OHEMISTBY AND PHY8I08 OF PLANTS. 77 



the proper place, through which to introduce water. Weigh the 

 whole at intervals of a few hours. The loss will be the amount of 

 evaporation ^approximately). By adding weighed quantities of 

 water at intervals the experiment may he continued indefinitely. 



(i) Cutoff a rapidly growing leafy shoot of the apple or geranium 

 and place the lower end in a bottle of water. Close the bottle by 

 pressing soft wax into tlie mouth of the bottle around the stem. On 

 account of the upward movement of the water through the shoot its 

 level in the bottle will be perceptibly lowered. This will be more 

 evident the smaller the diameter of the bottle. 



(j) Cut oil the stem of a rapidly growing sunflower a couple of 

 inches above the ground; slip over it the end of a tightly fitting 

 india-rubber tube 8 to 10 cm. long. Slip into the other end a small 

 glass tube 5 to 10 mm. in diameter, being sure to make the joints 

 water-tight. The " root-pressure" will cause the water to rise into the 

 vertical tube. Note the effect of a change of temperature of the soil. 



(k) Cut oS a small branch of a maple-tree on a cold winter day; 

 bring it into a warm room. As soon as the temperature of the branch 

 rises, the sap (water) will begin to flow from the cut surface. Lower 

 the temperature and the flow will cease; raise it again and the flow 

 will be resumed. 



146. Plant-Food. — The most important elements which 

 are used in the nutrition of plants, or which, in other 

 words, enter into their food, are Carbon, Hydrogen, Oxy- 

 gen, Nitrogen, Sulphur, Iron, and Potassium. These all 

 appear to be necessary to the life and growth of the plant, 

 and if any of them are wanting in the water, soil, or air 

 from which the plant derives its nourishment, death from 

 starvation will soon follow. 



147. There are other elements which are made use of by 

 plants, but, as life may be prolonged without them, they 

 are regarded as of secondary importance. In this list are 

 Phosphorus, Calcium, Sodium, Magnesium, Chlorine, and 

 Silicon. 



148. The Compounds Used. — With the single exception 

 of oxygen, the elementary constituents named above do not 

 enter into the food of plants in an uncombined state; on 



