24 PLANT PHYSIOLOGY 



magnesium are much more poisonous ; a 0-15 per cent, solution of MgCl 2 was fatal 

 to Phloeum. Many of the nutrients, therefore, must be injurious even in the 

 concentrations in which they occur in nutritive solutions, unless their poisonous 

 influence be counteracted by other substances. Thus, according to LOEW (1892), 

 the poisonous effect of magnesium salts is restrained by the addition of calcium 

 carbonate. BENECKE (1904) demonstrated a corresponding action of chalk, 

 and OSTERHOUT (1906) has brought forward similar evidence. One thus 

 appreciates how difficult it is to arrive at accurate conclusions with regard 

 to the functions of the individual elements with the aid of the water-culture 

 method. Every withdrawal of an element may have two effects, one due to 

 actual absence of the element itself, the other due to the poisonous activity 

 of another element whose action was previously inhibited by the absentee. 



Recently, it has been observed that poisons which even in weak con- 

 centrations are injurious, aid the development of the plant when administered 

 in still weaker doses. RAULIN (1869) had already shown that manganese 

 stimulates the growth of Mould Fungi, yet there is no doubt that these organ- 

 isms can exist for generations without manganese, and that it must not be 

 looked upon as a nutrient. Still more noticeable were RAULIN 's discoveries 

 with regard to zinc, discoveries which have been recently completely con- 

 firmed by RICHARDS (1897). RAULIN showed that the addition of 0-0005 per 

 cent, of ZnSO 4 to a nutritive solution materially aided the growth of Fungi, 

 and that a 0-003 per cent, solution of the same salt brought about a doubling 

 of the plant's weight. The greatest effect was observed with this concentra- 

 tion ; any further increase not only inhibited growth but had a directly in- 

 jurious effect. There are quite a number of substances which behave in a 

 similar way, acting favourably in dilute solutions but injuriously in stronger. 

 CoSO 4 gives an optimum effect with a concentration of 0-002 per cent. ; NiSO 4 

 acts best in a 0-033 per cent, solution. ONO (1900) found that an acceleration 

 of growth took place after the addition of minute quantities of LiNO 3 , K 2 As0 3 , 

 and NaFl to Algae and of HgCO 2 and CuSO 4 to Fungi. In Phanerogams also 

 similar results have been obtained with the use of CuS0 4 , ZnS0 4 , and NaFl 

 (LoEW, 1903 ; KANDA, 1904). A comprehensive exposition of the whole 

 phenomenon will be found in LAFAR'S Mykologie (I, p. 342), from the pen of 

 BENECKE. Without going into a discussion of the original papers themselves, 

 \ve may conclude from this account that such effects are produced by many 

 kations, but that certain anions, e.g. Cl and Si0 2 , are also potent. Organic 

 poisons, such as morphine and amygdalin, have also the same effect, and 

 a stimulating action has been shown to be possessed even by certain unknown 

 metabolic products of certain Mould Fungi on the organisms excreting them 

 (NIKITINSKI, 1904). 



RAULIN, on account of the fact that they promoted development, regarded 

 Zn and Si as direct nutrients to Fungi, a view which cannot be accepted 

 nowadays. It is more probable (PFEFFER, 1895) that these substances act as 

 stimulants only, and that they further growth, though in a manner as yet 

 unknown to us (Lecture XXV). One thing is clear, however, that to decide 

 whether a substance is a necessary food-stuff or only a stimulant is by no means 

 an easy task ; doubtless, the genuine nutrients also act as stimulants. The 

 facts already quoted explain also the effect of copper on the higher plant, an 

 effect which has been demonstrated in agriculture in the employment of the 

 so-called ' Bordeaux mixture ' (a mixture of copper sulphate and calcium 

 carbonate). If plants like potatoes or tobacco, liable to suffer badly from 

 diseases due to Fungi, are sprayed with Bordeaux mixture they are thereby 

 rendered immune, for copper is extremely poisonous to the lower plants. 

 NAGELI (1893) has shown that it is fatal to Spirogyra even when in a solution 

 so dilute that it cannot be detected chemically. The fungicidal effect of 



