V Movements of Plants 8i 



whole is covered with a bell-jar and placed in the dark. 

 The young stem grows straight up, the young root grows 

 straight down into the water. Then light is allowed to fall 

 upon the plant, in one direction only, through a narrow slit. 

 In a few hours the young stem has curved towards, the 

 young root away from the light. 



Darwin's experiments led him to conclude that the 

 sensitiveness to light was localised in the tips, e.g. of the 

 cotyledons of Phalaris and Avena, of the young stems of 

 Brassica and Beta, and of the radicles of Sinapis. There 

 seemed to be a transmission of influence from the tip to the 

 other parts, causing them to bend. " It is an interesting 

 experiment to place caps over the tips of the cotyledons of 

 Phalaris, and to allow a very little light to enter through 

 minute orifices on one side of the caps, for the lower part of 

 the cotyledons will then bend to this side, and not to the side 

 which has been brightly illuminated during the whole time." 



The commonest position of leaves and cotyledons during 

 the day is one more or less transverse to the direction of the 

 light, and this also Darwin believed to be due to a modified 

 circumnutation ; but few would now agree with him in this 

 interpretation. In some plants in which the leaflets are 

 provided with little swollen cushions or pulvini at their 

 base, they move upwards or downwards or twist laterally 

 when the sun shines very brightly upon them, as in the 

 well-known " compass-plant " (Silphium) ; by directing their 

 edges towards the light they avoid the injurious effects of 

 too intense illumination. 



It is easy to understand that it is advantageous for most 

 plants to bend towards the light, for thus their leaves are in 

 a better position to use the power of the sunlight on which 

 the life of the plant so much depends. On the other hand, 

 it is also advantageous that the aerial rootlets of the ivy or 

 the tendrils of the vine should turn away from the light. 



G 



