , it first changes 

 the cane sugar into a simpler sugar by means of a sugar- 

 changing enzym, and then this simpler sugar is fermented. 



54. Reproduction by Division. The yeast cells reproduce, like 

 those of bacteria and Protococcus, by division, but the method of 

 division is somewhat different. The first indication that it is to occur 

 is the appearance of a small swelling like a bud at one side or one end 

 of the cell (Fig. 13, B). This is caused by the pushing out of the 

 protoplasm and the cell wall at that point. The bud grows rather 

 rapidly. The nucleus of the cell divides into two, and one of the 

 daughter nuclei moves out into the bud. When the bud has reached 

 a certain size it is cut off by a wall, so that there are now two cells, of 

 which one (the former bud) is smaller than the other. The smaller 

 cell grows to full size. Both cells may then form new buds. The 

 cells may remain attached after a division, so forming a colony of two 

 cells ; but sooner or later they are likely to be separated. If growth 

 and division are going on rapidly, we frequently see not merely two 

 cells but a row of several cells (Fig. 13, C, D), and sometimes even 

 branched rows due to the formation of buds on two or more sides of a 

 particular cell. 



55. Reproduction by Spore Formation. Some yeasts under un- 

 favorable conditions, as when they are starved or when the liquid in 

 which they live dries up, form two, three, or four thick-walled cells 

 within the original cell wall (Fig. 13, ). These thick- walled cells 

 are called spores, and like the spores of bacteria they are able to endure 

 heat, cold, and dryness better than the ordinary yeast cells. Spore- 

 formation in yeasts, however, is quite different from spore-formation 

 in bacteria, because it results from cell division and is therefore a 



