INTRODUCTION 33 



degree of temperature, a requisite supply of oxygen 

 and water, these modes of activity are started into 

 action and the plant grows. Light and a supply of 

 food are also necessary conditions. Growth depends 

 on the activities of living cells, and at the growing 

 point the cells must be in a condition to divide and 

 multiply. 



Owing to the existence of seasonal changes in 

 temperature growth is not continuous, and is accele- 

 rated or started, after the quiescent stage of winter, 

 in spring, when the above conditions first begin to be 

 realised, i. e. those that undergo change such as heat 

 and light. 



A temperature which is most suitable for plant- 

 growth is about 28° centigrade, though plants can 

 grow below this, and above about 40° C. plant 

 growth is not usually possible. The first tempera- 

 ture is the optimum or most suitable temperature, 

 the second is the maximum, while there is also a 

 low temperature or minimum below which growth 

 is reduced or impossible. These values differ for 

 every plant. Above a temperature of 56° C. plants 

 usually die. 



The amount of water largely regulates these points 

 of temperature. Seeds can resist higher and lower 

 temperatures than the plant itself. 



The presence of water in a plant is necessary for 

 growth, for the filling of the cell with cell-sap, other- 

 wise the plant collapses, as partly happens in wither- 

 ing or wilting. The size of plants depends on the 



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