GROWTH 193 



made with the stem, we shall see that there is not the 

 same tension in the root as in the stem. If we split a 

 length of root into two halves (like the stem in fig. 53, r), 

 no curvature will be noticed in these halves ; if it be 

 cut into three parts (53, s) no elongation of the middle 

 part, nor shortening of the outer part, will be noticed. 

 To sum up, the root has none of the tension of tissues 

 peculiar to the stem ; its external parts grow as quickly 

 as the inner ones. This is also evident from one more 

 property of the root : a young root generally elongates 

 more quickly than a stem, and this is why it does not 

 show any tension, which is nothing but restrained 

 growth. 



Thus if the force of gravity does not cause the 

 upward curvature of the growing root-tip, this is partly 

 explained by the absence of a mechanical condition 

 necessary for the purpose: it lacks the corresponding 

 tension of tissues. Our model shows how, given merely 

 a difference in the structure of two organs, we may get 

 entirely opposite results from a similar action of the 

 force of gravity. It is useful to remember this whenever 

 physiological facts are discussed. If one and the same 

 external factor causes different effects in different 

 organs, we must admit either a difference in the pro- 

 perties of the organs or a complexity in that same 

 factor. This second alternative is impossible with 

 regard to the force of gravity ; but the difference in 

 the tension of tissues is certainly not the only possible 

 difference between the properties of the stem and root. 



Our explanation would be perfectly satisfactory could 

 we further prove that the lower half of a root lying 

 in a horizontal position grows more quickly than its 

 upper half, as is the case with the stem ; and that 

 in spite of this, owing to its own weight, it passively 

 bends down like our spiral d', II in fig. 54. Some 

 experiments seemed to prove this, but doubts have 

 arisen in connexion with them, because contradictory 



