A MANUAL OF BOTANY 109 



dioxide. Where does the plant obtain those substances '/ 

 From them starch or oil is made. Observe whether tlie 

 bubbles are formed when the alga is in a dark corner of the 

 room or early in the morning. From the results of these 

 observations what seems to be necessary for the manu- 

 facture of the gas (which is a waste product) and the 

 process of food making which produces the gas ? 



2. The water and carbon dioxide for the manufacture of 

 starch or carbohydrate food is taken into the cell of the 

 alga through the cell wall according to the laws of absorp- 

 tion or osmosis, a physical phenomenon somewhat difficult 

 to understand. A number of experiments tend to illustrate 

 the principle, but all are rather elaborate and may with pro- 

 priety be demonstrated before the class. You ought to 

 consult the textbooks to find out what experiments they 

 give for this purpose. Some of them you may be able to 

 perform without aid. This simple procedure you certainly 

 can carry out : Soak a well-dried raisin, bean, prune, or 

 currant in water and observe the result. This in a homely 

 way illustrates food absorption by algae and all other 

 plants. How did the water get into the fruit or seed? 



3. In all these experiments you will find that a mem- 

 brane (fruit or seed coat in the last) separates a thin liquid 

 (of small comparative density) from a much denser, com- 

 monly liquid, substance, and that a marked flow of the thin 

 liquid through the membrane into the denser substance is 

 the first phenomenon apparent to the eye. 



4. In the algae we have the dense cell cytoplasm — the 

 cell wall and the surrounding water with the dissolved 

 minerals and other food materials. 



Habitat. The jjlants studied in this exercise are common 

 on the rocky shores of New England. In the interior they 

 ^ Ascophyllum or Fucus. 



