316 PRACTICAL COURSE IN BOTANY 



is the bubbles of gas that were seen rising in the liquid which 

 cause beer to effervesce and bread to rise. They permeate 

 the dough and by their expansion produce the sponginess 

 peculiar to leavened bread. Look for a cell with a bud form- 

 ing on it ; from what part does it appear to grow ? Where a 

 number of buds remain for some time attached to the mother 

 cell (Fig. 449), they form a colony. Make a sketch of a 

 single cell and of a colony of two or more adherent ones, 

 labeling all the parts. If the cell wall cannot be made out 

 clearly, run a little glycerine, or salt water, under the cover 

 glass with a medicine dropper. What causes the contents of 

 the cell to contract and leave the wall ? (56, 59.) 



358. Reproduction. -- From time to time buds break away 

 from the mother cell and form new individuals or colonies 

 of their own. This process is called multiplication by bud- 

 ding, and is only another form of cell division. 



Whenever reproduction takes place by other means than 

 seeds or spores, it is said to be vegetative. This sort of repro- 

 duction is not confined to unicellular plants, but exists also 

 among the phanerogams, the propagation of species by means 

 of buds, tubers, rootstocks, runners, grafting, and the like 

 being variations of the same process. On the other hand, 

 yeasts and bacteria and the unicellular algae have the power, 

 under extreme conditions, to form resting spores, which 

 sometimes lie dormant for years and resume their activity 

 when favorable conditions return. 



Practical Questions 



1. When is fermentation useful to man? 



2. What is the effect on canned fruits and vegetables if yeast cells get 

 into them? 



3. Why does milk turn sour in warm weather? (350, 351; Exp. 96.) 



4. Why do buttermilk and clabber spoil if left standing too long ? 

 (345, 356.) 



5. What causes bread to be "heavy"? (356, 357.) 



6. Why will dough not rise unless kept in a warm place ? (Exp. 96.) 



7. Why is beer kept cold during fermentation? (350, 356.) 



