GROWTH 



187 



It follows that the attraction of the earth is the force 

 which determines the permanent direction of the growth 

 of the stem and root. But it is one thing to indicate 

 the force to which a phenomenon is due and quite a 

 different thing to explain just why and how this force 

 acts in that way. 



As a matter of fact it would be quite easy to under- 

 stand that the force of gravity would make the root grow 

 towards the centre of the earth ; but how are we to 

 understand that, under the influence of the very same 

 force of gravity, the stem tends on the contrary away 

 from the centre of the earth ? This is, nevertheless, 

 exactly what does happen. It is not only that a stem 

 placed vertically continues to grow in this direction, 

 but even a stem laid horizontally bends sharply upwards. 

 Here is a small seedling of cress, which some hours ago 

 was placed flat on a glass plate (a, b, fig. 53). Its little 

 stem has turned up as you notice, has reached the posi- 

 tion of n from that of m. Here is some cress, grown up 

 on a piece of felt. First the felt lay horizontally, later on 

 I placed it on its edge, then successively upside down, on 

 the other edge, and horizontally again. In this way the 

 stems have changed 

 their position four 

 times with regard 

 to the horizon and 

 after having de- 

 scribed a whole 

 circle, and twisted 

 themselves into a 

 knot they continue 

 to grow upwards. 

 It is clear that under the force of gravity the stem turns 

 away from the direction of its action. How is this to be 

 explained ? We must, of course, always keep in view 

 not the stem only, but also the root. Only such an ex- 

 planation can be considered satisfactory as will explain 



FIG. 53. 



