268 



FOUNDATIONS OF BOTANY 



-b 



EXPERIMENT XXXIX 



Can Yeast grow in Pure Water or in Pure Syrup ? Put a bit of 

 compressed yeast of about the size of a grain of wheat in about four 

 fluid ounces of distilled water, and another bit of about the same size 

 in four fluid ounces of 10 per cent solution of rock candy in distilled 

 water ; place both preparations in a warm place, allow to remain for 

 twenty-four hours, and examine for evidence of the growth of the 

 yeast added to each. 



322. Size, Form, and Structure of the Yeast-Cell. The student 

 has discovered by his own observations with the microscope that the 

 yeast-cell is a very minute object, much smaller than most of the 

 vegetable cells which he has hitherto examined. The average diam- 

 eter of a yeast-cell is about ^-$^-3 

 of an inch, but they vary greatly 

 both ways from the average size. 



The general form of most of 

 the cells of ordinary yeast is some- 

 what egg-shaped. The structure 

 is extremely simple, consisting of 

 a thin cell-wall, which is wholly 

 destitute of markings, and a more 

 or less granular semi-fluid proto- 

 plasm, sometimes containing a 

 portion of clearer liquid, the vacu- 

 ole, well shown in the larger cells 

 of Fig. 197. 1 



323. Substances which compose the Yeast-Cell. The cell- wall is 

 composed mostly of cellulose; the protoplasm consists largely of 

 water, together with considerable portions of a proteid substance, 2 



1 This is not the ordinary commercial yeast. 



2 It may be found troublesome to apply tests to the yeast-cell on the slide, 

 under the cover-glass. Testing a yeast cake is not of much value, unless it 

 may be assumed that compressed yeast contains little foreign matter and con- 

 sists mostly of yeast-cells. Still the test is worth making. Millon's reagent 

 does not work well, but the red or maroon color which constitutes a good test 

 for proteids is readily obtained by mixing a teaspoonful of granulated sugar 

 with enough strong sulphuric acid to barely moisten the sugar throughout, 

 and then, as quickly as possible, mixing a bit of yeast cake with the acid and 



FIG. 197. Yeast (Saccharomyces ellip- 

 soideus) budding actively. 



A, a single cell ; B, group of two budding 

 cells ; C, a large group ; b, buds. 



