LEAVES WITH BEGABD TO LIGHT. 

 Fig. 3. 



427 



Diagram illustrating the rotation of a plant on the klinostat. 

 kk, the spindle of the klinostat; B, the box in which the seedling grows; 

 h, the hypocotyl of seedling ; x and y, its cotyledons. 

 Fig. 4 represents the change which occurs after the klinostat has rotated 

 through half a turn. The arrows show the direction of light (see text). 



In this and all subsequent drawings where k k and B occur, the plane of the 

 paper represents a vertical plane ; the plane of the table on which the klinostat 

 stands would therefore be at right angles to the plane of the paper. 



equivalent to illumination parallel to the axis of rotation. In the 

 greater number of experiments the klinostat was placed close 

 to a (north) window, outside which a large (92 X 92 centims.) 

 oblique mirror was fixed, so as to reflect as much light as possible 

 horizontally into the room. When seedling radishes or cabbages, 

 which are highly sensitive to lateral light, are placed on the 

 klinostat in the position shown in figs. 3 and 4, the hypocotyls 

 continue to grow parallel to the axis of rotation, showing that 

 the efficient light is practically parallel to that axis. And in this 

 sense I shall use the expression "light parallel to axis of 



rotation." 



Zenith position.— The position shown in figs. 3 and 4 is there- 

 fore equivalent to what would be illumination from above if the 

 plant were growing in the normal position. I shall therefore call 

 this position (when the axis of the plant is parallel both to the 

 axis of rotation and to the incident light) the " zenith position." 

 If the under surfaces of the leaves are illuminated while the 

 axis of the plant is parallel to the axis of rotation and to the 

 light, the plant is said to be in the nadir position. 



