RESPIRATION AND FOOD PRODUCTION 21 



3. Other things being equal, which would have the cooler, pleasanter 

 atmosphere in summer, a well-wooded region or a treeless one? (20.) 



4. Could you keep a bouquet fresh by giving it plenty of fresh 

 air? (14). 



5. Why does a withered leaf become soft and flabby, and a dried 

 one hard and brittle? (9, 14.) 



6. Why do large-leaved plants, as a general thing, wither more 

 quickly than those with small leaves? (14-19.) 



7. Is the amount of water absorbed always a correct indication of 

 the amount transpired ? Explain. (20, 21.) 



8. Why must the leaves of house plants be washed occasionally to 

 keep them healthy? (15-18.) 



9. Why is it so hard to get trees to live in a large manufacturing 

 town? (15, 1 8.) 



RESPIRATION AND FOOD PRODUCTION 



MATERIAL. A green aquatic plant of some kind in a glass of water ; 

 two wide-mouthed glass jars ; a bent glass or rubber tube, and a shallow 

 dish of water ; boiled bean or tropaeolum, or other green leaves ; a half 

 pint of alcohol ; some tincture of iodine ; a strip or two of tin foil. 



22. Leaves give off Oxygen. Place in a glass of water 

 a green aquatic plant of any kind ; the common brook 

 silk (Spirogyra) found in almost every pool will answer. 

 Set it in the sunlight and place beside it another similar 

 vessel containing nothing but water, and also a third ves- 

 sel containing a piece of the same plant immersed in water 

 from which the air has been expelled by boiling. After a 

 time bubbles will be seen rising from the first vessel. Air 

 bubbles will usually form on the bottom and sides also, 

 but these are caused by the expansion of gases contained 

 in the liquid, as will be evident on comparing them with 

 similar phenomena in the jar containing only water, and 

 must not be confounded with the gas given off by the 

 plant. Remove the vessel from the light, and the bubbles 

 will soon cease to appear, but will begin to form again if 

 restored to the sunshine, thus showing that their produc- 

 tion can take place only in the light. Do any bubbles at 

 all appear in the glass with the boiled water ? 



It has been proved by chemical analysis that these 

 bubbles are oxygen, which the plant has been separating 



