sect, ii PHYSIOLOGY 173 



has taught that when they are brought into contact with the leaves (by sprink- 

 ling the plants with solutions to prevent the inroads of insects), they exercise a 

 beneficial influence on the formation of chlorophyll, and increase assimilation, 

 transpiration, and the length of life. 



The nutritive substances are, naturally, not taken up lay 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 green 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, although the 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. 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 

 consideration. 



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 set free the nitrogen of nitrogenous compounds and thus render it unservice- 

 able for the nutrition of green plants. On the other hand, other forms of bacteria 

 convert the free nitrogen of the air into compounds (amides ?) which serve 

 not only for themselves, but also for the higher plants as convenient nitrogenous 

 food material. This remarkable nitrifying power of bacteria .has led to a life 

 partnership (symbiosis) between them and some of the higher plants (Leguminosac). 

 In such symbiotic relations the bacteria provide the higher plants with nitrogen in 

 a form in which it may be assimilated, while, in turn, they are supplied with the 

 carbon compounds essential for their nutrition (c/. p. 211 and Fig. 186). 



SULPHUR and phosphorus form, like nitrogen, important constituents 

 of protoplasm. All proteid substances contain 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 forma- 

 tion 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 potassium, participating in the most important 

 synthetic processes of plants, is found in combination with various 

 acids, particularly in reservoirs of reserve material (in seeds to the 

 extent of 2 per cent) and in growing points (in leaves only | per cent). 

 Calcium also is taken up in the form of one of its abundant salts, and 

 in considerable quantities (2-8 per cent). Calcium plays an im- 

 portant part in the metabolic processes of plants, not indeed as an 

 actual constituent of protoplasm, but as a vehicle for certain other 



