136 BOTANY: PRINCIPLES AND PROBLEMS 



419. The internal temperature of plants is not far from that of their 

 surroundings; in the higher animals it is usually much above that of 

 their surroundings. Explain. 



420. Why do land animals need to have lungs for inhaling and exhaling 

 air, when such structures are unnecessary in plants? 



421. Which will weigh more, a piece of sound wood or a piece of 

 decayed wood of the same volume? Which will yield more heat when 

 burned? Explain. 



422. Why do plants in glazed pots grow poorly? 



423. Why should pebbles or bits of broken pottery be placed in the 

 bottom of a flower pot in which a plant is to be grown? 



424. In propagating a plant by "cuttings", a small shoot is cut off 

 and the cut end placed in damp soil, where it takes root. Sand is much 

 better than a heavy clay soil for this purpose. Why? 



425. Cut flowers will keep longer in a refrigerator than at ordinary 

 room temperatures. Why? 



426. In cranberry bogs which have been flooded in the fall to protect 

 them from frost, the unripe berries suffer much more from "smothering," 

 due to lack of oxygen caused by the flooding, than do the mature, fully 

 ripe ones. Why? 



427. Fermentation often generates a considerable gas pressure, but 

 respiration does not. Explain this difference. 



428. What causes yeast bread to "rise"? 



429. Why is it important to boil down maple sap as soon as it is drawn 

 from the tree rather than to let it stand? 



430. When jars of preserved fruit "spoil," why do the covers some- 

 times blow off? 



431. A foundation of stable manure under a hot-bed will "heat" 

 and thus keep the soil warm. Explain. 



432. Why must the manure under a hot-bed be moist before it will 

 "heat?" 



433. Give at least two reasons for lifting the glass from a hot-bed in 

 the middle of a sunny day? 



434. What danger is there in putting insufficiently dried hay into a 

 barn? Explain. 



