RESPIRATION AND OXYGEN. 19 



to the presence of chlorophyll or to included air. Bergmann 

 (1920 : 17) and Cannon and Free (1920 : 62) have likewise shown 

 that the formation of root-hairs is dependent upon oxygen. Raci- 

 borski (1905 : 338) found that the ability of the root-surface to 

 oxidize easily oxidized substances about it is controlled by oxygen. 



Stoklasa and Ernest (1905 : 723) pointed out that roots act strong- 

 ly upon rock through the excretion of CO 2, as is often seen in nature 

 on smooth stone-walls corroded by roots. This effect has often 

 been ascribed to organic acids produced in the roots, but it is now 

 clear that this role is played only by the CO2, and it is particularly 

 noticeable in young root-systems. This makes it possible to under- 

 stand how the plant prepares its necessary mineral nutrients in so- 

 lution through its root-system. This results from the excretion of 

 CO 2 by its own root-system and through the micro-organisms of the 

 soil, the number of which is materially increased in cultivated soils. 



Brizi (1906 : 89) found that the roots of rice are not of the aquatic 

 type, and demonstrated by means of water-cultures that oxygen is 

 absolutely essential for their growth. He concluded that the algse 

 of the rice-fields are doubly useful, in that they utilize a large amount 

 of CO2 and set free a corresponding quantity of oxygen available for 

 the roots. 



Day (1906 : 37) has grown wheat, barley, oats, and peas in jars, 

 one set of which was aerated daily during the growing season by 

 drawing enough air through to completely change the air in the jars. 

 In germination there was no marked difference between the two sets, 

 except in the case of the peas. The aerated jars germinated 81 per 

 cent and the unaerated ones 61 per cent. Moreover, the growth of 

 the pea plants was nearly twice as great in the aerated as in the un- 

 aerated jars for the first period. There was but little difference 

 between the grains grown in the aerated and unaerated conditions. 

 In a second study of aeration (1907 : 36), alfalfa and soy beans were 

 grown in addition to wheat, barley, oats, and peas. There were four 

 jars of each species and air was forced through two of them once a 

 day. The results for wheat and barley were negative, while oats 

 and peas yielded slightly better when aerated. This lack of response 

 was thought to be due to the fact that frequent rain produced good 

 natural aeration in all jars. One striking fact was that the pea 

 plants grown from seed from the aerated jar of the previous year 

 gave double the yield given by the plants from seed from unaerated 

 jars. Alfalfa and soy beans did not mature, but the beneficial 

 effect of increased aeration was very noticeable. 



Hall (1906 : 57) concluded that from several points of view it is 

 not necessary to assume the existence of an excretion from the roots 

 of a plant of a permanent acid, organic or inorganic, to attack the 

 solid mineral particles of the soil and to bring them into solution for 

 the nutrition of the plant. The growing portions of a plant-root are 



