EFFECT OF WEEDS ON CROPS 
275 
too many plants on the same area, or in other words to plant too 
thick, even if a crop is grown simply to eradicate weeds. 
In each case the dry weight of the pure culture of wheat alone 
or of oats alone is greater than is the case with mixed cultures. 
With one exception, it is found that the total transpiration for 
the mixed culture is greater than for the pure wheat and oats 
cultures. The present study shows that wheat transpires more 
than oats during a growing season. 
FIEED STUDIES 
A series of field tests were made to determine the extent to 
which weeds would encroach upon such a crop as corn. 
In these series, an attempt was made to ascertain the effect of 
various cultivation treatments. Plots a meter square were used. 
The field work was carried on by Mr. R S. Kirby during the 
summer of 1917. Mr. Kirby determined the amount of green 
weight produced but neglected to ascertain the water requirements 
or the amount of water necessary for the production of a unit 
amount of green weight. It is known that the amount of water 
given off in twenty-four hours is approximately equal to what is 
absorbed during the same period and that a negligible amount is 
used in the increased material of the tissues. The authors re- 
alized this, but it was not until the summer of 1921* that they 
were able to conduct experiments to determine this water rela- 
tion. At this time a series of plants — purslane, crab grass 
and pigweed — were grown in galvanized iron containers or 
pails of the same dimension as given previously, without sealing 
over the soil surface. 
Of course there was some error due to this but deductions were 
made by having the soil only, in one container. An average was 
made from the seven cultures having crab grass, purslane and 
pig weed of the amount of water used at the time the plants were 
fully headed out and seed set. This was on August 15. The 
data obtained here were used as a basis for the amount of water 
transpired. It will be noticed that the collections were made 
from August 8 to August 16. 
From what is known concerning the structure and development 
of corn, the data supplied from the weed cultures during the sum- 
mer of 1921 cannot be used for corn. However, it is the water 
relation of the weeds which prompts the attention in this particular 
study. 
* It is with pleasure that the authors hereby recognize the assistance of Mr. Oliver 
Miller. 
