160 A TEXTBOOK OF GENERAL BOTANY 
EFFECT OF EXTERNAL FACTORS ON GROWTH 
Water. An abundant supply of water is very essential to the 
most rapid growth of plants. It is a well-recognized fact that 
plants which lack water grow more slowly and produce smaller 
quantities of dry material than do plants that are abundantly — 
supplied with water. ) 
Any part of a plant, in order to grow, must contain enough 
water to’ be turgid. When cells or plant organs lose their tur- 
gidity, they decrease in size. Leaves, fruits, or stems may lose. 
more water on a dry, sunny day than they absorb, and actually 
become smaller than they were during the preceding night. It is 
therefore necessary for normal growth that a plant should be able 
to obtain not only enough water to replace that which is lost 
through transpiration but, in addition, enough to fill the newly 
formed tissue. The amount needed for growth is very small as 
compared with that lost through transpiration. 
The actual rate of transpiration is not so important as is the 
ratio between water lost and water absorbed. It is highly im-_ 
portant that the daily rates of transpiration and of water absorp- 
tion should be so balanced that the water absorbed is sufficient 
to replace that lost by transpiration and to support new growth. 
Excess of water loss over water absorption may be brought 
about either by a high rate of evaporation or by a low moisture 
content of the soil, or by a combination of these two. This con- 
dition can usually be remedied by either decreasing the rate of 
evaporation or increasing the water content of the soil. In,agri- 
culture the method most generally practiced is to supply water, 
as by irrigation. 
Heat. At very low temperatures plants do not grow. The 
minimum, or lowest, temperature at which a plant will grow 
varies with different species, but 4° C. may be regarded as an 
average minimum temperature for many plants. As the tempera- 
ture increases beyond the minimum the rate of growth becomes 
more rapid until the optimum temperature is reached. ‘This 
optimum temperature also varies with different species, but 

