Select the correct code. The primary root is:
1. Positively geotropic
2. Positively hydrotropic
3. Negatively geotropic
4. Negatively hydrotropic
A) 1, 2, 3 correct
B) 1, 2 correct
C) 2, 4 correct
D) 1, 3 correct

Hint: Tropism is the property of being attracted or repelled by a certain environmental stimulus. Tropic movements and nastic movements are a part of the nature of a plant and they are generally defined by the needs of plants.

Complete answer: In plants, the phenomenon of tropism is observed in response to various stimuli. These stimuli include water, gravity, sunlight, chemicals, air, etc. In plants, the stimuli exhibited by the primary root is positively geotropic and positively hydrotropic. When a root grows in the direction of the force of gravity, it is known as a positive geotropism. Whereas, a positive hydrotropism is one in which the organism tends to grow towards the moisture. This movement towards the water table is in order to absorb water for the requirements of the plant. Apart from these, some plants show some other tropisms as well. For example: touch me not plant shows negative tropism (negative tropism means the tendency of a plant to move away from a stimulus) towards the touch. Such attraction towards water is termed as positive hydrotropism. Apart from tropic movements, another type of movements shown by plants is called nastic movements. These are the movements exhibited by plants in response to certain stimuli, the response (or movement) is not towards or away from the stimulus. The response is a generalized one. The nastic movements are non-directional responses to stimuli like humidity, temperature or turgor pressure. The types of nastic movements are epinasty, hyponasty, photonasty, nyctinasty, chemonasty, hydronasty, thermonasty, seismonasty, geonasty, gravinasty, thigmonasty and haptonasty.
Hence, the correct answer is option B.

Note: TTropisms occur in three sequential steps. First, there is a sensation to a stimulus, which is usually beneficial to the plant. Next, signal transduction occurs. And finally, the directional growth response occurs.