3G JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



is no such force-action from above (other than light), as gravity 

 acts from below.* 



Eepulsion by Obstruction. — According to Darwin, the tip 

 of a root will try to turn away from an obstruction. He showed 

 this by fastening a piece of card to one side of root-tip, as well as 

 by other methods. This gave rise to a good deal of criticism, 

 the idea of his critics being that the motion was a result of injury 

 to the delicate tip. The root, however, is deflected away from 

 the obstruction, so that it looks as if the irritation causes a 

 greater growth to take place on the same side as the obstruction. 

 If so, this would imply an increased vigour at the spot. Moreover, 

 this would be quite in keeping with what takes place in climbing 

 stems, &c, that as soon as pressure is exerted at any point the 

 tissues begin to thicken in response to it. The enormous power ' 

 which roots acquire to overcome obstructions, as, e.g., when grow- 

 ing under a wall, is probably the result of the effort induced by 

 the obstruction itself, which causes it to develop tissues in 

 response to the pressure. 



If, therefore, Darwin's peas had been able to have a fulcrum, 

 instead of growing in air, results perhaps might have been 

 different ; but this suggestion requires experimental verifica- 

 tion. 



Climbing Plants. — These supply an abundance of illustra- 

 tions of motion. The climbing property of stem-twiners is the 

 result of their circumnutation, perhaps coupled with some slight 

 degree of sensitiveness to contact. In the greenless stem of the 

 parasitical dodder both properties are in evidence, and some 

 branches, as of Strychnos, are known to be modified like tendrils,f 

 which both circumnutate and are sensitive ; but it is not certain 

 whether the latter property is possessed by ordinary green- 

 stemmed twiners. Circumnutation is* well seen in long slender- 

 stemmed plants as the hop. The motion appears to be the 

 result of a combination of forces. First, there is, or believed to 

 be, the spiral growth of the stem itself, so that the apex is, so to 

 say, being continually pushed over to the opposite side by growth 

 at one point at a time ; as the successive points of growth are 

 supposed to take place in order round the stem, the ultimate result 



* I would refer the reader, for a further discussion of this subject, to 

 my Origin of riant Structures, p. 197 acqq. 



f See a paper by P. Muller, Journ. Linn. Soc, ix. p. 314. 



