REVIEW AND CONCLUSION 421 



Response to gravity also causes the stem of twiners 

 to twist round their supports, and the lateral branches 

 of a tree to grow horizontally and so spread the leaves 

 to the light, while, if the main stem is broken, a branch 

 will lose this property and instead grow upright to take 

 its place. All these reactions, being due to differences 

 in the rate of growth of either side of the organ, take 

 place only in parts that are young and still growing. 

 This power of reacting to the external influences, 

 and not always in the same way, but some organs 

 in one some in another ; and even the same organ 

 differently under different conditions, is one of the most 

 important attributes of life, which we may now add 

 to those given in the introductory chapter of feeding, 

 growing and reproducing. 



