188 UNICELLULAR PLANTS. 



Physiology. Like all other organisms the yeast-plant occu- 

 pies a definite position in space and time; it possesses an en- 

 vironment with which it must be in harmony if it is to live, 

 from which it derives an income, and to which it contributes an 

 outgo of matter and energy ; it manufactures its own substance 

 from foods (anabolism), and like all living things it wastes by 

 oxidation of its substance (katabolism). It is not obviously con- 

 tractile or irritable, but it is highly metabolic and reproductive. 



Yeast and its Environment. Yeast is an aquatic form, and, 

 as might be supposed, cultivated yeast thrives best in its usual 

 habitat, the juices of fruits, such as apples or grapes, and the 

 watery extracts of sprouted seeds, such as barley, corn, and 

 rye (wort, mash, etc.). It lives, however, more or less success- 

 fully in many other places (such as the dough of bread), and can 

 even endure much dryness, as is shown by the commercial 

 " dried - yeast. " It appears to prefer a temperature from 

 20 to 30 C. ; it is usually killed by boiling, but if dried, it can 

 endure high temperatures. Its action is inhibited by very low 

 temperatures, but like most living things it endures low temper- 

 atures better than high. It is killed by many poisons (anti- 

 septics). 



Income. Owing to its industrial importance yeast has been 

 perhaps more thoroughly studied in respect to its nutrition than 

 any other unicellular organism. And yet it is impossible to 

 give accurate statistics of its normal income and outgo. It is 

 believed that the ordinary income of a yeast-cell living in wort 

 (the watery extract of sprouted barley -grains) consists of #, dis- 

 solved oxygen / i, nitrogenous bodies allied to proteids, but diffusi- 

 ble and ablo to pass through the cellulose wall ; , carbohydrates, 

 especially sugary matters ; and d, salts of various kinds. 



It was supposed for a long time by Pasteur and others that 

 yeast could dispense with free (dissolved) oxygen in its dietary. 

 It now appears that this faculty is temporary only, and that if 

 yeast is to thrive it must, like all other living things, be sup- 

 plied, at least occasionally, with free oxygen. 



Metabolism. Out of the income of foods just described yeast 

 is able to build up its own peculiar protoplasm (anabolism}, and, 

 doubtless, to lay down the droplets of fat which often appear in 

 it. There is good reason to believe that its substance also breaks 



