ABSORPTION BY THE ROOT 205 



more slowly. There are -minimum, optimum, and maximum 

 temperatures for absorption (see page 194). During summer 

 roots absorb more rapidly than in winter, for one reason, 

 because the soil is warmer. 



Strength of the solutions. — If we make the culture-solution 

 gradually stronger and stronger, we find that, after a certain 

 strength is attained, the root absorbs more and more slowly as 

 the strength of the solution increases, till finally it practically 

 ceases to take in any liquid. We thus see that the root can 

 take up only weak solutions of salts. 



Transpiration. — Again, carrying the plant into a better- 

 lighted spot, or where there is a drier atmosphere, absorption 

 increases in rapidity (see next chapter). 



CHEMICAL ELEMENTS ESSENTIAL TO PLANT-LIFE, 

 AND THEIR ABSORPTION. 



It has already been stated that a plant may be successfully 

 cultivated if supplied with the simple inorganic culture-solution, 

 mentioned on page 191, if the atmospheric air is also accessible. 

 In the solution and the atmosphere together there are the 

 elements carbon, oxygen, nitrogen, hydrogen, sulphur, phos- 

 phorus, iron, calcium, magnesium,, potassium (as well as common 

 salt, which the plant can dispense with). Every plant requires 

 all these elements : if it be deprived of one of them it cannot 

 continue to live and grow. The first six elements are essential 

 to build up protoplasm. Iron is required for the formation of 

 chlorophyll. The carbon is taken in as carbon dioxide by the 

 green parts. The last six elements must be absorbed by the 

 roots, because they are not found in the atmosphere : they are 

 absorbed in the form of salts in solution. Hydrogen and 

 oxygen are taken in as water (HgO). But oxygen and nitrogen, 

 so far as we have yet learned in this book, might be absorbed 

 from the air or from the soil, as they occur in both. The 

 oxygen will be dealt with in a future chapter. There only 

 remains nitrogen. 



Absorption of Nitrogen. — Does the plant absorb the free 

 nitrogen from the atmosphere by its shoot, or does it take 

 nitrogen in by its roots ? If we take away all the combined 

 nitrogen (nitrates) from the culture-solution already described, 

 and endeavour to grow a seedling of a Sunflower in the culture- 

 solution thus impoverished, the plant remains stunted and soon 



