THE PHYSIOLOGY OF PLANTS. 77 



a shoot of Sunflower after it has become slightly wilted. Pour mercury in 

 the free arm and the pressure will force the water into the shoot causing it 

 to regain its rigidity. 



6. If a vigorously growing young plant be cut off near the 

 ground, water (sap) will exude. This " bleeding " is due to 

 what is termed root pressure. It is a consequence of the ab- 

 sorbent activity of the root-hairs. The root pressure of peren- 

 nial plants is greatest in the spring while the extensive leaf 

 surface and high temperature are wanting. In some plants 

 the root pressure is manifest in the exudation of drops of 

 water from the margins and tips of the leaves, especially the 

 younger ones, as in case of some Aroids, Grasses, etc. 



Cut off a strong young Grape-vine (or other plant, as Sunflower, Dahlia, 

 Maple, Birch, etc. ) in middle spring, a short distance above the ground. 

 To the stem remaining in the ground attach an upright glass tube by means 

 of a short rubber tube. Note the rise of the sap and the variation in the 

 height of the column during the day. 



7. Comparatively few of the elements found in nature are 

 concerned in the nutrition of plants. Those found by chem- 

 ical analysis of the tissues are the food elements required. 

 Another method of determining which of the elements are 

 necessary in plant nutrition, is the employment of " water- 

 cultures."' These are arranged as follows: A clean flask or 

 wide-mouthed bottle with distilled water is used. Very small 

 quantities of soluble substances which will furnish the ele- 

 ments supposed to be necessary for plant growth are added. 

 A seed is then germinated and fastened in the mouth of the 

 bottle or suspended so that the root will grow downward 

 into the liquid from which it can absorb the food materials 

 needed. The plant can then be grown to maturity — flowers 

 and seed being produced as when grown in soil under ordi- 

 nary conditions. 



Dissolve in two litres (about four pints) of distilled water the following : 

 Two grams potassium nitrate ; one gram sodium chloride ; one gram calcium 

 phosphate ; one gram calcium sulphate ; one gram magnesium sulphate ; 

 and add a few drops of a solution of ferric chloride or ferrous sulphate. Fill 

 a glass jar of one pint capacity or less, with the nutrient solution. Fasten 



