Question

Summer is changing to fall. The temperatures drop and the days are shorter. Tree leaves are changing from green to yellow and red and orange. The trees responding to what type of stimulus? internal external

Answer 1

Organisms respond to external and internal stimuli.

External stimuli are things that occur in the environment or come from other organisms, such as what organisms can touch, see, smell, taste, hear or feel; while internal stimuli refer to things like blood pressure, concentrations of ions, etc.

Changes in tree leaves in fall are a response to changes in the temperature and daylight duration, both of which are environmental stimuli and hence, external stimuli.