4H THE FOOD OF PLANTS 



but also for the organic food, for the action of light, and indeed for all 

 factors or agencies which produce a perceptible effect only at a certain 

 degree of concentration, above which their action gradually increases until 

 the optimal effect is produced. 



From the complicated interactions and correlations which exist between 

 the different parts of the vital mechanism, it is only to be expected that the 

 optimal amount of any given substance may within certain limits be a variable 

 quantity. This is shown by O. Wolff's l empiric results, obtained while estimating 

 the minimal quantity of potassium or phosphorus, necessary for the full develop- 

 ment of an oat-plant when all the other elements were present in sufficient 

 amount. All the elements cannot however be simultaneously reduced to this 

 lowest possible minimum. It was not found possible to develop a normal oat- 

 plant containing less than 3 to 4 per cent, of ash, whereas the latter would only 

 amount to 2 per cent, could all the elements be reduced to their minimum amount. 

 This may possibly be due to the fact that the place of any particular essential 

 element may be partially taken by others when these are present in sufficient 

 amount. Non-essential elements may also aid in this, for apparently silicon may, 

 like calcium, help to further reduce the minimal amount of potassium or phos- 

 phorus necessary for development. 



In the older organs the percentage of ash usually increases, and its composition 

 alters. The young organs contain for the most part only what is absolutely 

 necessary, and hence their ash retains an approximately similar composition " in 

 different plants. On the average 3 grammes of the ash of embryonic tissues 

 contain the following amounts of the different elements : phosphorous pentoxide 

 i-i g., potash i-o g., magnesia 0-35 g., lime 0-25 g., sulphur trioxide o-i g., ferric 

 oxide 0-03 g., soda 0-08 g., chlorine 0-04 g., silica 0-05 g. 



In adult organs the calcium and silicon increase to a marked extent and may 

 frequently form 50 percent, of the ash, so that the harvest from a hectare (2! acres) 

 may remove annually 200-300 kilos of the mineral constituents of the soil :1 . 



An essential element must always be presented in the form of 

 appropriate chemical combinations, for upon this its nutritive value largely 

 depends, as has already been seen in the case of nitrogen and carbon. 

 This is not of so marked importance as regards the ash constituents, 

 probably because the salts usually absorbed split up into their ions when 

 they dissolve in water. Acids as well as bases can be used as food by 

 higher plants only when highly oxidized, but fungi can assimilate 

 sulphurous (H 2 SO 3 ) and hyposulphurous (H 2 SO 2 ) acids, while a few 

 bacteria are able to utilize sulphur and sulphuretted hydrogen, and many 

 fungi seem able to assimilate the lower oxides of phosphorus. It is still 



1 O. Wolff, Versuchsst, 1877, Bd. xxx, p. 387. 



2 \Jarreau, Ann. d. sci. nat., 1860, iv. ser., T. xni, p. 179. In Wolff's tables the influence of 

 manuring is given. On fungi, see Sieber, Bnct. Centralbl., 1892, Bd. x, p. 78, and Zopf, Pilze, 1890, 

 p. 114. 



3 Cf. Ebermayer, Physiol. Chem., 1882, p. 761. 



