352 EXPERIMENTS WITH PLANTS 



HEAT. At the beginning of the chapter it was re- 

 marked that to give the plant too much of any of the 

 things it requires is as bad as to give it too little. 

 This is, perhaps, more strikingly seen in the case of 

 heat than in other cases. We all know that too much 

 heat quickly kills both animals and plants. On the 

 other hand, if they have too little, they stop growing, 

 and may die. 



In general, the less water the plant contains, the 

 more resistant it is to cold. This is well shown by the 

 behavior of greenhouse plants; excessive watering 

 makes them over -sensitive to heat and cold (and light 

 as well; such plants are liable to " sunscald")- Plants 

 which have just been transplanted are said to be less 

 sensitive to frost, since they contain less water. Frost 

 does not injure buds in winter when they are compara- 

 tively dry, but in spring, when they are full of sap, it 

 kills them. Frost, during the period when sap is run- 

 ning, causes the bark to adhere to the wood, and it 

 does not again become separable until the return of 

 milder weather. In such cases, the health of the tree 

 may not always be affected; but the crop of fruit falls. 



Dry seeds which contain very little water (about 10 

 to 15 per cent) stand extremes of temperature which 

 would kill them if full of water. Seeds have been kept 

 at the temperature of liquid hydrogen (minus 238 C., 

 or minus 396 F.) for some time, and carefully thawed 

 out again, when they grew quite normally. 



