THE ROOT | ha 
TROPISMS. 
The Greek word trope means a turning. When a 
plant organ turns towards the thing that causes 
the turning the tropism is positive, and when it turns 
away from the thing the tropism is negative. 
Geotropism. We have already seen that, in an 
ordinary plant, the stem grows upwards into the air 
and the root downwards into the soil. Thus the root 
is positively and the stem negatively geotropie, (t.e., 
the former turns towards and the other away from 
the centre of the earth). Many experiments may be 
made to demonstrate this fact. 
Stick a few mustard seeds on a thick piece of moist 
black cloth fixed in the bottom of a large glass capsule 
(i.e., a round glass box with a lid to it) and then wire 
the lid on and hang the capsule vertically. In the 
course of a day or two the seeds will germinate. The 
root will travel unerringly downwards, and the plumule 
as unerringly upwards along the surface of the 
cloth, against which the mass of root hairs presents 
a strikingly beautiful sight (Fig. 40). 
If broad bean seedlings that are well established 
are taken from the soil and planted upside down so 
that the roots point upwards and the stems downwards, 
it will be found that the roots and stems will bend 
round till they are again travelling in their 
original directions, the former towards the centre 
of the earth and the latter away from it 
(Fig. 36). The great advantage, indeed the 
necessity of these particular tropisms of root and stem 
must be obvious, for no matter in what position the 
seeds may germinate, the organ that is concerned in 
the light and air relations makes its way to the region 
where these things are most abundant, while that which 
relates the plant to the soil, places itself in the best 
position for that purpose. 
