IONS IN PLANTS 33 



the ions which are so much more readily absorbed 

 than the molecules. 



All the food required by plants must be in 

 solution, excepting carbon and oxygen, and, except- 

 ing the latter, they are all absorbed by the roots. 

 They are absorbed chiefly as sulphates, phosphates 

 and nitrates. That they are injurious to plants if 

 absorbed in a high state of concentration is known ; 

 on the other hand, their absorption at all by osmosis 

 necessitates that the degree of concentration in the 

 solution outside shall be higher than that of the 

 interior of the plant ; whereas the more dilute the 

 solution the greater is its ionic concentration and 

 velocity, and consequently the greater the osmotic 

 pressure and rapidity of absorption. 



Among the mineral substances, calcium appears 

 to be essential to the life and functions of all proto- 

 plasm, yet it is unknown in what way it is combined 

 with the essential basis of life. Another important 

 element, especially of plant life, is potassium ; like 

 calcium, it is obtained from the soil ; without it the 

 absorption of C0 2 and assimilation of carbon does 

 not go on, and the plant does not increase in weight. 

 All plants which are rich in starch or sugar contain 

 an abundance of potash salts ; and it appears that 

 chlorophyll is unable to take up C0 2 and fix the 

 carbon, except in the presence of potassium. 

 Chlorophyll is only formed by the leaves of plants 

 when iron is being absorbed from the soil ; and it is 

 the combination of a ferric or ferrous ion with the 

 chloroplastid which enables the plant to utilise the 

 CO 2 absorbed by the leaves. Under the influence of 

 the sun's rays C0 2 absorbed from the air and H 2 O 



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