CH. Vl] STABILITY. 91 



This is what Knight found: the stems of the young 

 plants grew towards, the roots grew away from, the centre 

 of the wheel. 



Geotropism is not only valuable in enabling a plant to 

 take the shortest line in its upward growth, but it is also 

 important in another way, it plays the part of the 

 plummet to the builder. If a tree had no power of 

 vertical growth it might grow upwards in an oblique 

 direction, and would therefore fall by its own weight. 



The question how stability is gained in plants, how 

 they come to be strong enough to stand upright, is of 

 considerable interest. In the first place it should be 

 noted that the herbaceous plant, such as a seedling 

 sunflower, has a stability of a different order from that 

 which enables the oak to rear itself into the air. It 

 is well known that a delicate seedling plant withers if 

 exposed to the sun on a hot, dry day : it loses its stability 

 and droops towards the ground, but, if its roots are 

 supplied with water, it will recover when the damp 

 air of evening checks the evaporation from the leaves. 

 A woody stem, such as that of an oak sapling, is not so 

 affected, it does not collapse when dried. 



The reason of this difference may be discovered by 

 experiments. Any juicy, actively growing leaf-stalk or 

 flower-stem will serve as material. A stem of this sort 

 cut from the parent-plant and allowed to lie on the table 

 in the dry air of a room soon loses its stiffness and 

 becomes flaccid. Or we may place it in a 5 per cent, 

 solution of common salt, which robs it of its water by 

 osmosis, just as dry air robs it by evaporation. After it 



