156 knight's experiment. [CH. VII 



We use an apparatus designed by Mr H. Darwin. 

 A disc covered with a thick layer of cork is attached to a 

 horizontal axis turning on bicycle ball-bearings. It turns 

 with ease and is driven at considerable velocity by an 

 endless band from a turbine. The experimental plants 

 are kept damp by a bell-jar which is not attached to the 

 revolving disc, but fits by its broad ground edge against a 

 fixed vertical metal plate, through which the axis passes. 

 The space in which the plants rotate is not therefore 

 absolutely closed, but the air can be kept sufficiently 

 damp for practical purposes. The most serious drawback 

 to the apparatus is that the plants are practically 

 subjected to a current of air produced by their own 

 rotation. This evil has been fairly well overcome by a 

 four-armed fan attached to the disc, and dividing the 

 space inside the bell into four compartments ; as the fan 

 rotates, the air within the bell-jar is carried round with 

 the plants. 



To use the apparatus it is only necessary to pin 

 seedling beans so that the root lies tangentially ; each 

 bean must be fixed on two pins firmly driven into the 

 cork. They should be fixed near the circumference of the 

 disc, but it must be remembered that the roots will curve 

 away from the centre of rotation; allowance must 

 therefore be made for their growth in this direction. 

 Roots curve perfectly well even when covered by a layer 

 of wet sponge pinned completely over them, an arrange- 

 ment which insures their being kept damp. 



The scapes of Taraxacum or cabbage shoots may be 

 used for apogeotropic curvature. Each shoot is fixed in a 



