60 PLANT BIOLOGY 



2. (Optional.) Put the plant prepared as above on one of the 

 scale pans of a balance and with it a stoppered graduate 

 or a stoppered bottle. On the other pan put weights 

 enough to equalize the balance. At the end of 24 

 hours put enough water into the graduate or bottle lo- 

 calise the weights on the two pans to be equa 1 



a. Describe the preparation of the experiment. 



b. What volume of water was necessary to equalize the weights 



(i.e. how much water was given off from the plant in 

 24 hours) ? 



c. In a similar way add water each 24 hours for a week or 



until the plant shows signs of wilting. What is the total 

 amount of water given off during the experiment ? 



d. Bearing in mind the relative amount of leaf surface of this 



plant and on the trees of a forest of even a few acres, 

 what would you infer as to the quantity of water given 

 off by the trees of a forest during a summer season ? 



70. Leaves as organs for food manufacture. While 

 studying the manufacture by plants of carbohydrates (sugar 

 and starch) we showed that the raw materials necessary for 

 this process are water and carbon dioxid. We have proved 

 that water enters the root-hairs by osmosis and travels in a 

 system of ducts through the woody portion of roots and stems 

 out to the leaves. Here the ducts of the stems connect with a 

 network of veins containing similar ducts by means of which 

 the soil-water is supplied to the cells that are to manufacture 

 the food. The carbon dioxid that is needed is secured from 

 the air. This gas passes through the openings (stomata) 

 in the epidermis, and enters the air spaces in the mesophyll, 

 and finally reaches the cells containing the chlorophyll 

 bodies. 



The sunlight acting upon the chlorophyll bodies enables 

 them to combine the elements found in the water and carbon 

 dioxid to form carbohydrates. These, as we have seen, are 



