26 AERATION AND AIR-CONTENT. 



trast to plants without aeration, and continued to live throughout 

 the 3 weeks of the experiment, while the unaerated ones died. Root- 

 hairs developed on some roots of the aerated plants. Coleus showed 

 wilting in 1 or 2 days when its roots were submerged, and lost nearly 

 all its leaves, while geranium did not show ill effects for more than a 

 week. In the case of Vicia faba, plants in garden soil, peat, and 

 Sphagnum with submerged roots began to wilt in 4 to 5 days. 



The determination of the root-pressure of plants of Coleus and 

 Fuchsia in soil and submerged pots showed it to be 2 to 3 times 

 greater in the former. If the plants with submerged roots were 

 aerated by means of bubbling air or by placing Philotria or Spiro- 

 gyra in the water, the root-pressure was nearly as great and as well 

 maintained as in the soil. The effect of submergence on the trans- 

 piration of geranium was to greatly increase it at the outset, but it 

 quickly fell off in 2 days to a point below that in moist soil. In 2 

 days more the leaves began to turn yellow, and at the end of 8 days 

 the transpiration had fallen to less than 30 per cent of the normal and 

 in 11 days to less than 15 per cent. Seedlings of Quercus rnacrocarpa 

 in moist and submerged soil showed similar results. Seedlings in 

 moist soil gave a transpiration rate of 7 to 13 gm. from the second to 

 the twenty-fourth day, while those in the submerged soil lost but 2 

 to 4 gm. per day. 



Schley (1920 : 79) has found that the respiration of a root geotrop- 

 ically stimulated is greater than that of one unstimulated. The 

 respiration-rate decreases as the time of stimulation increases. 



Knudson (1920 : 379) concluded from experiments with the roots 

 of Pisum arvense and Zea in solutions containing sucrose that the in- 

 crease in reducing sugars in the latter is due to the excretion of these 

 from the roots and not to the excretion of invertase. 



Bergman (1921 : 50) has recently studied the relation between 

 the oxygen-content and the injury of the cranberry vine due to 

 flooding. Injury is most apt to occur during cloudy weather, when 

 the oxygen is lowest. When submerged cranberry vines are shaded, 

 injury results as a consequence of the reduction in the oxygen-content 

 of the water. No essential difference was seen when the shaded 

 vines were in pond-water or bog-water. The flowers and growing 

 tips were most affected, owing to their higher rate of respiration and 

 consequent greater demand for oxygen. 



Summary. — The essential features of the respiration of plants were 

 established in the short period from Ingenhousz (1779) to Saussure 

 (1804). It was shown not only that all organs of the plant possessed 

 this function in common, but also that roots respired in exactly the 

 same manner as stems and leaves, in spite of the difference in medium. 

 This is supported by practically all the evidence drawn from germi- 

 nation, since the early stages of this have to do with the radicle. In 



