ABSORPTION OF MINERAL ELEMENTS BY PLANTS 243 



the inside with a paraffin layer, in order to prevent the solution 

 of elements found in the glass. All of these substances, like iron, 

 are required by plants only in exceedingly small quantities. 

 With the usual method of water cultures, there is no need to 

 introduce them into the nutrient solution. Traces are always 

 found in the common "chemically pure" reagents or are leached 

 from the glass walls of the container. But some plants are so 

 sensitive to a deficiency of these elements that for a long time 

 they could not be grown successfully in water cultures. As such 

 may be mentioned flax and tobacco, which require the addition 

 of boron salts to the nutrient solution. It is not improbable 

 that further investigations with more carefully purified reagents 

 will reveal the necessity of still other elements for the normal 

 development of the plant, those that now are regarded as acci- 

 dental and useless admixtures. 



It must be noted that the relative amount of one element or 

 another in the ash of the plant by no means indicates the degree 

 of its importance. The plant can do without sodium, an element 

 always constituting a perceptible part of the ash of plants and 

 even accumulating abundantly in some of them. On the other 

 hand, the presence of traces of manganese and boron is absolutely 

 necessary. Likewise, though a plant always contains a con- 

 siderable amount of sihca in its ash, only an insignificant part of 

 this is really indispensable, all the rest evidently being unneces- 

 sary. Sometimes, however, the accumulation of superfluous 

 elements may play an accessory role. Thus, the sihcic acid 

 accumulating in the cell walls of cereals serves to stiffen and 

 protect them from attacks of fungi. 



The common elements used in nutrient solutions are absolutely 

 necessary for the proper growth of plants. The exclusion of any 

 of them leads to a check in development and finally to death. 

 Figure 73 shows clearly what conspicuous differences may be 

 obtained in the development of plants in a complete nutrient 

 solution and in one from which a single element is absent. It 

 seems to make little difference which of the elements is excluded, 

 all of them being equally necessary. The slight difference that 

 may be observed when various elements are excluded depends 

 not so much upon their comparative value as upon the amount 

 of the particular element stored in the seed and upon the quantity 

 used for the development of the plant. 



