III.] ELEMENTS NECESSARY TO PLANT GROWTH 51 



shown in the illustration, from which it will be seen that 

 only soda and perhaps chlorine can be omitted without 

 some injury to the plant. When either nitrogen, 

 phosphoric acid, or potash is omitted, growth ceases as 

 soon as the food supply in the seed is exhausted ; in the 

 absence of sulphuric acid, or magnesia, or lime, growth 

 continues, though it becomes rather abnormal ; but soda, 

 silica, and sometimes chlorine can be left out without 

 causing any difference to the growth. If no iron is 

 added to the solution, the leaves of the seedling plant 

 soon become very pale in colour; the new shoots in 

 particular will be of a pale straw colour and possess no 

 substance, until in a short time the whole plant perishes. 

 But if a few drops of ferric chloride are added to the 

 solution, on the following day the veins of the leaves 

 will be seen to be turning green again, and the whole of 

 the leaf will rapidly follow suit. In some way an iron 

 salt is necessary to the formation of the chlorophyll — the 

 green colouring matter of the leaf — without which no 

 assimilation can take place. We may extend the 

 method of water cultures to ascertain in what state of 

 combination the various elementary constituents must 

 be presented to the plant if they are to be utilised by 

 the plant. In our experiment, for example, we have 

 used a nitrate as the source of nitrogen, and seen that 

 the plant can take in this compound of nitrogen and 

 elaborate from it all the proteins and other complex 

 nitrogenous bodies it contains ; which of the numerous 

 other compounds of nitrogen can be similarly utilised ? 

 As a matter of fact, only a very few, and those of the 

 simplest type of construction, can be so utilised ; in 

 addition to the nitrates, plants will take in the salts of 

 ammonia and a very few bodies like asparagin — i.e., simple 

 amino bodies such as they build up themselves at a very 

 early stage from the still simpler nitrates and ammonia 



