(d) : Plants require a day length or light period for flowering, this light period is called as photoperiod. It was first studied by Garner and Allard (1920) Short day plants (SDP's) flower in photoperiods less than critical day length, e.g., Nicotiana tabacum, Glycine max (Soybean), Xanthium strumarium. Further these plants require long uninterrupted dark period and hence are called long night plants. Long day plants (LDP's) flower in photoperiod more than critical day length, e.g., Hyocyamus niger (Henbane), radish, Beta, spinach, Plantago, etc. Day neutral plants flower in any photoperiod, e.g tomato, maize, cucumber, etc.