ROOTS V. STONES 



winding cracks and crawl round stones with an ingenuity 

 that makes one wonder if they can possibly be without some 

 sort of intelligence. 



It is the very tip or end of the young root that seems to 

 be responsible ; for if, in the course of its journey ings under- 

 ground, it should strike a stone or something hard, the root 

 does not grow on and flatten itself. 



But some sort of message is sent back from the tip to the 

 growing part which is a short distance behind it. After 

 this message has been received, the growing part begins to 

 curve sideways, so that the tip is brought clear of the obstacle 

 and can probably proceed triumphantly upon its way. The 

 inexplicable part is that the growing part which curves has 

 never been touched at all, but simply answers to the message 

 from the tip.^ 



This is perhaps the most reasonable and intelligent be- 

 haviour found in the whole vegetable world, and it is not 

 surprising that Darwin compared the root-tip to a brain. 



These extraordinary responses fill one with astonishment, 

 but there are others still more interesting and remarkable. 

 It will be remembered that we have already shown how 

 different the soil is at different levels. The subsoil, soil, and 

 uppermost layers are all quite different from one another. 



This may explain why it is that many plants seem to 

 prefer to develop their roots at one particular depth below 

 the surface. Not only so, but they find their own favourite 

 level in the most persevering way. 



If, for instance, you sow a barley-corn at too great a 

 depth, the seed germinates and forms a few roots, but it im- 

 mediately sends out a stem which grows upwards towards the 



^ If the growing part itself touches a stone it curves round the stone, 

 not away from it — the reverse of the reaction at the tip ! 



90 



