240 BOTANY PART I 



mixture of the elements H and is not a substitute for water, it is 

 not sufficient to supply the plant with the elements contained in the 

 nutrient salts either as elements or in any of their combinations. 

 Thus metallic potassium or pure sulphur are of no use. The plant 

 requires particular salts or, since these in part dissociate in water, 

 particular ions. Necessary kations are K + , Ca ++ , Mg++, Fe ++ (or 

 Fe +++ ), while S0 4 ~ ", H 2 P0 4 ~ arid N0 3 ~ are necessary anions. While 

 phosphorus and sulphur can only be utilised in these combinations, 

 the nitrogen can also be obtained, although not always so usefully in 

 the form of the kation NH 4 +. 



The method of water culture has not only shown the necessity for 

 certain salts, but also that many substances, especially sodium, chlorine, 

 and silicon, which the plant usually absorbs can be done without. 



Even in halophytes, in which it is present in greatest quantity, sodium is not 

 indispensable. These plants live in soils rich in sodium chloride not because this 

 substance is necessary to them but because they bear it better than other plants 

 do. The concurrence of these in such localities is thus prevented. The character- 

 istic succulent construction of halophytes (Fig. 195) is more or less completely lost 

 in the absence of common salt. Sodium appears to be indispensable to the Diatoms 

 and some Seaweeds ( <2 ). 



Silicon is not indispensable to Equisetum and Grasses which contain considerable 

 quantities of Si0 2 ; on the other hand, it is requisite to the Diatoms, the cell walls 

 of which are almost entirely composed of silicic acid, and owe their permanence 

 to this. The cell walls of Diatoms form considerable geological deposits of siliceous 

 earth or kieselguhr. Aluminium ( 21 ), while generally distributed in small quantities, 

 is only absorbed in considerable amount by a few plants (e.g. species of Lycopodium) ; 

 whether useful or indispensable to these is not established. On the other hand, 

 although scarcely a trace of iodine can be detected by an analysis of sea-water, 

 it is found, nevertheless, in large quantities in seaweeds, so much so that at one 

 time they formed the principal source of our supplies of this substance. Whether 

 it is essential to these plants is not known. 



The substances which, as culture experiments show, are not indispensable for 

 the life of the plant are, however, of use in so far as they can replace for some 

 purposes (such as the neutralisation of free acids, etc.) essential elements of plant 

 food. The latter are thus available for the special purposes for which they 

 are indispensable. Thus K can be partially replaced by Na, and Mg by Ca. 

 Certain other substances, although not indispensable, are of use in the plant 

 economy and of advantage to growth. For example, Buckwheat flourishes 

 better when supplied with a chloride, and the presence of silica is advantageous 

 as contributing to the rigidity of the tissues. It has also been found that the 

 presence of certain substances which are not of direct use may inhibit the poisonous 

 action of other substances some of which are necessary. 



Absorption of Nutrient Salts. The nutrient salts can only be 

 absorbed by the superficial cells of the plant when in solution. The 

 question has to be considered in what way the dissolved substances 

 reach the vacuole through the cell wall and the protoplasm. It was 

 seen in connection with plasmolysis (p. 226) that the protoplasm is 

 semi-permeable, i.e. permeable to water but not to dissolved substances. 



