The Relative Sensitivity of Xanthium Leaves of Different Ages to Photoperiodic Induction.

if transpiration is rapid, but the plants recover if the soil drains promptly. If the soil remains saturated more than 24 hours permanent injury occurs. Stem elonga tion usually ceases, epinasty often occurs, the leaves turn yellow and die, beginning at the base and progressing up the stem, and the roots and base of the stem begin to die and decay. Injury develops more rapidly in warm than in cool soil. The initial wilting is caused by a sudden decrease in permeability of the roots to water. Prolonged flooding causes injury and death of the roots because of deficient aeration, but this is aggravated by the activity of microorganisms which destroy the roots and plug the xylem in the base of the stem. It also is possible that toxic substances produced by microorganisms or in the dying root cells contribute to injury of the shoots. The role of microorganisms in flooding injury deserves further investigation. Wilting and injury to flooded plants can be prevented by forced aeration of the soil, and plants growing in soil at field capacity can be caused to wilt and show symptoms of injury by displacing the soil atmosphere with CO2 or N2. Use of CO2 produces wilting sooner than N2. Although injury to plants in flooded soil is caused by deficient aeration it is more complex in nature than injury from 02 deficiency alone because in most instances microorganisms appear to be involved. LITERATURE CITED