100 GENERAL BOTANY 



These experiments with stem and root show that 

 there is in these organs a power of discerning the 

 direction of the vertical. It is not its weight that makes 

 the root bend downwards, for it will do so even in 

 mercury, on which it would easily float, but there is 

 a sense of direction in these organs akin to that by 

 which we ourselves are able to stand upright even 

 on a steeply-sloping hillside and is not anything of the 

 nature of instinct but is due to the action of gravity 

 on something inside the plant. 



In the case of roots, this sense is in the extreme 

 tip only, for if the experiment described above be 

 repeated, but with the last yV of the root cut off there 

 will be no bending. 



The downward tendency of roots is termed geo- 

 tropism, and the upward tendency of stems apo- 

 geotropism. 



These tendencies are usually only noticeable in the 

 main portion of the stem or root, being suppressed 

 and apparently absent in the side branches. The 

 branches from the main root are, in fact, fairly hori- 

 zontal, as you will see if you grow a Broad-bean with 

 its roots in water (and covered up from the light) ; 

 and in them the tendency is rather to lie across the 

 vertical. This is called dia-geotropism. 



In many trees the branches slope upwards, but 

 in some they are almost horizontal, being, like the 

 secondary roots, diageotropic. 



2. Light has also a great influence on the position 

 of the shoot-axis. Take any small actively growing 

 plant that has a fairly straight vertical stem, and put it 

 in a box open on one side but covered in above, so 



