94 



plant growth and development are impossible. Thus very 

 little iron is required, but unless the necessary minimum 

 is present no chlorophyll is developed. Of these various 

 elements the carbon is obtained from the carbon dioxide 

 of the air, oxygen is obtained partly from the air and 

 partly from water, hydrogen from water and the others are all 

 Availably, absorbed in solution by the roots from the sc-il. Large quanti- 

 ties of these necessary substances, however, may be present in 

 the soil and yet not be available for the plant, because they 

 occur in a form in which the plant can make no use of them. 

 As only substances in solution can penetrate the cell-wall and 

 thus enter the absorbing root-hairs, only such substances as 

 are soluble can be utilised by the plant. Although it appears 

 that some of the higher plants can directly utilise ammonium 

 salts, most of them can only obtain their nitrogen readily 

 .Soil when it occurs in the form of nitrates. It a solution of organic 



Absorption. an( j mor g an j c substances is poured on ordinary soil, it will 

 be found that the water which filters through is usually 

 different in composition to that originally added. This is due 

 to the power possessed by the soil of absorbing and retaining 

 various substances, partly on account of chemical combi- 

 nations taking place, and partly on account of the attraction 

 exercised by the particles of soil. Soils vary considerably with 

 regard to their absorptive power ; those containing humus 

 absorb the most and are thus able to store up large 

 quantities of valuable plant food-materials, while sandy soils 

 have very little absorptive power. All substances, however, 

 are not retained with equal vigour ; ordinary soils allow the 

 valuable nitrates to filter through and pass away in drainage 

 water, while phosphates, potassium salts and ammonium are 

 usually strongly held. Such absorbed substances can, however, 

 be gradually washed out again if sufficient water is continually 

 added to the soil, so that from the stored substances the 

 dilute solutions required by plants are thus made available 

 in a moist soil. 



Living active root-hairs are continually excreting carbon 

 dioxide as a product of their respiration, and this, passing into 

 the water which surrounds them, aids materially in dissolving 

 and making available for the plant useful substances in the 

 soil. Calcium carbonate, for example, is readily soluble in 

 water containing carbon dioxide and, if a plant is allowed to 

 grow with its roots touching a polished marble plate, the 

 latter will be corroded and etched where the roots have come 

 in contact with it. 



Power of 

 Plants to 

 obtain 



Necessary 

 -Substances. 



