14S Elementary Botany 



CHAPTER XIII. 



INFLUENCE OF HEAT AND LIGHT UPON GROWTH. 



The various vital phenomena can only be carried on within 

 a certain range of temperature. There is a minimum below 

 which and a maximum above which the activity ceases. The 

 maximum and minimum vary with different plants and for the 

 different processes of the same plant. Roughly speaking, how- 

 ever, from 0° C. to 50° C. may be looked upon as the range. 

 Between the maximum and minimum there is always a tem- 

 perature, the optimum, at which the vital function is most 

 active. 



Thus, taking the germination of Maize, g'5° C. is the lowest 

 temperature at which it is possible ; as the temperature is in- 

 creased the Maize germinates more readily until the optimum 

 is reached at 337° C. Beyond this the germination is less 

 and less active until the maximum is reached at 46-2° C. 



Comparing Wheat with this, we find that the minimum of 

 germination is 5° C, the optimum 28'7° C, and the maximum 

 377° C. 



The injurious action of high and low temperatures upon 

 plants depends to a great extent upon the amount of water 

 present. Dry peas do not lose their power of germination even 

 if exposed to a temperature of 70° C. for an hour, whilst if wet 

 a temperature of 54° C. will kill them. In the case of low 

 temperatures the injury is due to the water being separated from 

 the protoplasm. It never freezes within the cell, but is separated 

 and forms crystals of ice outside the cell wall. These crystals 

 often considerably injure the tissues. If afterwards the thaw- 

 ing is gradual the tissues may recover, but if it is too rapid the 

 water cannot be reabsorbed, and either rotting ensues, or the 

 water evaporates and the plant dries up. 



