Nastic movements differ from tropic movements in being

The correct option is Nondirectional.

In plant biology, plants respond to environmental stimuli through different types of movements. Two primary categories of these responses are tropic movements (tropisms) and nastic movements (nastics). The key factor that distinguishes them is how the direction of the movement relates to the direction of the stimulus.
Let us break down the differences step-by-step:

1. Tropic Movements (Directional Responses):
Tropic movements are growth movements whose direction is determined by the direction of the external stimulus. If the plant grows towards the stimulus, it is called positive tropism (e.g., phototropism, where stem tips grow towards light). If it grows away from the stimulus, it is negative tropism. Hence, tropic movements are always directional.

2. Nastic Movements (Nondirectional Responses):
Nastic movements, on the other hand, are plant responses where the direction of the movement is independent of the direction of the stimulus. No matter from which direction the stimulus (such as touch, light, or temperature) comes, the plant response occurs in a predetermined direction. For example, the folding of leaflets in the sensitive plant (Mimosa pudica) when touched occurs in the same way regardless of the direction from which you touch the leaf. Therefore, nastic movements are nondirectional.

Consequently, nastic movements differ from tropic movements primarily in being nondirectional.