ic March, 1912.] Wheat and its Cultivation. 189- 



on the under surface, transpiration is more active from that surface. The 

 guard cells of the stomata regulate the amount of diastomatic transpiration' 

 and the relative position of these cells is largely dependent on their tur- 

 gidity. The more turgid the guard cells the more they curve away from- 

 one another, the wider the opening and the greater the transpiration. 

 Conversely when they become flaccid they straighten out and completely 

 close the aperture. Transpiration is therefore largely influenced by the- 

 turgidity of the guard cells. 



The most important external conditions governing the processes are — 



{a) Temperature. 



{b) Humidity. ' 



{c) Intensity of light. 



{d) Air movements. 



{e) Water content of soil. 



Some of these influence transpiration through the agency of the guard' 

 cells and some of them act independently. 



The absorption of the dilute mineral solution by the root hairs from 

 the soil grains and the mode of its transference to the manufacturing 

 organs of the plant — the leaves — has already been described. It is now 

 necessary to consider the nature of the food thus absorbed. 



This may be determined synthetically by means of water cultures, i.e., 

 by growing wheat plants in vessels containing distilled water to which 

 certain mineral substances of known chemical composition have been added. 

 Carbon we have already seen is obtained from the air, hydrogen and' 

 oxygen, which are also necessary, are obtained from water. Water cultures- 

 show that besides these three there are other seven chemical elements neces- 

 sary, and that these can only be obtained from the soil. 



These seven are nitrogen, phosphorus, sulphur, iron calcium (lime), 

 magnesium and potash. If the soil is wanting in but one of these elements 

 thf^re is no crop. Three other elements drawn from the soil are alwa\'S 

 found in the ash of plants but they are regarded not as essentinl, but as 

 accidental elements. These are silicon (silica), sodium (soda), and' 

 chlorine. A number of other accidental elements may be found in crops 

 from special soils. 



For the farmer practical interest centres in the seven essential con- 

 stituents of the ash enumerated above. Each of these is usually present 

 in sufficient quantity in Australian wheat soils save phosphoric acid. Even 

 on such soils there is usually from fifty to one hundred times more phos- 

 phoric acid than is required for one wheat crop. But the roots have no- 

 openings and they must absorb their ash constituents in solution and the- 

 great bulk of phosphoric acid in soils is insoluble in water. Given suf- 

 ficient soil moisture and good physical condition it is the most deficient 

 soil constituent which determines the size of the crop, and for this reason 

 it is usual to supply a little soluble phosphoric acid in superphosphates to 

 improve the soil in its weakest point. 



Soils may be classed as sand, clay, lime and peat or humus soils. As^ 

 regards the grain the ash varies little in composition according to the 

 soil. The straw however varies considerably — £.g-, on clay soil the ash 

 may contain about 70 per cent, of silica, on peat hardly any. A 20-bushel 

 crop of wheat may contain 35 lb. nitrogen, 14 lb. phosphoric acid, and: 

 20 lb. of potash, the first two being chiefly in the grain and the last-named. 

 in the straw. 



