SAP PRESSURE 



29 



and in this will be found the six elements sulphur, phos- 

 phorus, potassium, calcium, magnesium, and iron in the form 

 of incombustible salts (sulphates, phosphates, and so on). 



The plant gets its carbon and oxygen from the air, as will 

 be explained in Chapter xn. Deprived of air, all green plants 

 soon die. The hydrogen is obtained 

 from water. 



The importance of the six ash- 

 forming constituents mentioned 

 above is most readily studied by 

 means of water cultures in which 

 plants are grown with suitable 

 proportions of dissolved salts. If 

 any one of the six elements is 

 omitted from a solution, the 

 plants grown in it are dwarfish 

 and unhealthy. 



Ordinary soil water contains 

 sufficient salts in solution for the 

 nutrition of plants, but not always 

 enough to stimulate rapid growth. 



38. Sap pressure. Not only 

 does much water gain admission FIG. 24. Apparatus to measure 

 to the plant through the roots, sap P ressure 



Vmf nnrlpr nrrlirmrv rirrnrrmran^ps 7 '' large tube fastened to the stum P of 



the dahlia stem by a rubber tube ; 



it is found forcing its way 011, r, r, rubber stoppers ; t, bent tube 



into, and through the stem (for ?, 7Si differ 

 explanation see Sees. 48-51). The Sachs 



force called sap pressure with which the upward-flowing current 

 of water presses may be estimated by attaching a mercury gauge 

 to the root of a tree or the stem of a small sapling. This is best 

 done in early spring after the thawing of the ground, but before 

 the leaves have appeared. The experiment may also be per- 

 formed indoors upon almost any plant with a moderately firm 

 stem, through which the water from the soil rises freely. 



