||<p/>The growth of leaves of some butterhead type varieties of lettuce has been investigated under different light intensities and temperatures, with special reference to the process of head formation. Most experiments were carried out with the varieties 'Meikoningin' and 'Rapide' in climatized growth rooms.<p/>In Chapter 1, a typical feature of lettuce leaves is demonstrated, <em>viz</em> ., that lamina extension may largely exceed that of the corresponding midrib, yielding the caracteristic folds and crinkles of the leaf blade (Plate II). Therefore, length and greatest width of the leaves have mainly been chosen as criteria for differences in leaf growth.<p/>In Chapter 3, the effects of different light intensities, light duration (daylength) and their interaction with temperature are presented. Leaf production increases both with light intensity and with temperature (fig. 3.1), but appears to remain fairly constant with plant age. Since subsequent leaf development occurs at a lower rate, a number of leaf primordia and young leaves of the plant accumulate with time (figs. 3.4 and 3.5). Results suggest that primordial growth is more affected by temperature than by light intensity (fig. 3.6).<p/>In many respects clear-cut differences are found between the response of leaf length and leaf width under various experimental conditions. Based on the maximum leaf dimensions reached, leaf width generally responds positively to increasing light energy, either given as a higher light intensity or a greater daylength (figs. 3.7 and 3.9.) In both cases relationships are represented by saturation curves which, for light intensity, tend to go through the origin. Effects of daylength become particularly evident for periods shorter than 12 hours.<p/>For leaf length, a positive relation to light energy is only found at a low intensity level, since at high light intensity midrib elongation appears clearly suppressed. Effects of different daylengths, also are only evident at a low light intensity (fig. 3.9).<p/>Temperature effects greatly depend on the prevailing light intensity: a negative response observed at low light intensity changes into a positive one at high light intensity, in particular for leaf width (fig. 3.11). This implies that the effect of temperature on leaf width is small at intermediate light intensities (fig. 3.12). It further appeared that light intensity effects on leaf width are especially manifest at high temperature, whereas for leaf length they are more pronounced at low temperature (fig. 3.11).<p/>Growth-time relationships appear to be quite different for leaf length and leaf width. Leaves elongate fast at low light intensity, but growth is maintained for a longer period at high light intensity. As a consequence hereof light intensity effects on the final length of the leaves remain restricted. In contrast to this, the effects on leaf width are much more pronounced, since both growth rate and growth duration are greatly reduced at low light intensity (fig. 3.16).<p/>From the linear, but greatly different length-width relationships, measured during leaf expansion at different light intensities (fig. 3.21), it may be concluded that an important factor in determining the ultimate shape of the leaves is the moment at which leaf blade expansion is initiated during primordial leaf development.<p/>In Chapter 4, the foregoing results are examined on the base of differences in number and size of epidermal cells in the midrib and the leaf blade. Both for the midrib and the leaf blade there is a positive relation between light intensity and cell number. Differences in cell number largely determine differences in leaf width, whereas in the midrib differences in cell length are much more important (figs. 4.3 and 4.7). In general, average cell length in the midrib decreases with increasing light intensity. This may explain the reduction of the midrib observed at high light intensities. Cell division appears restricted mainly to the early stage of growth.<p/>In chapter 5, results of some additional experiments are presented, concerning defoliation, extra CO <sub><font size="-1">2</font></sub> , and gibberellin application. Defoliation causes a temporary reduction of leaf width of subsequent leaves (fig. 5.1) which can also be brought about by a temporary reduction in light intensity (fig. 5.3). At a higher CO <sub><font size="-1">2</font></sub> concentration larger leaves are produced than in normal air. In all these cases, differences in leaf width are closely related to changes in cell number. Gibberellin, in particular, induces a strong elongation of the midrib which eventually may go at the expense of leaf blade development.<p/>It has been suggested that differences in leaf growth may be understood on the base of a balance between energetic and non-energetic processes. Both may operate as a limitation for further growth. Energetic processes (i.e. photosynthesis) seem to control to a large extent cell division, consequently expansion of the leaf blade, whereas non-energetic processes (e.g. hormon activity) seem important in particular for cell extension, and therefore play a major rôle in midrib elongation. In this respect it is tempting to assume that at high intensity a relatively higher hormonal activity is required to keep the cells in optimal condition for extension.