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:

answer: grassland biomes are closer to the equator and receive more rain.

grasslands are biomes that are dominated by grasses rather shrubs and trees. savanna are biomes that exhibit grasses, shrubs and small tree as vegetation. grasslands are located near the equator of the earth. these are found in north america. savanna biomes are found in africa, south america, australia and india. grassland exhibit moderate climatic conditions, it receives   amount of rainfall higher than savanna.   therefore, savanna exhibit dry and hot climatic conditions.