Q. Assertion : In some plants flowering depends only on a combination of light and dark exposure.
Reason : The site of perception of light or dark duration are the shoot apices of plants.

 1821  223 Plant Growth and Development Report Error

Solution:

Some plants require a periodic exposure to light to induce flowering. It is now also known that not only the duration of light period but the duration of dark period is also of equal importance. Hence, it can be said that flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations. This response of plants to periods of day/night is termed photoperiodism. The shoot apices modify themselves into flowering apices prior to flowering, shoot apices cannot perceive photoperiods by themselves. The site of perception of light/dark duration are the leaves.