FORCE OF GRAVITATION : GEOTROPISM 255 



cardboard. Afterwards plug the holes gently with cotton wool so as to 

 prevent the seedlings from slipping, and then place the cardboard over a 

 beaker of well water so that the roots of the plant may dip vertically into 

 the water. 



Place the whole in the darkened box with a hole in the side as described 

 above, and allow the seedlings to grow : examine in a day or two and note 

 if the root and stem are vertical as arranged when first put into the box. 



Ex. 158.— Examine fuchsias, geraniums and other plants growing in 

 windows, and note the bending of the stems towards the light. 



Note that the leaves have their upper surfaces towards the light. 



Observe the leaves of ivy shoots and other plants growing close to a wall ; 

 their upper surfaces are towards the light. Do the leaves grow out all on one 

 side of such stems ? Have the petioles curved in any way ? 



(iv) Movements in response to the force of gravitation : 

 geotropism. — All bodies on the earth behave as if they were 

 attracted towards the centre of the earth by a force which is 

 spoken of as the force of gravitation. This force exerts a 

 peculiar stimulating influence upon the various members of 

 living plants. Most primary stems grow vertically upwards 

 against the force and away from the earth ; when displaced 

 into a horizontal position, the growing regions near the ends 

 of the stems slowly bend upwards until they are again vertical. 

 Primary roots, on the other hand, grow downwards with the 

 force and towards the centre of the earth : when the primary 

 roots of seedlings which have been allowed to grow straight 

 down are placed horizontally, their growing parts soon curve 

 through a right angle and take up a vertical position with 

 the tips pointing downwards. 



Roots are described as geotropic or positively geotropic, while 

 stems which grow away from the earth are spoken of as apogeo- 

 tropic or negatively geotropic. 



The rhizomes of couch-grass, potatoes, and other plants are 

 generally diageatropic ; they grow in a horizontal position and 

 when placed vertically begin to slowly curve to one side until 

 the growing regions and tips are parallel with the surface of 

 the ground. 



