These movements are a result of the response to an external stimuli and the direction of movement is controlled by the direction of the stimulus. It is of the following type Photonastic response to light
Thermonastic response to temperature
Chemonastic response to chemicals
Depending upon the nature of the stimulus there are various types of tropic movements. The chief stimuli are gravity, light, water and chemical substances. The movements induced by them are geotropism, phototropism, hydrotropism and chemotropism respectively.
Secondary roots and branches grow at right angles to the force of gravity and are said to be plageotropic. Lateral roots grow oblique to the force of gravity and are known as diageotropic. Tertiary roots are not affected by gravity and are said to be ageotropic.
It will be seen that after a few days all the seeds have germinated in different positions. In all the cases the roots are found growing in the downward direction and shoots in the upward direction.
But in roots, the greater concentration on lower side inhibits growth and the low concentration on the lower side favours the bending of roots downwards towards gravity.
If a plant is rotated on a special apparatus called clinostat, it neutralizes the effect of gravity and the shoot continues to grow horizontally without any curvature.
Take a well watered plant and keep it in a dark chamber, into which light enters by means of a tiny opening. Leave the plant in the chamber for a few days and then observe.It will be seen that the shoot is found bending towards the source of light, showing that shoot is positively geotropic.
Example: Growth of pollen tube through the style towards the embryo sac.