THERMOTROPISM IN ROOTS 137 



spacious room with three large windows on one side. The room temperature re- 

 mained fairly constant at 12° C. A plate of sheet iron, 65 cm. square was used as 

 the source from which the heat radiated. It was fixed in an upright position on a 

 long table, which ran parallel with the windows. The plate was heated from behind 

 by four movable gas burners; in front it was besmeared with soot. The objects 

 experimented on were rotated on a clinostat in front of the iron plate, with the axis 

 of the clinostat perpendicular to it. In this position the light from the three win- 

 dows fell upon one side of the material under experimentation. In order to elimi- 

 nate resulting heliotropic reactions, a mirror was placed on the other side of the 

 apparatus reflecting the light from the windows opposite. This made the light 

 received from the two sides approximately equivalent. 



The spores of Phycoinijccs nilens were sown on four sides of a cubical piece of 

 bread which was fitted with a cardboard box, provided with holes. This arrange- 

 ment restricted the number of sporangiophores, as only a suitable number could 

 grow through the holes, and also prevented too rapid evaporation, while being 

 heated. Nevertheless the cardboard frame was sprayed with water every half hour. 

 The cube of bread was inserted on the axis of the clinostat and rotated before the 

 iron plate. The sporangiophores were shown by the experiments to be negatively 

 thermotropic ; that is, they bent away from the source of heat. They were subject- 

 ed to various temperatures in the neighborhood of 25°C. If they were moved so 

 far from the plate that the temperatures sank below 20°C., they failed to react. 



Experiments with seedlings of Lepidium sativum showed them likewise negative- 

 ly thermotropic as the first leaf of the epicotyl invariably bent away from the plate. 

 The time elapsing before the commencement of the reaction was found inversely 

 proportional to the intensity of the heat radiations falling on the plant. These ex- 

 periments also contradicted Van Tieghem's hypothesis, for above the optimum 

 temperature, which is 27°C. (De Vries) for Lepidium sativum, the bending was in- 

 variably and decisively negative. 



The seedlings of Zen Mays prove positively thermotropic and although they did 

 not react with the same sharpness and precision as did the Lepidium seedlings, yet 

 their behavior was nevertheless consistent and instructive. The optimum temper- 

 ature for Zea Mays shoots lies at 33.7°C., consequently if the seedlings were heated 

 to a lower temperature, they should according to Van Tieghem bend away from 

 the .source of heat. In every such case however, they reacted positively. Wort- 

 mann concluded that these reactions can have no more relation to the difference of 

 temperature on the two sides of the plant organ, than heliotropic reactions have 

 with a corresponding difference of light intensity. 



In 1S84, Barthelemy' published a short article "De Taction de la chaleur sur 

 les phenomenes de vegetation." He recognized three determining influences 

 which regulate the direction assumed by adventive roots; the difference of tem- 

 perature, the humidity and the nutritive value of the soil. In order to eliminate 

 the last two of these, he selected bulbs, with roots growing in water. He placed a 

 number of vases with the bulbs in a circle around an oven. The roots all grew to- 

 ward the oven, while the leaves faced the window. Again he divided a glas jars 

 into two compartments; in the one hyacinth bulbs floated in water of room temper- 



• Barthelemy, A., De Taction de la chaleur sur les phenomenes de vegetation. 

 Compt. Rend. 98: 1006-1007. 1884. 



