30 Gardening 
supply of the minerals that it needs. These minerals are 
dissolved in only small amounts in the soil water, and to 
get enough of them a plant must take in large quantities 
of water. The greater part of this is given off by the 
leaves into the air in the form of water vapor. This 
evaporation of water from the leaves is called transpira- 
tion. 
The advantage of transpiration to the plant is that 
it allows large amounts of water containing weak solu- 
tions of minerals to be taken in by the roots, and when 
the water is evaporated the minerals are left in the plant. 
The minerals in water remain in a kettle when the water 
is boiled away; similarly, the minerals remain in the 
leaves when the water is given off into the air. 
But in dry weather, transpiration is at times a disad- 
vantage to a plant. Sometimes the roots do not absorb 
water as fast as it evaporates from the leaves, and so 
the plant wilts. One of the chief tasks of the gardener 
is to provide enough water for the needs of his plants; 
for in hot, dry weather plants are often injured because 
the water is lost too rapidly from the leaves. 
The stem and its work. The siem supports the leaves 
so that they will be distributed widely in the air and can 
receive much sunlight. It also carries the sap between 
the roots and the leaves. The water and minerals that 
are taken in by the roots as raw materials pass upward 
through the stems to the leaves, where they are manu- 
factured into food for the plant. This food is then 
carried downward through the stem to the roots or to 
any growing and working part of the plant. By hold- 
ing up great numbers of leaves to the light, the stem 
