COMPARATIVE RESPONSES OF LONG-DAY AND SHORT-DAY PLANTS TO RELATIVE LENGTH OF DAY AND NIGHT

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Abstract
The discussion deals primarily with the problem of satisfactorily classifying plants into the long day, short day and indeterminate or neutral groups on the basis of response to relative length of day and night. In the first 2 groups there exists a fairly definite critical light period, which constitutes the dividing line between day lengths favorable to flowering and fruiting and those tending to produce a purely vegetative type of activity. Short day plants are those in which reproductive activity is favored by day lengths shorter than the critical and long day plants are those in which reproduction is promoted by day lengths in excess of the critical. Among annuals and herbaceous perennials the alternative vegetative stage in the short day type commonly is characterized by extensive elongation of the axis while usually a prominent feature of the vegetative stage in long day plants is a leaf-rosette form of growth. Plants of the less sensitive or indeterminate group have no clearly defined critical light period. Long day and short day plants show definite contrast in their responses to abnormal light periods with respect to initiation of reproductive activity but not as to maintenance of the general nutrition.

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