Cell Division and Expansion in the Growth of the Leaf

Abstract

Volumes and numbers of cells were determined at different stages of develop ment of the fifth leaf of Lupinus albus, and each of the second pair and the tenth leaf of Helianthus annuus. In the case of the second pair of sunflower leaves the values cover the whole life of the leaf from initiation to senescence. During both primordial development and the ensuing 'grand period of growth' division is the determinant of growth. About 10 per cent, of the cells in the fully grown leaf are laid down before leaf-emergence; the remaining go per cent, are formed during unfolding. Division does not cease in the lupin leaf or the second pair of sunflower leaves until they have reached half their maximum area. The tenth leaf, on the other hand, is as much as three-quarters fully grown before division ceases. Cell expansion commences soon after leaf initiation and continues throughout the life of the leaf. With lupin and the second pair of sunflower leaves there is a fourfold increase in the average volume of the cells before emergence from the apical region. During unfolding, there is a further tenfold increase in the average volume of the cells of the lupin leaf, and a twentyfold increase with the second pair of sunflower leaves. Expansion continues after the cessation of division but this further increase in volume is comparatively small. The data are discussed in relation to the 'two phase' hypothesis of leaf develop ment.