54 THE STORY OF PLANT LIFE 



nected. The requisite temperature at which each 

 plant thrives differs according to species. With an 

 increase in temperature growth is promoted. But if 

 it is too high plants become lank or too tall and do 

 not grow laterally. The seedling requires a lower 

 temperature than the growing plant, and either than 

 a plant in the fruiting stage. 



Water, especially in the case of terrestrial plants, 

 is of vital importance. There is a medium of supply 

 below or above which plants do not flourish. The 

 salts in solution must also be of the right kind, and 

 neither in excess nor the reverse. With excess of 

 water there is a reduction of available oxygen. With 

 too little the plant cannot be maintained in a state of 

 turgescence or distension. 



If the nutrient salts in solution are very concen- 

 trated, the loss by evaporation in the cell-sap is 

 reduced. If the food is increased water is economised. 

 The necessary water of transpiration must frequently 

 exceed the amount available by absorption. 



When water is not abundant the output of leaves 

 is less and transpiration is not so rapid. Excess of 

 water tends to vegetative growth, whilst the reverse 

 or drought induces the hastening forward of flower- 

 ing and seeding. 



The organic substances of which the plant is com- 

 posed contain compounds derived from the foregoing 

 elements. They are either non-nitrogenous, or 

 nitrogenous. To make the account of the processes 

 by which these substances are built up in the 



