The stomatal index of the trifoliate leaves of the Cowpea is a function not only of the total radiation received but also of variations in radiation during the 6 d preceding the major phase of the unfolding of the leaf lamina. Hence, for sun plants a whole day in the shade can cause a decrease in the stomatal index provided this day occurs during the period defined above. For shade plants it is shown that a single day of high light intensity during this critical period will cause an increase in the stomatal index. The latter is thus a characteristic for each individual leaf. It is postulated that the site of photoperception controlling the stomatal index is located in the expanded adult leaves. Excision and transfer of still unfolding leaves to a culture medium partially prevents the influence of the older expanded leaves on the stomatal index of the excised leaf.
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SCHOCH et al. (1980) studied this question.