THERMOTROPIC REACTIONS OF SHOOTS 99 



would lessen danger from falling snow and ice and would also 

 decrease transpiration and retard radiation of heat. 1 



153. Thermotropic Reactions of Shoots. Place a long table with 

 one end toward a window, and set upright near the window a 

 plate of sheet iron which has been smoked on the inner side by- 

 means of a candle flame. Place two or more gas jets back of 

 the plate nearer the window in such position that the plate will 

 be warmed over its whole surface. At the farther end of the 

 table set a large mirror which will reflect light directly toward 

 the plate. Adjust the ventilation and heat of the room to se- 

 cure a temperature of about 12 or 13 C. Secure a number of 

 seedlings of Lepidium in small pots. The seedlings should be 

 about 3 or 4 cm. high and should be grown in two-inch pots. 

 Set a pair of the seedlings at a point on the table where the air 

 over the pot is.at 35° C, and a second pair farther away where 

 the temperature is 30 C. Maintain these temperatures for a 

 period of four hours and note position of shoots in both pairs. 

 The optimum temperature for Lepidium is about 33 C. and the 

 shoots should tend to curve toward the source of radiation or 

 away from it as they lie below or above the maximum. 



Repeat the test with Zea seedlings two or three cm. in height, 

 and set them in pairs at such distances as to secure temperatures 

 of 30° C. and 25° C. over the middle of the pot. Several kinds of 

 seedlings may be used at once, but it will be found that not all 

 are thermotropic, either negatively or positively. 2 



154. Influence of Temperature on the Opening or Closing of 

 Flowers. Select a cool cloudy morning and cut a flower of the 

 tulip and fix the stalk in a bottle of water by means of a cork. 

 Attach a fine filament or thread of glass to one of the outer perianth 

 segments by cementing it with shellac to the groove on the outer 

 surface of the organ. Fasten a similar filament to the opposite 



1 Harshberger, J W. Thermotropic movement of the leaves of Rhododendrum 

 maximum L. Proc. Acad. Nat. Sciences of Philadelphia. 219. 1899. 



2 Wortmann, J. Ueber den Einfluss der Strahlenden Warme auf wachsende Pflan- 

 zentheile. Ber. Deut. Bot. Ges. 41 : 457, 473. 1883. 



