[Vol. 2 



526 ANNALS OP THE MISSOURI BOTANICAL GARDEN 



hours exposure, however, the exosmosis was pronounced, even 

 when the roots were not in direct contact with the vapor. 



An exposure of only 5 minutes t^ illuminating gas pro- 

 duced no effect, but one of 10 or 15 minutes' duration caused 

 considerable exosmosis. That the 15-minute exposure should 

 result in less exosmosis than the 10-minute one is an inter- 

 esting point which finds an analogy, we shall see, at different 

 places throughout the work, where in isolated cases a briefer 

 exposure or milder treatment results in greater conductivity 

 of the medium than a somewhat more prolonged exposure or 

 more severe treatment. Where such a condition exists it is 

 usually found near the boundary line of noticeable effect, and 

 not where the effect is either nil or very pronounced. At this 

 critical point the individual hardihood of the plants them- 

 selves seems the most plausible explanation of the difference. 

 As the manipulation methods were exactly similar for any 

 given series it is altogether unlikely that difference in tech- 

 nique was responsible for the variation. 



The only plants to sustain any injury were those of cultures 

 28, 29, and 31. The tops of those in No. 31 drooped imme- 

 diately after the treatment and soon died, though the leaves 

 remained green ; the roots, however, remained entirely normal 

 to all appearances and retained their turgor. This is an in- 

 teresting point and was referred to above. After 7 days Nos. 

 28 and 29 plainly showed some injury, but it was slight, and its 

 visible effects were slow in making their appearance. At that 

 time the tops of these cultures showed greater yellowing and 

 drying than did those in the controls, No. 29 being somewhat 

 more affected than No. 28; the roots of both, however, re- 

 mained normal in appearance. 



The greatest contrast between the treated plants and the 

 controls is seen in fig. 3. The effect on the treated cultures 

 corresponds to the duration of treatment, the curves espe- 

 cially showing the difference in the speed of initial exosmosis. 

 It will be seen that the conductivity curves of the controls rise 

 rather high during the first day. This is no doubt due to the 

 effect of rather prolonged exposure of the roots to the air in 

 the bell iar. even though it was saturated with water vapor. 



