478 PHYSIOLOGY OF GROWTH. 



vertically upright. They are curved ; the convex side is directed 

 towards the moistened strip of paper. 



If we put not a wet but a dry strip of cardboard in the neigh- 

 bourhood of the sporangiophores> they do not curve. The nutation, 

 therefore, is not, as supposed by van Tieghem, 2 to be regarded as 

 somatotropic, i.e. as due to the mass of the paper, but actually as 

 hydro tropic. 



1 See Wortmann, Botan. Zeitung, 1881. 



2 See van Tieghem, Extrait du Bulletin de la Societe botan. de France, T 

 xxiii. 



182. Thermotropism. 



It is of interest to satisfy oneself that growing plant structures 

 perform irritation movements when their opposite sides are ex- 

 posed to different temperatures. 1 We make the experiments in a 

 large room,with a north aspect if possible, and subject to the smallest 

 possible variations in temperature. At one end of a long working 

 bench a plate of sheet-iron, previously smoked, and about 60-70 

 cm. square, is set up on a strong support with its surface directed 

 vertically to the window panes. The back of the plate can be 

 warmed by means of four gas flames, the position of which can be 

 adjusted according to requirements. The objects to be examined 

 (seedlings of Lepidium and Zea grown in sawdust in flower-pots) 

 can now be placed at various distances from the plate. To elimin- 

 ate heliotropic nutations a large plane mirror is set up behind the 

 originally perfectly straight seedlings, and parallel with the win- 

 dow, through which softened light enters through curtains. Before 

 placing the pots in position, we warm the plate, and suspend 

 thermometers at different distances from it. When these indicate 

 the particular temperatures at which we desire to experiment, the 

 seedlings are brought into their immediate neighbourhood, and 

 the experiment begins. Experiment with Lepidium. Ten seed- 

 lings. Height 3-4 cm. Temperature of room 12 C. Tempera- 

 ture over the middle of the flower-pot 30-35 C. After a few 

 hours marked negatively thermotropic nutation. Experiment with 

 Zea. Two seedlings, 2-3 cm. high. Room temperature 12 C. 

 Temperature over middle of flower-pot 30 C. After a few hours 

 positively thermotropic nutation (Wortmann). 



The temperature optimum for the growth of Lepidium lies 



