THE SIMPLER ORGANISMS 97 



of structure of the single yeast cell. 3) Budding and the 

 aggregation of the yeast cells together into torulae. 



Study in the molds: i) The differentiation into my- 

 celium and sporophores; 2) The type of branching, with ab- 

 sence of cell divisions in Mucor, 3) The streaming of the 

 protoplasm in filaments of Mucor. 4) The germination of 

 the spores and the beginning of mycelia in Penicillium. 5) 

 The development of the spore clusters and of the arrange- 

 ment of the spores in Penicillium, and in any other fruiting 

 molds that may be available. 



The record of this study may consist in simple outline 

 drawings, and notes on the things observed. 



Bacteria. — These are the smallest of the chlorophylless 

 plants — indeed, they are the smallest of living organisms. 

 They feed upon much the same materials as do other fungi, 

 and while present nearly everywhere, they are sure to 

 abound wherever there are moist organic substances in which 

 they can multiply. Under favorable conditions bacteria 

 increase in numbers with extraordinary rapidity. Their 

 method of increase is already familiar — grow^th in size, 

 followed by cell division. A division may recur every half 

 hour, and at this rate something like 17,000,000 individuals 

 might appear as the offspring of a single one in the course 

 of twenty-four hours. Obviously, such a rate could not 

 long be maintained for want of food. Their reproductive 

 capacity, together with the readiness with which they may 

 be distributed, give them an important place in the economy 

 of nature. They are nature's chief agency of decomposition 

 and decay. They play a large role in restoring spent organic 

 materials to circulation. 



Certain bacteria at times develop spores. Usually but a 

 single spore is produced in each cell, the protoplasm of 

 which develops a resistant spore coat within the old cell 

 wall (fig. sSd). The spores are not injured by drying, and 



