[Vol. 2 



528 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



The 



The subsequent decline in the curve, however, is characteristic 

 for normal root tissues. It is also seen here that the 15- 

 minute exposure to illuminating gas resulted in a greater rise 

 in the conductivity curve than did a similar exposure in the 

 case of the cultures recorded in fig. 2. That this is due to the 

 different ages of the plants in the two cultures was borne out 

 by treatment of plants of different ages with other agents, 

 older the tissues the more resistant they become to the toxic 

 substance. McCool ( '13 ) was the first to point this out, in his 

 experiments with manganese chloride, and we see that it here 

 holds for anesthetics as well. 



Figure 4 shows the effect of illuminating gas at different in- 

 tervals when only the tops are exposed directly to the gas, the 

 roots meanwhile remaining in distilled water. The plants 

 were affected in proportion to the duration of treatment. The 

 tops of No. 39 were only very slightly injured, so that there 

 was practically no difference between them and the tops of 

 the controls ; No. 41 was affected more ; and No. 43 still more, 

 finally dying, after progressive drooping and yellowing. But 

 here again the roots of the treated plants were in all respects 

 similar to those of the controls and entirely unaffected, vis- 

 ibly, even though exosmosis was considerable. In such cases 

 it was also presumed that the excreted substances came in 

 part from the tops and that here we had an illustration of the 

 downward flow of food materials which occurs in plants under 

 natural conditions. This presumption was considered experi- 

 mentally as follows : 



Some cultures were placed under a bell jar and treated 

 with illuminating gas as before, the roots meanwhile being in 

 distilled water. The tops of one culture were not cut off, 

 while those of another were removed just before treatment, 

 and finally those of a third were removed just after treat- 

 ment. The controls were not treated, but their tops were cut 

 off immediately after the roots were placed in the distilled 

 water. The treated plants all gave approximately the same 

 exosmosis, which was considerably more than that from the 

 controls. A point to be noted here is that even though the 

 treated tops which were not cut off were very much affected 



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