The answer is it signals transmitted downward. Early research
on photoperiodism dealt with the flowering period of plants. They precise
short-day (late summer and fall, less than about 16 hours), long-day (summer, lengthier
than 9 to 16 hours), day-neutral (any length), and intermediate-day (not
classifiable as either long-day or short-day) plants in qualified to their
photoperiod.
