PLANT PITYSIOLOGY 151 
be found that both curve round, so that the radicle again 
points downwards and the plumule upwards. 
131. Experiment 26.—Pin a Bean seedling to a cork 
which has been previously fixed to the side of a dish 
containing mercury and a little water. The seedling 
should be pinned with the root horizontal. The root 
will soon turn and grow right down into the mercury. 
This shows that it is not the weight of the root which 
makes it turn down, since the tip forces its way into 
the mercury, in spite of the great resistance offered by 
this dense liquid. 
132. Water.—The direction of growth of roots is in- 
fluenced by moisture, and they are therefore said to be 
positively hydrotropic. This property is very useful to 
them, as it enables them to grow towards the damper 
parts of the soil. 
133. Experiment 27.—Get a large box and fill it with 
dry sand or sawdust. In the middle, place a flower-pot 
with the hole corked up, or any porous pot. Fill this 
pot with water. Then plant some seeds which have 
just germinated in the sand, at various distances from 
the flower-pot. After a few days pull them up and see 
if their roots have grown towards the flower-pot, i.e. 
towards the moisture. 
Another method is to obtain a box with a bottom of 
wire gauze, fill it with damp sawdust, plant seeds in it 
and hang it un. The roots grow out through the gauze, 
owing to the influence of gravity, and then turn back 
again because of their hydrotropism. If, however, the 
air is very dry, the roots may shrivel up in the air and not 
turn back, hence the first method is more satisfactory. 
