SYSTEMATIC BOTANY. 93 



class, as do also pulmonary consumption, diphtheria, etc. 

 In each of such cases it is a certain species only that invades 

 the human system and causes the disease. The species of 

 Bacteria are not directly injurious, but there are secreted 

 poisonous substances, called ptomaines. The organism may 

 survive the attack of the parasites, and in case of several of 

 the diseases immunity (as yet unsatisfactorily explained) is 

 thereafter enjoyed. 



10. Another family of plants belonging to the Schizophytes, 

 includes the Yeast plant (Saccharomyces cerevisise) and other 

 species, which produce fermentation in sugar solutions. The 

 transparent cells are round, oval, or elongated, and multiply 

 by budding (Fig. 118). When the supply of nourishment is 

 less abundant (as when Yeast is grown on slices 

 of potato or carrot), the cells are larger and divide 

 internally into four new cells. These may be 

 called spores; they escape and grow into cells 

 of the ordinary kind capable of multiplication 

 by budding. Saccharomyces cerevisise produces the -pia. lis. 

 alcoholic fermentation ; that is, the sugar in the 



solution is converted into alcohol with the escape of carbonic 

 dioxide. Its agency in bread-making is well known. S. ellip- 

 soides, S. conglomeratse, and others live on grapes and find their 

 way into the juice in the manufacture of wine, and cause the 

 fermentation of the latter. S. mycoderma is found on the sur- 

 face of spoiled beer or wine ; it does not produce fermentation 

 like the others, but putrefaction instead. 



A supply of the Yeast plant for study can be found in vigorously growing 

 yeast, but the use of the compound microscope is indispensable for its ex- 

 amination. 



11. Very common, abundant, and beautiful fresh-water spe- 

 cies belonging to the group of Algse (easily recognized among 

 the lower plants by uniform presence of chlorophyll), are the 

 members of the genus Spirogyra. They are many-celled, fila- 

 mentous, having the chlorophyll in spiral bands, and a con- 

 spicuous nucleus in each cell, which is imbedded in the small 



