1922] CURRENT LITERATURE 241 
effect of stomatal movement, makes the problem an especially difficult one. 
It is true, however, that the effect of external conditions on transpiration, 
determined usually by comparing transpiration at different places at different 
portions of the day or year, is now much better understood than at the time 
of publication of the first volume. These results have been obtained usually 
with the help of either painstaking direct weighings, or by the use of automatic 
transpiration records, the evaporation, temperature, sunlight, wind, and wet 
bulb depression having been simultaneously determined. 
Water requirement of plants used in its narrow sense is the amount of water 
consumed by a plant during its period of growth in the production of a unit 
weight of dry matter. It is evident, therefore, that any factor which affects 
relative consumption of water during growth and the effect of environmental 
conditions on the water requirement are less easily determined than the effect 
of environmental conditions on transpiration, since not only do the conditions 
control the rate of transpiration, but also affect the relative rate of growth for 
production of dry matter 
though very little work has been done on the effect of insufficient soil 
moisture on the transpiration of plants, a great mass of data has been 
accumulated on the amount of moisture in the soil at the time plants wilt. 
Although results are somewhat conflicting, it has been found that there is a 
relatively definite percentage of soil moisture content, beyond which the 
movement of moisture in the soil is so slow as to make it practically impossible 
for a plant to supply its transpiration demand from the mass of soil through 
which its roots ordinarily extend. This moisture content has been referred 
to as the wilting coefficient. 
Many papers have dealt with structure and morphological investigations 
for lessening transpiration. Although the value of transpiration in reducing 
the temperature of leaves has been brought out by a number of investigators, 
its value in relation to the nutrition of plants has not fully been admitted. 
The difficulty of getting acing so large a volume of English literature, 
especially at a period when war made access to literature from Englis! 
speaking countries difficult or ea ety must have been very great. This 
work is a valuable summary, and no investigation of transpiration is feasible 
without first consulting it. It is impossible to bring into the work all of the 
material contained in the original papers, and these should always be consulted. 
A careful perusal of this work shows clearly that there is not a single line of, 
investigation at the present time which does not afford a good starting point 
for further research.—H. L. SHANTz. 
