WATER-ABSORPTION AND TRANSPIRATION. Ill 



sulphate in place of magnesium sulphate ; of sulphur by using magne- 

 sium chloride instead of the sulphate ; of nitrogen by using sodium 

 chloride and calcium sulphate in place of calcium and potassium 

 nitrates ; of iron by omitting the iron salt (which should be added in 

 all other cases). 



146. Essential Soil-Substances. A soil must contain 

 the same essential elements as the full culture solution in 

 order to allow plants grown in it to come to maturity, and 

 these elements must be in a form available for use by the 

 plant. Soil- water contains most of the essential elements in 

 the form of dissolved compounds. The composition of the 

 water present in a soil can be learned by analysis of drainage 

 water, and we can test soils for soluble substances by letting 

 distilled water drain through them and then applying to it 

 tests for the essential elements given above. 



* (a) Compare the growth of seedlings (of the same kind of plant) 

 which have been supplied with (1) distilled water ; (2) tap water ; (3) 

 culture solutions (some complete, others with one element or other 

 omitted in each case). If you grow the plants in washed sand, water 

 with (1), (2), or (3) daily. In either case compare also with plants 

 grown in good garden soil. After, say, six weeks, dry thoroughly and 

 weigh the seedlings, and compare their dry weights ; then burn them 

 and compare the ash weights. 



* (6) Place some dry, fine garden soil in a funnel, plug the tube lightly 

 with cotton- wool, and pour water slowly on the soil. Collect the first 

 few drops that ooze through and repeat the test with diphenylamine. 

 The presence of nitrates is shown by the production of a deep blue 

 colouration. A fertile soil always contains nitrates. 



* (c) Continue to pour water through the soil in the funnel, and after 

 collecting what comes through, evaporate it to dryness and compare 

 the residue with what is left after evaporating an equal volume of 

 the water used. 



147. Formation of Nitrogenous Substance. The elaboration 

 of nitrogenous substance is not so clearly understood as that of carbo- 

 hydrates. Perhaps the simplest process, leading to the formation of 

 soluble nitrogenous organic compounds (amides or amido-acids, con- 

 taining carbon, hydrogen, oxygen, and nitrogen), that one can suggest 

 is the inter-action of sugar and nitrate of potash, the chief products 

 being an amide called asparagin (widely distributed in plants) and 

 oxalate of potash. The latter, like oxalic acid itself, is poisonous, but 

 it combines with the lime brought up in salts from the soil, forming 

 oxalate of lime, which is insoluble in water, and therefore crystallises 

 out and is rendered harmless. This is probably one source of the 

 oxalate of lime which occurs so abundantly in crystals in plant- tissues. 



