GROWTH 191 



upper half of the horizontal stem is shorter and the 

 lower longer than the vertical stem, as was to be 

 expected. What is true of a horizontal stem can be 

 applied also to one in an inclined position : as soon as 

 a stem deviates from the perpendicular, the force of 

 gravity by causing increased growth of the lower side 

 of the stem brings it back to its vertical position. 



Now we understand why it is that owing to the 

 force of gravity the stem curves in a direction con- 

 trary to the direction of this force. But a question 

 arises : Why is it that the same thing does not happen 

 to the root ? You see now that our closer investigation 

 of the phenomenon has reversed the question. At first 

 we thought it quite natural and comprehensible that 

 the root should grow along the line of the force of 

 gravity, and incomprehensible that the stem should 

 grow in a contrary direction ; whereas now we under- 

 stand why the stem grows exactly as it does, and find 

 it difficult to understand why it is that the root grows 

 differently. Let us turn for an explanation of this 

 apparent contradiction to the following model. Let us 

 imagine two wooden discs (fig. 54, L, c and d), joined 

 by means of springs to a cross bar (6) . Two pliable india- 

 rubber rods (a, a) joined by a transverse handle (e) are 

 passed through holes in this bar. Their ends press against 

 the centres of the discs c and d. If we push these rods 

 in the direction of the arrow, we shall separate the coils 

 of the spirals and bring the two springs into a state of 

 tension. The india-rubber rods in our figure are meant 

 to represent the rapidly growing axial parts of organs, 

 while the compressed coils represent the slower growing 

 external tissues of these organs, distended by the growth 

 of the inner parts. This movement demonstrates sym- 

 metrical growth, and the tension of tissues resulting 

 from it. Let us now try to show by means of the same 

 figure unsymmetrical growth, such as is caused by the 

 action of the force of gravity, in which the lower part 



