20 THE FLOWERING PLANT. 
a more or less oblique one. If a seedling is placed with its 
primary root in a horizontal direction, this will very soon curve 
round and grow downwards, while the stem curves upwards. By 
keeping seedlings in a state of slow revolution in a vertical plane, 
the effect of gravity is neutralized. The result is that root and 
stem do not in this case grow downwards and upwards, but have 
a tendency towards the horizontal direction. Heliotropism is the 
term employed to designate the influence of light on direction 
of growth. Roots are generally negatively heliotropic, 7.e., turn 
away from the light. Hydrotropism refers to the influence of 
moisture. Roots are positively hydrotropic in that they grow 
towards moisture. This is commonly the case with trees. If 
seedlings are placed in a perforated vessel full of damp moss, and 
the apparatus is then hung up, the primary roots will grow down 
as a result of positive geotropism, and make their way through 
the perforations. If the surrounding air is moist, the roots will 
continue to grow downwards, but if dry, they will bend up towards 
the damp moss. The tips of roots are also sensitive to contact, 
which causes them to curve. 
It is evident that reaction to all the above stimuli is of such a 
nature as to fit the root for its share in nutrition. 
