Studies on the Expansion of the Leaf Surface: III. THE INFLUENCE OF RADIATION ON CELL DIVISION AND LEAF EXPANSION

The numbers of leaves and the areas and cell numbers of leavesfrom the first five nodes of the cucumber were determined throughouttheir development with three different amounts of daily radiationand two conditions of nutrient supply. The rate of leaf production was found to be constant with timefor any one amount of radiation and to increase with increasedradiation. The transition from the rate in darkness to thatin light was sharp and occurred more quickly the higher theradiation. Provided the nutrient supply was high the ultimate areas ofindividual leaves were greater the higher the radiation; ifmineral nutrients were depleted the maximum area of a leaf occurredwith an intermediate amount of light. This arose because thesefactors exercised a differential effect on the various phasesof growth. Each leaf commenced its life as a mass of dividing cells andthe mean rate of division remained constant until it unfoldedfrom the terminal bud. The mean rate of division was much greaterat high than at low levels of radiation and was interpretedas being regulated by carbohydrate supply. Although expansionof some cells was likely before unfolding, after this stagethere was a marked decrease in the proportion of cells proceedingto division. The duration of division in the leaf after unfoldingwas independent of radiation; although 70–98 per cent,of the final number of cells were formed after unfolding, theultimate number was effectively determined by the rate of divisionprior to unfolding. Low nutrient supply restricted the duration of division in theleaf to a significant extent and the expansion of cells to avery considerable degree. The smaller leaves on plants receivinghigh rather than medium radiation were due to their cells beingmuch smaller.