56 Principles of Plant Culture. 
sedums, cacti etc., have special water-storing tissue, from 
which transpiration is extremely slow. 
Experiments indicate that the transpiration from most 
leaves is between one-third and one-sixth as much as the 
evaporation from an equal area of water. When we take 
into account the immense leaf surface of a large tree, it is 
evident that the aggregate transpiration must be very great, 
as is often illustrated by the dwarfing influence of trees 
upon adjacent crops in dry weather. (Fig. 24). Transpira- 
tion is much more rapid during dry than during wet weather, 
and in the rare atmosphere of high altitudes than in the 
denser atmosphere of low lands. 
Excessive transpiration, as occurs in very dry weather, is 
detrimental to plants, since it reduces the water pressure 
within the cells below the point where healthful growth can 
take place (63); but normal transpiration, i. e., in amount 
not sufficient to interfere with healthful growth, is doubt- 
less beneficial, since it aids in carrying food materials from 
the soil into the leaves, where they are needed for assimila- 
tion (59). For this reason, plants native to regions having 
a rather dry atmosphere, do not thrive in greenhouses unless 
abundant ventilation is given to encourage transpiration. 
Fic. 24. Showing how aspruce hedge dwarfs an adjacent corn crop in dry 
weather. Madison, Wis. 
