152 HENRY D. HOOKER, JR. 



Although T' and T'" register a difference of thirty degrees, the 

 fall in temperature is but one degree every 2 mm. At the tip of the 

 root of Ervum Lens for instance, there would be barely a difference 

 of half a degree. The same condition exists in the agar-agar, so 

 that the roots here are under practically identical temperature 

 relations. All the other factors which are variable in the sawdust, 

 more especially the moisture difference, are included. The fail- 

 ure to obtain reaction from the roots in agar-agar shows that con- 

 ducted heat is not responsible for their bending iri sawdust. 



That hydrotropism might be the cause of Wortmann's ther- 

 motropic reactions was suggested by Klercker. The negative 

 bending is due to the drying out of the sawdust, which proceeds 

 rapidly, when being heated. Since roots are positively hydro- 

 tropic, they bend toward the moist sawdust, consequently away 

 from the source of heat. When the sawdust is covered, evapor- 

 ation takes place less rapidly, the psychrometric difference is not 

 so quickly established, and the reaction requires more time. Pos- 

 itive bending is produced likewise by a difference of moisture, 

 but since it is much smaller, the reaction is less definite and con- 

 stant. Warm air is capable of holding more water vapor than cold 

 air. The positively hydrotropic roots bend toward the warmer 

 sawdust, where the interstitial air-spaces offer a moister atmos- 

 phere. This accounts for the so-called positive thermotropism, 

 the irregularity and uncertainty of its appearance, as well as the 

 length of time necessary. The conditions of moisture depend 

 on how damp the sawdust is at the beginning of the experiment 

 and so vaiy materially. It is but natural that Wortmann and 

 Klercker should disagree as to the critical temperature of Pisum 

 satimim. The critical temperature is merely the point where 

 the increased capacity of the air for holding vapor is offset by 

 the evaporation. When the sawdust was made very wet, so that 

 it flowed, the moisture-conduit was equalized throughout the 

 mixture, and no reactions could be obtained. 



In all probability, the negative bending was assisted and ac- 

 celerated by a traumatropic stimulus. Wortmann often heated 

 the roots above the maximum temperature. In experiments with 

 Ervu7n Lens, Pisum satimmi and Phaseolus 7?iultiflorus, the roots 



