424 THE FOOD OF PLANTS 



probable that this is the case also with potassium and magnesium, for young 

 organs rich in protoplasm as well as isolated masses of the latter contain 

 potassium and magnesium, as well as nitrogen and phosphorus, in relatively 

 large amount. Moreover, water does not extract from a dead cell all the 

 potassium it contains, even though acids are present which form soluble salts 

 of potassium, and in aleurone grains compounds of magnesium and calcium 

 with proteids apparently occur. 



The different mineral elements may be used to form various organic 

 compounds, and hence the mineral constituents are not only present 

 in the protoplasm, but also in the cell-sap and even in the cell-wall 

 also. They may occur as salts of organic acids, and may also apparently 

 combine with carbohydrates and other bodies. The soluble organic and 

 inorganic salts of potassium and magnesium are very commonly utilized 

 in the maintenance of turgidity (Sect. 24). although other substances are 

 also of importance, and indeed since turgidity is only of accessory importance 

 to the actual vital phenomena, it is not surprising to find that under certain 

 cultural conditions sodium and calcium may partly replace potassium and 

 magnesium, and must indeed do so in those cases in which an absorption of 

 calcium or sodium does not cause any increase in the osmotic pressure '. 



It is certain that calcium may serve a variety of purposes in the plants 

 which need it, and if, as appears probable, it may be absent from the 

 primary meristem, it can only be of secondary importance, either to 

 neutralize oxalic acid or to aid in the formation of the cell-wall, although 

 when present it may enter into intimate relationship with the protoplasm 2 . 



The results of chemical analysis afford but an approximate indication 

 of the substances present in the living plant, for when death occurs various 

 substances in each cell come together which were formerly held apart. 

 The precipitation or discolouration which may then take place is the outward 

 sign of a few only of the various chemical reactions which are thus 

 induced, and it is probable that the different substances which form an 

 essential part of the protoplasm undergo more or less marked modification 

 or dissociation as soon as life ceases (Sects. 7, n). In the form of organic 

 compounds, magnesium and calcium, as well as phosphoric acid, may remain 

 unprecipitated in a neutral or alkaline solution, and hence also in the plasma. 



The function of an essential element is by no means directly indicated 

 by the results which its absence produces, for, owing to the complex 



1 [Copeland (The relation of nutrie.it salts to turgor, Hot. Gazette, Vol. XXIV, Dec., 1897, 

 p. 399) concludes that of the mineral constituents potassium salts play a far more important part in 

 the maintenance of turgor than is generally supposed, but it is certainly not the case that potassium 

 is necessarily of primary importance in all cases, viz. beet-root, onion bulbs, &c.] 



a [Bokorny, Bot. Centralbl., Bd. LXXIV, 1898, p. 258. Molisch (Ber. d. Wien. Akad., 1895, 

 Bd. CIV, p. 795) has shown that the partial absence of Ca causes an incomplete formation of the 

 transverse walls in Spirogyra.~\ 



