400 GROWTH OF PLANTS 



lettuce seed thrown into deep dormancy by a period in a germinator at 

 35° C (95° F) can be forced to germinate and produce normal plants at 

 this temperature by 96 hours' treatment wdth this gaseous mixture. Even 

 5 to 20 per cent CO2 with 20 per cent O2 will force freshly harvested lettuce 

 seeds to germinate within 17 hours in darkness at 26° C (79° F). High 

 concentrations of CO2 with 20 per cent O2 force the germination of intact 

 cocklebur seeds at unfavorable low temperatures. Kidd found that similar 

 concentrations of CO2 threw white mustard seeds into dormancy. 



Treating dormant potato tubers so, 83, 87 ^^{i]^ 40 to 50 per cent CO 2 and 

 20 per cent O2 hastens the germination. In this treatment 20 per cent O2 

 was better than 0, 5, or 10 per cent O2; consequently, the forcing was not 

 due to anaerobiosis. This fact is interesting, for as was shown in Chapter 7 

 on "Dormancy in buds," reduced O2 pressure without CO2 favored ger- 

 mination of dormant potato buds; moreover, dormant potatoes sprout 

 more quickly in 100 per cent CO2 or N2 than they do in 100 per cent O2. 

 The presence of high partial pressures of CO 2 raises the optimum oxygen 

 pressure for germination. In earlier papers Thornton found that high con- 

 centrations of CO 2 with normal O2 induced various metabolic changes in 

 the living potato tubers: it increased the O2 uptake, the alkalinity of the 

 juice, the reducing property of the juice for methylene blue, the iodine 

 absorption, the catalase activity, the content of reducing and cane sugar, 

 the permeabihty of the protoplasm, and even the glutathione content 

 some time after treatment. Several of these changes were worked out 

 more fully in later researches and they will be discussed further presently. 

 Some of the effects of CO 2 on the accumulation of reducing and cane sugars 

 in potato tubers are discussed in the first section of this chapter. 



Carbon dioxide renders living plant tissues more alkaline. One of the 

 most general effects of CO2 on living plant tissue is to make it more alkaline. 

 Up to the time this work was done it was assumed that CO 2 had the opposite 

 effect, for CO2 is an acid and when dissolved in water, a non-living medium, 

 or even dead plant tissue, it renders them all more acid. This alkalizing 

 effect of CO 2 on living plant tissue is an indirect effect, and is brought 

 about by a pronounced modification of the metabolism of the living tissue. 

 In the chapter on "Dormancy in buds" we have seen that ethylene chlor- 

 hydrin, a slightly acid substance, renders living plant tissue more alkaline. 

 In that case it causes the change by inducing the protoplasm to respire 

 citric and perhaps other organic acids and to consume nitric and sulphuric 

 acids in the synthesis of glutathione. Ethylene chlorhydrin and CO2 may 

 induce other as yet unknown metabolic changes in living protoplasm that 

 modify the pH in either dhection. If so, the net effect of all the changes 

 is to raise the pH of the tissues. 



Table 45 shows ''^ the amount of the rise in pH induced in various living 

 tissues when they are exposed to 50 to 70 per cent CO 2 for various periods 

 at 25° C (77° F). In a number of the less acid tissues the pH shift was 

 considerable, while in the four acid fruits and some other tissues the shift 



