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, i.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. 



