202 THE AMERICAN MONTHLY [November, 



from the single oval body of the inactive yeast first examined. In place 

 thereof oval bodies, sometimes several of them, have grown together. In 

 fact, there is now before us actively growing yeast. The process of growth 

 in yeast is called budding. The steps can be readily traced. There grows 

 at first on one spot a little pimple, which enlarges until eventually it rivals 

 the size of the original cell from which it derived its origin. Not uncom- 

 monly two of these little pimples or buds, as they are named, arise at the same 

 time on a yeast plant. Both then increase in size. Before they have grown 

 to full size, one or both of the buds may in turn have produced a bud, con- 

 stituting a third generation derived from the original yeast cell. In actively 

 growing yeast this goes on so rapidly that one may not improbably find a cell 

 carrying two buds, which in turn each carry one or two. These in turn also 

 bear buds. In an example before us twenty yeast plants are thus connected 

 which plainly have grown from a single cell. All the separate ovals are like 

 the original yeast plant in three particulars. They all have a cell-wall, a 

 protoplasmic substance, and some fat droplets. But the older ones differ 

 from the younger ones in two particulars — ( i ) they are larger than the youngest 

 ones, and (2) the vacuole more nearly occupies the whole of the space inside 

 the cell-wall. 



7. Yeast from the rain-water. — While the yeast from the sugar has 

 grown extensively by budding, and the bulk of yeast has been vastly increased, 

 that left in rain-water has done nothing of the kind. It has not budded, but 

 is, in shape, made up of single oval cells, with some carrying one bud or at 

 most two. They do not look like the luxuriantly budded plants of the sugar 

 and water solution. Close obsei^vation will, however, reveal a still more sig- 

 nificant fact than the absence of budding. The yeast plants prove on careful 

 scrutiny to be only emptv cell-walls containing only a few very small bodies, 

 which may perhaps be the remains of the fat droplets. The protoplasm has 

 gone and the yeast is all dead. It has died of starvation ! That the yeast 

 is dead could be proved by putting some of it in the sugar solution and notic- 

 ing it did not bud. But to do this certain precautions would need to be taken 

 to prevent the introduction of any live yeast. This would require consider- 

 able skill, and cannot be attempted in an elementary course. 



S. General conclusion. — There have now been observed, with the micro- 

 scope and with the naked eye, the evidences of life and growth in the yeast 

 plant. This sketch will be completed by noticing briefly the meaning of 

 some of the observations. One would be likely to reason that sugar in the 

 water kept the yeast alive by furnishing it with food. That this is the case is 

 shown by the fact that the sugar gradually disappears, and if not renewed the 

 yeast dies. In getting its food from the solution the yeast lets loose from the 

 sugar the alcohol which it does not require. It has increased very consider- 

 ably in aggregate bulk, and the substance for this increase has been made in 

 part from the sugar. Since this substance is made only in part from the 

 sugar, something else should be furnished in the solution for food. Unless 

 it is, the yeast will only thrive for a few days. This is not a time to go 

 more fully into the life-processes of the yeast plant. One w^ord will 

 suffice by way of comparison with Protococcus, which can live in pure 

 rain-water. It has a power, because of its green coloring matter, which the 

 colorless yeast plant does not possess — that of making its food cut of gases 

 which abound in the air. 



The growth of yeast in a sugary solution is known as fermentation ; it is 

 attended with the evolution of carbonic acid gas. The report remarked above 

 was due to the fact that this gas accumulated in the vials until it produced a 

 high pressure there and finally forced out the cork. There are many other 

 plants which in their mode of life and growth closely resemble yeast. 



