158 BOTANY FOR BEGINNERS CHAP. 



(3) That plants which receive blue rays bend towards the 

 side where the strongest light falls, just as plants do which grow 

 in white light. 



(4) That the curvature of positively heliotropic organs is due 

 to the rays from the blue end of the spectrum. 



"Why Heliotropic Movements take Place. The 

 movements which plant organs show when acted on by light is 

 due to the elongation of the side in the shade. Either the side 

 in the shade grows faster than that exposed to the brighter 

 light, or a different distribution of water occurs in the cells of 

 the organs. It must be distinctly understood, that no matter 

 what the external agency may be which produces the curvature, 

 it is the protoplasm of the plant cells which responds to it. 



Geotropism. We have seen that most shoots either grow 

 erect or bend towards the light, but that roots grow away from 

 the light. There is, however, another external agency in addi- 

 tion to light, which acts on the various parts of plants. This force 

 plays a very important part in determining the direction of the 

 organs of plants and is spoken of as gravitation. 1 



The property which enables plants to take up a definite position 

 under the influence of gravitation is called geotropism. Some 

 organs grow in opposition to the attractive force of the earth, 

 others grow in the same direction as gravitation acts. As in 

 the case of heliotropism, we use the terms negative and positive 

 in describing the two conditions of growth. 



Positive Geotropism. Those parts of plants which grow 

 towards the centre of the earth are said to be positively 

 geotropic. Tap roots, aerial roots, and a few cotyledonous 

 sheaths, grow downwards and are positively geotropic. Lateral 

 roots and stems grow outwards, and are described as being 

 diageotropic. In some cases the tap root has become 

 injured, and one of the secondary roots has developed a posi- 

 tively geotropic growth. 



Negative Geotropism. All those parts of a plant which 

 grow upwards or away from the centre of the earth are said to 

 be negatively geotropic. This is the rule with erect stems, 

 flower-stems, and a few leaves. 



1 As the student will probably know, by gravitation is meant the mutual attraction 

 between material bodies separated from one another. This mutual attraction between 

 the earth and bodies near it gives rise to the weight of bodies. 



