Growth and Movement. 131 



than the others, causing the apex to bend toward the west ; 

 then the region of greatest growth is on the north side, 

 resulting in the bending of the apex toward the south, and 

 so on. 



By movements of this sort the plant is able to feel about 

 in space for some support, and having found it to twine 

 about it. If growth ceased in the coils as fast as they 

 were made, the plant would rise upward little or not at all, 

 and the successive coils of the stem would lie near together 

 or over each other. This, however, is prevented by the 

 continued growth of the stem for some time after the coils 

 have been laid down, the growth then being most rapid on 

 the under side of the coils, causing them to rise upward 

 and become spread apart in the form of a spiral ; at the 

 same time the stem is brought into closer contact with the 

 support. 



The benefit of the twining habit is easy to see, for those 

 plants which possess it are able to raise their leaves to the 

 sunlight with a very small expenditure of energy and 

 materials for the construction of strong stems. 



103. Source of Internal Energy. How the living proto- 

 plast is able to carry out the processes of growth and 

 movement is a mystery. We know in regard to it, how- 

 ever, that the protoplast ceases its activity and dies unless 

 a certain amount of internal energy is available, resulting 

 from the oxidation of the substance of the protoplast and 

 of reserve materials ; no amount and no form of external 

 energy can take the place of this. The more rapidly 

 growth and movement take place the greater the amount 

 of materials consumed. This oxidation of the substance 

 of the protoplast and of the reserve materials is the essen- 

 tial process of respiration, in plants as in animals. The 

 active, or kinetic, energy resulting from respiration is in 



