244 GROWTH 



Thus it is seen that plants may be too hot or too cold for 

 growth, and between these extremes there is an optimum or 

 best temperature where they make the most satisfactory 

 progress. 



The optimum temperature for most common farm and garden 

 plants is about 28° C, while the maximum usually lies between 

 38° and 43° C. ; the optimum for maize, scarlet-runner, bean and 

 cucumber is about 33° or 34° C., the maximum about 46° C. 



It may be conveniently noticed here that although ordinary 

 plants in an active state of growth have their development 

 stopped at the temperatures indicated above, the death of the 

 protoplasm does not usually take place until the higher tempera- 

 ture of about 56° C. is attained or until it has been cooled to 

 freezing-point or several degrees below the latter. 



The power of withstanding heat and cold depends very largely 

 upon the amount of water which the plant contains. 



Well-ripened shoots and buds containing little water do not 

 suffer so much from the effects of frost during winter as sappy 

 immature shoots which contain a larger proportion of water. 

 Turgid seedlings, buds just opening and recently unfolded 

 leaves, plants watered in the evening, succulent roots and all 

 parts containing considerable amounts of water are often injured 

 by exposure to sharp frost for a few nights. 



Usually when a plant is subjected to a temperature of 2° to 5° C. 

 the cytoplasm allows a certain amount of pure water in the vacuole 

 to ooze out of the cell into the surrounding intercellular spaces 

 where it freezes into small crystals of ice : death in such cases 

 is somewhat analogous to death by drying. Although plants are 

 sometimes killed in the process of freezing, this formation of ice 

 is not always fatal, for in many cases, if the frozen part is thawed 

 very slowly, the cells re-absorb the water and the tissues assume 

 their normal state. If, however, the frozen part is thawed rapidly 

 the water does not re-enter the cells and death takes place. 



Frozen potted plants should not be exposed to the direct rays 



