THE FOOD OF THE PLANT 13 



A series of seedlings should be grown, one in a 

 normal culture solution, one in a culture solution with 

 potassium omitted, another in a solution with calcium 

 omitted, and so on : in every case one of the elements 

 should be omitted and all the others included. By this 

 means it is possible to see which of the elements are 

 actually necessary for the full development of the 

 plant. 



All this is simple enough in principle, but, unfortu- 

 nately, the experiments are not easy to carry out. Very 

 large vessels are needed for the solutions, and they 

 are only with great difficulty kept free from fungus- 

 growths. 



To avoid disappointment the beginner is advised to 

 wait until experience has been gained in experimental 

 methods before attempting to work with culture 

 solutions. 



Some general results are therefore given here. 



For full development, all the elements present in the 

 list are necessary. A seedling grown without nitrogen 

 will be very small ; one without iron will not be of a 

 normal colour; and, in general, the absence of any 

 element will cause the seedling to suffer in some way. 

 But, given all the elements present in the list, the 

 seedling will thrive as well in the culture solution as in 

 the soil, always provided that its roots will adapt 

 themselves to the watery medium in which they are 

 growing. It will be noticed that no carbon was included 

 in the culture solutions ; it can thus be proved that the 

 plant does not require anjr carbon from the soil, although, 

 as seen before (page 2), it is often present there in the 

 form of carbonates. 



The Force that produces Root-absorption. — The 



next step is to determine what it is that causes the 

 absorption of water by the root. It is not possible 

 actually to see the process directly, but an experiment 

 can be set up in which a similar phenomenon can be 

 observed, and which will serve to make clear the force 

 by which the absorption is brought about. 



