14 GROWTH OF WHEAT SEEDLINGS. 



of a wheat seedling is greatest during the first few days of its 

 growth, this being especially true of the oxydases and peroxydases 

 of the root tip. These enzyms are particularly sensitive to acids, 

 and their destruction would mean a cessation of growth. The first 

 crop had evidently passed the critical period with reference to this 

 enzymic activity before the acidity of the solution had developed to 

 such a degree as to prohibit it. When the second crop was placed in 

 the solution the acidity was high enough to practically prohibit the 

 action of the enzyms and the plants were unable to tide over this 

 critical period. The effect of toxic bodies upon the oxydases and 

 peroxydases, that are so active during the first few days of a plant's 

 growth, is an important factor both in agricultural practice and in 

 scientific research and one that is often overlooked. 



Many substances that are classified as plant stimulants or depres- 

 sants no doubt derive their reputation in a great measure from their 

 effect upon the enzyms. Their beneficial or injurious effects would 

 only be noticed during the first few days of the plant's growth when 

 its enzymic activity is so pronounced. 



It will be noticed in the case of the second crop that the alkalinity 

 of the solution increased both the green and the dry weight of the 

 tops and the dry weight of the roots. In the case of the residual seeds 

 that were removed from the plants the reverse is true. The presence 

 of an aoid in the solution prevented the food material that was stored 

 up in the seed from being transported into the plant. The residual 

 seeds were therefore much lighter when calcium carbonate was pres- 

 ent than when it- was absent. 



After the second crop had been grown in the solutions described 

 for five days the plants were harvested, separated into tops, roots, 

 and residual seeds, and each part dried and weighed separately. The 

 respective solutions were likewise all tested for acidity and the pres- 

 ence of nitrates and potash. Tables 3 and 4 (see page 12) show 

 the results obtained, and a comparison with Tables 1 and 2 (pp. 7 

 and 9), respectively, will bring out some interesting changes in the 

 residual solution. After the first cropping solution 1 was slightly 

 acid, but after harvesting the second crop it was three times as acid. 

 Solutions 3 and 13 after the first cropping were alkaline, but after 

 the second one both solutions had become quite acid. Analysis of 

 solution 3 showed that while at the beginning there were 40.5 parts 

 per million of sodium (Na), after gathering the second crop only 

 8 parts per million remained in solution. The difference shows the 

 probable amount of sodium absorbed by the two crops. Not only 

 was the amount thus reduced from 40.5 to 8 parts per million, but 

 the presence of this small amount of sodium seemed to stimulate an 

 acid excretion on the part of the plants to such an extent that the 

 alkaline solution was not only neutralized but was made even more 



