10 WATER REQUIREMENT OF PLANTS. 
name serves to indicate a citation and to identify the paper to which 
reference is made. 
In the general subject of transpiration an immense amount of 
work has been done, and it is possible in this review to consider only 
that phase of the subject which related directly to the water require- 
ment. The term " water requirement" is here used to indicate the 
ratio of the weight of water absorbed by a plant during its growth to 
the dry matter produced. In most of the investigations described the 
plants were grown in open pots, and in some cases no correction for 
the water evaporated directly from the soil was attempted. In such 
instances we have designated the ratio of the water lost to the dry 
matter produced as the " water requirement including evaporation." 
Much of the work on the effect of the soil or of the nutrient solu- 
tion on plant growth and transpiration has been done with seedlings. 
In the measurement of the water requirement, however, the exact 
determination of the amount of growth is as important a factor as 
the measurement of the amount of water transpired. When the 
period of growth is short and the plants are harvested during the 
seedling stage it is difficult to determine the amount of dry matter 
produced. If the total dry weight of the plant at the end of the 
experiment is taken to represent the increase in dry matter, the water 
requirement will be too low, owing to the error of including much 
of the original dry matter contained in the seed. If a correction is 
made for the initial weight of the seed, the final weight may show a 
loss rather than a gain in dry matter (Le Clerc and Breazeale, 1911, 
p. 10), and even if a gain in weight is recorded it is impossible to 
determine how much weight was lost during the early stages of ger- 
mination. If only the aerial portions of the plants are weighed, the 
recorded increase is largely due to translocation of material and may 
bear no direct relation to actual increase of dry matter. It is there- 
fore necessary to lengthen the growth period of the plant until a 
sufficient amount of dry matter has been produced to make the error 
due to the initial weight of the seed so small as to be inconsequential 
if water-requirement measurements or transpiration measurements 
based on the increase in weight of the plant are to be of value. For 
the above reasons it was decided, after summarizing the data, to 
omit from the following discussion, except in special cases, water- 
requirement measurements based on the weight of seedlings. 1 
Investigations like that of Hohnel (1881), in which the water 
requirement of forest trees was based on the weight of the leaves 
i The following publications (arranged chronologically) contain water-requirement measurements based 
on the weight of seedlings: 
Sorauer, Paul. 
1880. Studien iiber Verdunstung. Forschungen auf dem Gebiete der Agrikultur-Physik, Bd. 3, pp. 
351-490. 
1883. Nachtrag zu den Studien iiber Verdunstung. Forschungen auf dem Gebiete der Agrikultur- 
Physik, Bd. 6, pp. 79-96. 
285 
