PHYSIOLOGY 189 



It has also been discovered that by the presence of certain substances, in them- 

 selves of no nutritive value, the absorption of actual nutritive matter is increased. 

 In minimal doses poisonous substances often have a favourable effect ; they 

 lead to better utilisation of the substances at the disposal of the plant and 

 increase the "economic co-efficient." Their effect is, however, often injurious even 

 when diluted so as to be imperceptible to chemical tests; thus by such "oligo- 

 dynamic" influence copper sulphate, even in the proportions of 1:25,000,000, has a 

 fatal effect on Spiroyyra, and on Peas in a water culture ( 9 ). 



The nutritive substances are, naturally, not taken up by plants 

 as elements, but in the form of chemical compounds. CARBON, the 

 essential component of all organic substances, is obtained by all green 

 plants solely from the carbonic acid of the atmosphere, and is taken 

 up by the leaves. All the other constituents of the food of plants 

 are drawn from the soil by the roots. HYDROGEN, together with 

 OXYGEN, is obtained from water; oxygen is derived also from 

 the atmosphere and from many salts and oxides. NITROGEN 

 is taken up by the higher plants only in the form of nitrates or 

 ammonium salts ; certain Fungi, Algae, and carnivorous plants, how- 

 ever, obtain it in the form of peptone, amido-acids, amides, or even 

 urea. As the ammonia of the soil formed by the soil bacteria from 

 organic decaying matter is transformed by the help of other so-called 

 nitrifying bacteria into nitrites, and eventually into nitrates, only 

 the nitrogen combined in the nitrates need be taken into con- 

 sideration ( 10 ). 



Bacteria, as contrasted with the higher plants, are particularly characterised by 

 their attitude towards nitrogen. In addition to the bacteria, which, by their 

 nitrifying capability, are of service to green plants, there are other soil bacteria 

 which in presence of organic compounds of carbon set free the nitrogen of nitro- 

 genous compounds, and thus render it unserviceable for the nutrition of green 

 plants. On the other hand, other forms of bacteria (e.g. Azotobactcr chroococcum, 

 Clostridium pasteurianum), other species of the same group of bacteria, and some- 

 times also mixtures of certain soil bacteria ( u ) convert the free nitrogen of the air 

 into compounds whicli serve not only for themselves, but also for other organisms in 

 the soil and for the higher plants as nitrogenous food material. Whether some 

 Fungi have the same power is still ah unsettled question. From the comparison 

 of the crops obtainable from plots of land with and without the addition of 

 manure, J. KUHN has concluded that a very considerable fixation of nitrogen takes 

 place in the soil ; according to WORMBOLD this is in part independent of the action 

 of micro-organisms ( V2 ). 



SULPHUR and PHOSPHORUS form, like nitrogen, important con- 

 stituents of protoplasm. All proteid substances contain A-2 per cent 

 of sulphur. The sulphur is taken into plants in the form of 

 sulphates ; phosphorus in the form of phosphates. POTASSIUM, 

 unlike sodium, is essential to plant life, and is presumably active in 

 the processes of assimilation and in the formation of protoplasm ; it 

 is introduced into plants in the form of salts, and constitutes 3-5 

 per cent of the weight of their dried substance. MAGNESIUM, like 



