222 DIRECTION OF GROWTH 



To a certain extent many roots are affected also by the direction 

 of the light falling on them; like the stems, they tend to 

 place themselves (or rather their young growing parts) in the 

 same line as the rays of light, but, unlike the stems, they grow 

 directly away from the source of light. This influence of the 

 direction of the rays of light upon the direction of the growth of 

 parts of plants is described as heliotropism. The roots and 

 stems thus affected are said to be heliotropic; the stem is 

 described as positively heliotropic, because it grows towards 

 the light, whilst the root is said to be negatively heliotropic, 

 because it grows directly away from the source of light. 

 Many roots, however, display only very slight, if any, helio- 

 tropism ; many stems which are not erect are not positively 

 heliotropic. 



Geotropism. — We know that all bodies are attracted to the 

 centre of the earth by the force of gravity. This force also 

 influences the direction of the growth, of stems and roots. If 

 a bean-seed be germinated in absolute darkness, in whatever 

 manner the seed be placed the main root will bend downwards, 

 whereas the young stem will curve and grow vertically upwards. 

 This behaviour is evidently not caused by the directive influ- 

 ence of light, because the seedling is in darkness. The end of 

 the root does not bend downwards because of its weight, nor 

 does it bend downwards because it is attracted by solutions 

 of salts or water, for the root will force its way down into 

 mercury which is much heavier than itself If the bean-seed- 

 ling be germinated on a slowly rotating vertical wheel, the roots 

 and stems will not grow in any definite fixed direction, because 

 gravity no longer acts on them in a constant direction. The 

 influence which gravity exerts on the direction of growth on 

 plants is described under the word geotropism. A main root 

 is said to be positively geotropic, because it tends to place itself 

 in the direction of the action of gravity and grows towards the 

 centre of the earth. Many main stems and flowering axes are 

 said to be negatively geotropic, because they tend to place 

 themselves in the direction of the action of gravity but grow 

 away from the centre of the earth. Lateral roots and lateral 

 stems are influenced by gravity in a manner which cannot be 

 discussed here. 



Hydrotropism. — -Many roots grow towards moisture in the 

 soil, and are said to h& positively hydrotropic. 



