What is the external stimulus in thigmotropism?

Thigmotropism is defined as a negative or positive directional response in a plant or part of a plant such as a plant organ, that is caused by an external stimulus such as touch or contact with any solid object. The factor that causes the stimulus is a hard surface that can change the direction of growth of the plant or part of the plant.  Hope this helps :)

Explanation:

•there must be variation for the particular trait within a population.

•the variation must be inheritable (that is, it must be capable of being passed on from the parents to their offspring).

•individuals with one version of the trait must produce more offspring than those with a different version of the trait.good

explanation:

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.