Transpiration as a Factor in Crop Production. 25 
time, it is necessary to measure the water applied during growth 
as well as the difference in initial and final moisture content of 
the soil. A number of self-recording arrangements have been 
devised for securing a continuous record of water loss. These are, 
however, not essential. 
CHARACTER OF PLANT. 
Quantitative measurements with potometers may be divided 
into two general groups, according to whether plant cuttings or 
rooted plants are used. Rooted plants are much to be preferred 
and in fact are indispensable for a serious study of transpiration. 
We cannot rely upon conclusions drawn from other than normally 
functioning plants, unless the object is to study the abnormal 
condition as such. 
Plant Cuttings. — The great source of error with the use of 
cuttings is that the entrance of solutions does not take place in 
a normal manner. The tests must also be of short duration and 
under protected conditions, so that the rate of water loss is suffi- 
ciently suppressed for the stem to take in the moisture as quickly 
as it is lost from the leaves. Especially in experiments correlating 
transpiration and the production of dry matter, it is essential to 
use rooted plants functioning in their normal manner under the 
environmental conditions of the test. 
Rooted Plants. 1 — Investigations with rooted plants may be 
grouped according to the stage of maturity upon which results 
are based. They fall naturally into three main classes, viz: 
Seedlings, mature plants, and intermediate developments. Briggs 
and Shantz (1913 b) pointed out the chief source of error from the 
use of seedlings when transpiration is correlated with dry matter, 
and they have eliminated such experiments from serious con- 
sideration. We are in hearty accord with them and believe that 
important conclusions relative to dry weight cannot be drawn 
from seedlings. Sufficient growth must be made to overcome the 
effects of mere translocation of substance from the seed. This 
stage may be reached long before the plants are mature, but with 
few exceptions, in the case of certain agricultural crops, the water 
requirement should doubtless be determined up to that stage at 
which they are commonly harvested in farm practice. It has 
been observed that the relative effect of two treatments on 
the early growth of plants is sometimes very different in degree 
from the final comparative growth at maturity. 
It is entirely possible to establish certain interrelationships 
1 A list of investigators who determined transpiration from plants grown 
beyond the seedling stage by loss in weight of the potometers is given on 
page 36. 
