8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



tion was applied in a solution of KH2PO4 at four dififerent times 

 after the plants started to head out. The total quantity added was 

 about 10 grams. 



When the plants were harvested, the number to the hill could not 

 be determined. For this reason the data have been expressed as total 

 for each plot rather than the average per plant, as in table 3. 



On May 10 the leaves of the plants in plots 2 and 3 showed a slight 

 yellowing. This yellowing of the carbon-dioxide-treated plants during 

 their early growth was also observed in the previous year's experi- 

 ments. Later the plants overcame this initial handicap and outgrew 

 the plants of the control plots. By June 7 plants in plots i and 4 had 

 started to head out, but no signs of heading were in evidence in plots 

 2 and 3 (those receiving extra carbon dioxide) until a day or two 

 later. This was also in keeping with observations made the previous 

 year. By June 27 vegetative growth had practically ceased, and the 

 carbon-dioxide treatments were discontinued. 



So far as vegetative growth and the amount of tillering are con- 

 cerned, this experiment showed a beneficial effect of the carbon- 

 dioxide treatment. The weight of straw was increased, as was also 

 the number of heads produced. Although the weight of grain was 

 greater on the carbon-dioxide-treated plots, the greater number of 

 grains produced reduced the average dry weight per grain of these 

 plots to practically the same value as the enclosed control, approxi- 

 mately 0.03 grams. The number of grains to the head was but slightly 

 greater in the enclosed control plot, whereas in the previous year's 

 experiment it was considerably greater. 



SUMMARY AND CONCLUSIONS 



Three dift'erent experiments were carried out with Marquis wheat 

 to study the effects in sunlight of increased carbon-dioxide concen- 

 tration (in most cases about four times that of normal air) of the air 

 surrounding the plants during their growth. In one experiment 8- 

 inch pots were used, and in the two other experiments plots 2x2 feet 

 were employed. Commercial carbon dioxide of high purity was mixed 

 with the air surrounding the plants. The carbon dioxide was confined 

 to the space about the plants by cylinders of clear cellulose acetate in 

 one experiment and by square glass sides in the others. 



The main conclusions to be drawn from these experiments are 

 that air enriched with carbon dioxide (i) increased the tillering of the 

 wheat, (2) greatly increased the weight of straw, increased (3) the 

 number and (4) weight of heads, (5) increased the number of grains 



