CHEMISTRY AND PHYSICS OF PLANTS. 17 
Weigh the whole at intervals of a few hours. The loss will ve the 
amount of evaporation (approximately). By adding weighed quan- 
tities of water at intervals the experiment may be continued iudefi- 
nitely. 
(¢) Cut off 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 the 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 off 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 temperatre of the soil. 
(4) Cut off a small branch of a maple-tree on a cold winter day; 
bring it intoa 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 
