THE PHYSIOLOGY OF PLANTS. 79 



processes of the plants is obtained from water (HjO), and from 

 various oxygen-containing salts. Nitrogen, though small in 

 quantity, is a very important plant constituent. It is derived 

 from ammonia (NH3) and its salts, from nitrates and from 

 such organic substances as contain nitrogen, which the roots 

 absorb. Nearly four-fifths of the atmosphere is free nitrogen, 

 but it was long ago proven that plants are incapable of taking 

 up this free element. It should be added, however, that recent 

 experiments show that leguminous plants and possibly a few 

 others, which hear root-tubercles, are capable of using the free 

 nitrogen of the air. It is in some way connected with the 

 activity of the micro-organisms (Bacteria or other simple 

 plants) that are found in the tubercles. 



10. Sulphur is obtained by the plant from sulphates — espe- 

 cially the sulphates of ammonia, potassium, magnesium, and 

 calcium. Phosphorus is furnished by the phosphates. There 

 are several compounds from which the plants obtain potas- 

 Bium — the sulphate, phosphate, chloride and probably the 

 silicate. Calcium is supplied by the sulphate, phosphate, 

 nitrate and carbonate. Similarly, magnesium is furnished 

 by all its salts, except the chloride. Any of the soluble salts 

 of iron furnishes this element — extremely minute quantities 

 being sufficient for the needs of the plant. When iron is with- 

 held in culture solutions, the plants do not become green. 

 Sodium, furnished by its several salts, is necessary to plant 

 nutrition, but it is found in very small quantity. 



Repeat the experiment with the water-culture as detailed under paragraph 

 7, but omit the iron and notice the chlorotic effect, i. e. the green does not 

 develop. Omit other ingredients and their necessity to plant nutrition will 

 be proven. When all are omitted (the roots growing in pure water), devel. 

 opment will cease when all the food stored up in the seed is exhausted. 



11. Chlorine and silicon are always found in the ash of 

 plants, but they do not appear to be essential to "plant nutri- 

 tion. In marine plants iodine and bromine are present. 

 Several other elements are occasionally found in plants in 

 small quantities when they are abundant in soluble form in 

 the soil. All plants do not have the same proportional 



