416 THE FOOD OF PLANTS 



presence of small amounts of cobalt, zinc, manganese, fluorine, lithium, &c., 

 and thus the dry weight of the crop formed in a certain time during the 

 culture of a fungus may be much increased. It appears that the higher 

 plants can react in the same manner, for according to Frank and Kriiger 

 a trace of a copper salt favours the growth of the potato, while in some 

 such stimulating action may lie the explanation of Salm-Horstmar's con- 

 clusions that for the full development of summer barley the presence of 

 fluorine and lithium is essential ] . 



The increased growth appears to be due to a general power of reacting 

 against injurious influences possessed by living organisms, for similar 

 results are produced by ether, alkaloids, &c., not only upon growth, but 

 also upon respiration and fermentative activity 2 (Sect. 104). Richards 

 has shown that besides its nutritive importance, an iron salt, when present 

 in sufficient amount to act as a feeble poison, may exercise a similar 

 stimulating effect to that which any other poison would do. A very strong 

 poison produces its optimal stimulating effect when extremely dilute, and 

 growth may be retarded by doses above the optimum, while substances 

 which act as poisons only when highly concentrated produce no perceptible 

 result at all. 



The presence of a non-essential substance may therefore be useful or 

 even necessary under special conditions, as is the case for instance when 

 the absorption of a nutrient material is accelerated or rendered possible 

 by the action of an enzyme or an acid. Moreover, it must be remembered 

 that the forced non-performance of any accessory function, even though 

 it does not form an essential part of the actual vital mechanism, may 

 ultimately lead to a more or less marked inhibition of the general vital 

 activity (Sect. 64). Calcium is perhaps essential to the higher plants for 

 some such reasons, and it is possible that in certain cases silicon may 

 become essential in a similar manner. Plants growing wild are exposed 

 to severe competition, and hence a very trifling cause may determine 

 whether a given species will survive or die out (Sects. 76, 92). For this 

 reason the presence of silica may be of considerable importance, in con- 

 ferring a special means of protection against the depredations of animals, 

 and against the penetration of parasitic fungi (Sect. 75). 



From what has been said it is clear that it is not always easy to decide 

 with certainty whether an element is essential or not. Moreover, if but little 

 of a substance is required, the presence of the merest traces as impurities in 



1 The stimulating action of poisonous substances was first observed by Raulin (Ann. d. sci. 

 nat, 1869, v. ser., T. xi, p. 252), but their general importance was established later. Cf. Richards, 

 Jahrb. f. wiss. Bot., 1897, Bd. xxx, p. 665, and Pfeffer, 1895, Bd. xxvm, p. 238. See Sect. 66; 

 Frank u. Kriiger, Ber. d. Bot. Ges., 1894, p. i; Salm-Horstmar, Journ. f. prakt. Chemie, 1861, 

 Bd. i.xxxiv, p. 140. On Fl, see also G. Tamman, Zeitschr. f. physiol. Chemie, iSSS, Bd.xn, p. 322. 



! These are physiological reactions, and are quite distinct from the acceleration of a chemical 

 reaction, which may be caused by the addition of certain substances. 



