RESPIRATION AND OXYGEN. 



85 



be increased by greater amounts. While he found that the use of 

 CO2 has no effect upon the harvest, it seemed that in some cases the 

 development of roots was small. 



Lehmann (1911 : 90) showed that many plants, e. g., Vicia faba, 

 Brassica napus, Lupinus albus, Pisum sativum, and Cucurbita, stop 

 growth very quickly when oxygen is withdrawn and are unable to 

 grow even in sugar solution. He found no growth in epicotyls at 

 0.5 mm. pressure, though in some cases, e. g., Helianthus annuus, 

 slight growth occurred in 0.5 to 1 per cent sugar solution at 20° C, 

 and in distilled water at 25° C. 



Crocker and Davis (1914 : 312) have shown that seedhngs of 

 Alisma plantago grew 30 to 32 mm. in 21 days in vacua, or 11 to 12 

 times the length of the embryo, while those in the air grew 50 to 

 57 mm. or 18 to 22 times the length of the embryo. No leaf branches 

 were formed in the seedlings grown in vacua, and chlorophyll did 

 not develop. With the air-pressure below 5 mm. no chlorophyll 

 developed within a month. In the dark, an air-pressure of more 

 than 15 mm. was needed to develop leaf branches. The formation 

 of the primary root seemed to be dependent upon oxygen, as seed- 

 lings in drop cultures and on the surface of water produced primary 

 roots, while few showed them beneath a layer of water, and none 

 in an air-pressure much below 5 mm. 



Cannon (1916 : 75) determined that the exposure of roots of Pro- 

 sopis and Opuntia to pure carbon dioxid for 15 minutes did not 

 affect growth, but exposure during periods of over 30 minutes 

 inhibited it. Growth began again soonest after the shortest exposures, 

 and sooner at high than at low soil temperatures. Roots of Prosopis 

 recovered their normal rate of growth more readily than those of 

 Opuntia. In later studies upon the effect of low percentages of oxygen 

 (1917:82; 1918 : 82), he found that root-growth stopped sooner in 

 Opuntia versicolor than in Prosopis julifiora, as shown by table 26. 



Table 26. 



Root-growth regularly ceased in nitrogen, though it continued in 

 some cases for a day or two (1919 : 71). The effect upon the shoot 

 was less marked, in Eriogonum growth continuing at a rate of 8 to 



