Oct. IS, 1914 Water Requirement of Plants 47 



Sunflower, which made an excellent growth, gave a water requirement 

 of 705 ±8. Sunflower thus requires almost three times as much water 

 as pigweed and 86 per cent as much water as alfalfa. 



A comparison of the water requirement of pigweed during the three 

 periods of growth will show that the water requirement is not greatly 

 affected by the period of growth. 



If this holds for the other weeds, no great error will be produced in 

 comparing the water requirement of these plants without regard to the 

 period during which they were grown. The water requirement is, how- 

 ever, probably slightly less than it would have been if the plants had 

 been grown in midsummer. The results obtained, based on the produc- 

 tion of dry matter, are as follows: 



Variety of weed Water requirement 



Purslane (Portulaca oleracea) 292 ±ii 



Pigweed (Amaranthus retroflexus) 320it7 



Cocklebur (Xanthium commune) 432 ± 13 



Narrow-leaved sunflower from sand hills {Helianlhus pciiolaris) . . 570±ii 



Annual sunflower (Helianihus annuus) 705±8 



Narrow-leaved sunflower from near Akron (Helianlhus peiiolaris). 774±20 



Lamb's-quarters {Chenopodium album) 801 ±41 



Fetid marigold (Boebera papposa) 881 ±26 



Western ragweed (Ambrosia ariemisifolia) 948±66 



Purslane and pigweed, two introduced weeds, appear to be excep- 

 tionally efficient plants, their water requirement being only slightly 

 higher than that of Kursk millet and in practical agreement with the 

 sorghums. vSome of the indigenous weeds were also found to be fairly 

 efficient, cocklebur, a plant found in stream beds and about ponds, 

 having a water requirement 13 per cent less than wheat, while the narrow- 

 leaved sunflower from the sand hills had a water requirement 3 1 per cent 

 less than alfalfa. Lamb's-quarters, an introduced plant, and fetid 

 marigold (PI. VII, fig. 2) and western ragweed, indigenous plants, have 

 a slightly higher water requirement than alfalfa. 



It is evident, therefore, that the common weeds differ greatly in water 

 requirement. A growth of weeds in a crop or on summer fallow repre- 

 sents a tremendous loss of moisture, a thousand pounds per acre of the 

 most efficient weeds representing a loss of at least 1.5 inches of stored 

 rainfall, or from 4 to 5 inches of stored rainfall in the case of the weeds 

 having a high water requirement. The latter figures represent about 

 the maximum amount of moisture that can be stored in fallow land. It 

 is therefore easy to understand how the whole of the stored n)oisture 

 supply may be lost through the growth of a moderate crop of weeds, and 

 these varieties having a high water requirement are especially to be 

 dreaded. 



