88 METABOLISM 



doses and never accelerate growth, e. g. mercuric chloride, copper sulphate in 

 Algae (Ono), copper sulphate in Aspergillus (Richter, 1901). How are such 

 results to be interpreted ? Raulin considered zinc and silicon as direct nu- 

 trients for Fungi, a view which cannot be accepted nowadays ; it is more 

 probable that Richards (following on the preliminary investigations of Pfeffer, 

 1895) is correct in holding that these substances act as stimulants and not as 

 nutrients. Furthermore, on the presentation of the optimum amount of the 

 stimulant, development in the fungus is no longer normal, but an increase of 

 vegetative growth is induced and a retardation of the formation of conidia. The 

 normal correlation of growth in the organs is also interfered with, and the organism 

 as a whole ceases to thrive when such conditions are introduced. It is, as a 

 rule, quite possible to differentiate nutritive from stimulatory materials, for when 

 the nutritive substances, or only ong of them, are care fully ehminated, the develop- 

 ment of the organism comes to a standstill ; when the stimulants are omitted, 

 growth is retarded but is otherwise normal. This distinction is not readily 

 made out in all cases ; iron, for example, is a difficult element to deal with, be- 

 cause it is essential onlyin the minutest traces, and is possibly both a nutrient and 

 a stimulant. Further, it must be remembered that such well-known food-stuffs as 

 salts of potassium are tolerated by the plant only when in very low concentration, 

 while in higher concentration they act injuriously owing to their osmotic action. 



The facts which have now been put forward render intelligible a whole 

 series of observations which were previously obscure. For example, take 

 Benecke's observations on rubidium. We must bear in mind that the ru- 

 bidium presented to the plant may not have been quite free from potassium as 

 an impurity, so that we might regard the rubidium merely as a stimulant, while 

 the traces of potassium might be considered as nutritive. Rubidium acts, in 

 fact, like zinc sulphate ; in relatively small amounts it acts directly as a poison 

 and hinders the formation of conidia. Stimulus action further explains the 

 favourable influence often observed on the addition of silica to a water-culture, 

 perhaps also the good effect which carbon disulphide has on arable land 

 previous to the beginning of vegetative growth, by the action of sodium fluoride 

 on crops and possibly also that of copper on the higher plants. Copper is usually 

 very injurious to plants, and Nageli has shown that it is a deadly poison to 

 Spirogyra even in a state of dilution so great that it cannot be chemically detected. 

 Hattori has shown that 0-00005 per cent, is the extreme limit of concentration 

 for the pea, and 0-000005 per cent, for the maize, above which injury ensues. 

 All the same, sulphate of copper together with lime, under the name of 'Bordeaux 

 mixture ' has been used with success in combating diseases due to Fungi, and it 

 has a further and unexpected effect, for plants sprinkled with the mixture grow 

 more luxuriantly than control plants not so treated, provided that the latter 

 are free from infection by the fungus. Vines and potatoes treated with copper 

 show a greater development of chlorophyll and a more vigorous production of 

 organic substance. Why syringing with this solution should have this effect 

 cannot be explained at present, and the most varied views are held on the subject. 

 The favourable effect of this mixture is claimed to be due to the lime ; Ader- 

 HOLD (1899) recently advanced the view that it was due to the adulteration of 

 this substance with iron, but it is also possible that copper ^zs, the active cause. 

 At all events we must not draw the contrary conclusion that copper is not to be 

 identified chemically in the leaves, since if copper be useful it must obviously 

 enter in only in the very minutest traces. A full discussion of the copper 

 problem is impossible here ; a summary of our knowledge, along with new 

 experiments, will be found in Bain, 1902. 



In conclusion, we may mention that organic substances of various kinds 

 may act in a similar way as stimulants, just as do the inorganic salts above 

 mentioned. Thus in Richards' s cultures cocaine and morphine acted as weak 



