In some plants,flowering is induced by exposure to low temperatures,either quantitatively or qualitatively. This phenomenon is known as $........$.

Photoperiodism is:

$Nicotiana$ $tabacum$,a short-day plant,fails to flower when exposed to a light duration longer than its critical period. Explain.

In respect of photomorphogenesis,plants are most sensitive to:

The relation of flowering to the lengths of light and dark periods was investigated by two experiments. In one experiment,the plants were subjected to dark periods of various lengths while the light periods were kept at $4$ hours. In the second experiment,plants were kept in dark periods of various lengths while the light periods were kept at $16$ hours. The number of flowers formed in relation to these periods is shown in the table. These results indicate that flowering:

Dark periods (hours)	Flowers formed (Light: $4$ hrs / $16$ hrs)
$8$	$0$ / $0$
$10$	$0$ / $0$
$12$	$4$ / $6$
$14$	$5$ / $7$
$16$	$5$ / $8$

Phytochrome is a