122 RESEARCHES ON FUNGI 



plane of the protoplasmic septum. A similar spot of light must be 

 formed by the left-hand beam, also at a distance of 0* 25 mm. above 

 the median plane of the protoplasmic septum, but on the right-hand 

 side of the swelling exactly opposite to the first spot. Thus we 

 have two spots of light on opposite sides of the swelling and, accord- 

 ing to our theory, each of the spots affects the patch of protoplasm 

 on which it rests photochemically and causes it to send a stimulus 

 down to the motor region of the stipe. The stimulus arriving from 

 the patch of protoplasm illuminated by the spot of light on the 

 left-hand side of the swelling will tend to cause the motor region of 

 the stipe to grow faster on its left-hand side than on its right-hand 

 side ; and the stimulus arriving from the patch of protoplasm 

 illuminated by the spot of light on the right-hand side of the swelling 

 will tend to cause the motor region of the stipe to grow faster on 

 its right-hand side than on its left-hand side. Now it is very un- 

 likely that, under natural conditions, the two stimuli given by the 

 two spots of light falling on opposite sides of the subsporangial 

 swelling will be absolutely equal. Let us therefore suppose that 

 one of the stimuli, say that given by the spot of light on the left- 

 hand side of the swelling, is very slightly greater than the other 

 stimulus produced on the right-hand side of the swelling. The 

 two stimuli will travel downwards on opposite sides of the swelHng 

 to the motor region of the stipe. Since, according to our assumption, 

 the left-hand stimulus is very slightly greater than the right-hand 

 stimulus, the left-hand side of the motor region of the stipe will be 

 stimulated to grow a little faster than the right-hand side, with the 

 result that the stipe will bend slowly to the right and, in so doing, 

 turn the subsporangial swelling and sporangium to the right. As 

 the subsporangial swelling is being bent round to the right, the 

 left-hand spot of light must move down the side of the subsporangial 

 swelling, while the right-hand spot of light must move up the side 

 of the swelling ; and, doubtless, the left-hand spot of light, as it 

 goes farther and farther down the left-hand side of the sweUing, 

 gives a greater and greater stimulus to the left-hand side of the 

 motor region of the stipe which it is approaching, while the right- 

 hand spot of light, as it goes farther and farther up the right-hand 

 side of the sweUing, gives a weaker and weaker stimulus to the 



