186 



UNICELLULAR PLANTS. 



no chlorophyll is present and no starch. Until recently the yeast- 

 cell was supposed to be destitute of a nucleus, but it isnow known 

 that each cell probably possesses a large and characteristic nucleus. 

 This, however, can be demonstrated only by special reagents and 

 is rarely or never seen in the living cell (Fig. 96). 



Reproduction. The ordinary mode of reproduction of yeast 

 is by a modification of cell-division called budding. Under 



FIG. 96. The Nuclei of Yeast-cells and the Process of Budding. (Drawn by J. H. 

 Emerton from specimens prepared by S. C. Keith, Jr.) The upper left-hand figure 

 shows the nucleus in a specimen treated with Delafi eld's haematoxylin. The 

 other figures in the upper row and those in the lower (from left to right) show 

 cells in successive stages of budding, together with the appearance, position, and 

 movements of the nucleus. It will be observed that the bud is formed before the 

 nucleus divides. (Iron-hsematoxylin method.) 



favorable circumstances in actively growing yeast a local bulging 

 of the wall takes place, usually near, but not precisely at, one 

 pole of the cell. Protoplasm presses into this dilatation or 

 4 ' bud ' ' and extends it still further. At this time we have still 

 but one cell, although it now consists of two unequal parts and 

 the separation of a daughter-cell is clearly foreshadowed. Event- 

 ually the connection between the two parts is severed and the 

 daughter-cell or "bud ' is detached from the original or parent- 

 cell ; but detachment may or may not occur until after the bud 



