(i) Some plants require a periodic exposure to alternate light and dark for its flowering response and this phenomenon is called photoperiodism.
(ii) The requirement of light exposure is critical. The SDP plants when exposed to light period in excess of critical period fail to flower.
(iii) Those plants, which require exposure to light period at critical or more than critical period for its flowering response are called long-day plants.
(iv) Nicotiana tabacum fails to flower if exposed to more than the critical period of light
because it is a short-day plant.