Plants in their Relation to Disease. 169 



for the same genus also, their functional value is the same, but the more dis- 

 tant the relationship, the more widely will the physiological characteristics 

 of the plant be separated, and so will the special value of any element in the 

 processes of nutrition and growth become subject to wider variations.? This 

 appears in a general way in the varying proportion in which these elements 

 are taken up by the plant, so that while potash, for example, is demanded in 

 relative excess by certain plants, in others soda seems to be the special base 

 requisite to the given changes. And so with the muriate of potash, which 

 Nobbe has shown to be essential to the metastatic changes in buckwheat, 

 and which Dr. Goessmann has also shown to be necessary to similar changes 

 in the diseased peach. When, however, this salt is applied to the pear, 

 though belonging to the same family of plants, the result is not equally sat- 

 isfactory, for here the action of the chlorine appears to be replaced by sul- 

 phuric acid, and so far as we are at present able to determine, the sulphate 

 of potash — which has produced most favorable results in a number of cases — 

 is the best form of special fertilizer for the treatment of blight. Therefore, in 

 considering the relation of the nutrient elements to these changes, it is as 

 impossible as it is unwise to attempt to dogmatically formulate a law of gen- 

 eral application. Each subject must be treated according to its individual 

 or family characteristics. 



Of all the chemical elements known, somewhat less than one- fourth may 

 be regarded as embr.icing the various constituents of the plant. They are : 

 Mineral, as obtained from the soil, and constituting the inorganic portion of 

 the plant — iron, manganese, lime, magnesia, potash, soda, phosphorus as 

 phosphoric acid, sulphur as sulphuric acid, chlorine, silicon as silicic acid; 

 and combustible, as obtained from the soil and air, constituting the organic 

 portion of the plant — carbon, oxygen, hydrogen, nitrogen. 



Of these we may consider the last four as absolutely essential to the con- 

 stitution and growth of the plant, since they enter into the composition of 

 that which is of fundamental importance -the protoplasm — as well as into 

 the cellular framework. They can not be eliminated without disorganiza- 

 tion of the structure. The mineral constituents are all derived from the 

 soil in solution through the roots. The precise value of each is by no means 

 well known, but certain it is that iron is essential to the proper formation of 

 chlorophyl; silicon seems to exert an important influence as an element of 

 strength; potash is directly connected with the function of assimilation, and 

 so with the formation of starch and sugar, and so probably are both soda and 

 lime; while the acids in general may doubtless be regarded as essential to 

 those changes which are chiefly involved in the transformation of assimilated 

 material as incident to the direct nutrition of parts. The general importance 

 of these elements may be justly inferred not only from their more or less 

 constant presence, but from the injury which follows their exclusion. We 

 have also to consider that failure in the supply of any particular element 

 does not alone cause a direct effect through disturbance of the particular 



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