Question

the remarkable heliotropism of sunflowers makes these flowers worthy of their name. at sunrise, most sunflowers in the bud stage face east. over the course of the day, as the sun crosses the sky from east to west, the flowers turn accordingly. this motion is performed by a flexible segment of the stem called the pulvinus. as the bud stage ends, the stem hardens and sunflowers cease to be heliotropic. question the term \heliotropic\ (sentence 5) most probably means having a flexible pulvinus flowering in the bud stage having brightly - colored flowers turning toward the sun，

Explanation:

The text describes sunflowers' motion of turning as the Sun moves (heliotropism). It mentions that when the bud stage ends (and they stop this motion), they "cease to be heliotropic". So "heliotropic" relates to the turning - towards - the - Sun behavior described earlier. The pulvinus is the part that enables the motion (not the definition of the term). Flowering in the bud stage and flower color are not related to the motion described.

Answer:

turning toward the Sun