Natural History til 9 



even farther, so that they can tap the water supplies, which 

 are at a great depth. 



The root-system of a tree only one year old may have a total 

 length of 12 yards. 



Darwin showed that a root from which about one-twentieth 

 of an inch of the tip has been removed does not respond to gravity 

 if laid on its side ; it grows straight on. But if a root is first laid 

 on its side for a few minutes and then decapitated, it does, after 

 a few hours, curve downwards. He drew the conclusion that, 

 while the reaction takes place in the zone of most active growth 

 about one-fifth of an inch behind the tip, the effect of gravity is 

 registered by the extreme tip only. Later ingenious experiments 

 have confirmed this result. 



Plant Tropisms 



There is a close correspondence between plant-responses and 

 the tropisms exhibited by animals (see p. 78), and the plant's 

 answer-back to gravity is called geotropism. But while the moth 

 which flies into the candle flame perceives the stimulus of light by 

 special sense organs, the eyes, and moves by special motor organs, 

 the contractile muscles, and has these linked by special conduct- 

 ing organs, the nerves, the root has at its disposal no such 

 complex and highly specialised structures. Perception and 

 conduction take place in the general living matter; reaction is 

 due to a slight change in the growth rates of upper and lower 

 sides. Darwin's comparison of the root-tip to a brain is really 

 far-fetched. 



Ingenious attempts have been made to show that the plant 

 has sensory organs of a primitive kind, for the perception of 

 gravity, light, and touch. In a group of cells within the root-tip 

 there are numerous large starch grains which lie heaped up on the 

 bottom wall. If the root is laid on its side the starch grains fall 

 over and come to lie on the side wall, which is now lowermost. It 



