||<p/>The growth and development of sun and shade leaves was investigated in <em>Populus euramericana</em> 'Robusta', growing under various light intensities and temperatures and on a substrate optimal for the roots.<p/>Strong correlations exist between the dimensions of the various plant parts. The width of an individual growing leaf is always 2 cm smaller than its length. Leaf length and petiole length are strongly correlated. Mature leaf length is correlated with the diameter of the subtending internode, as measured at the earliest possible stage. The mature length of the internodes is similar in all treatments (Chapter 3).<p/>The mean mature size of the epidermal cells in the leaf lamina is practically constant, irrespective of treatment or of level of leaf insertion. The individual cells too tend to grow to a constant final size. The basipetal trend in the expansion of the leaf is the consequence of the shape of the leaf: where the leaf is broadest, the expansion lasts longest and the cells mature last (Chapter 4).<p/>In the linear phase of growth, the growth rate of the length of the individual leaf is correlated with its final size. The relative growth rate of leaf length depends only weakly on irradiation level but rather strongly on temperature. The average relative growth rate of total leaf length, of total leaf width and of total leaf area per plant is fairly independent of irradiation level and leaf length. The differences in growth rate have to be ascribed to differences in the number of cells involved in leaf growth. This number of cells is determined in phases preceding the linear phase of growth (Chapter 5.1.1).<p/>The difference in length of successive, growing leaves is more or less constant, irrespective of treatment or of the age of the plant and these lengths form a descending arithmetical progression with a common difference of about 2 cm. This is due to the fact that the linear growth rate of the leaves is proportional to the initation rate of the leaves (Chapter 5.2).<p/>The rather rigid relation between leaf length, leaf width and initiation rate allows to relate total growing leaf area and the growth rate of total leaf area per plant to the cube of the length of the youngest mature leaf (Chapter 5.3).<p/>The logarithms of the lengths of the primordia along the growing axis are linearly related to their serial number. The slope of this relation is steeper in shade plants than in sun plants, when they are both in their stationary state of growth. This indicates that the primordia grow exponentially and that a high growth rate of the plant is accompanied by accumulation of primordia in the apex. It is shown that this is due to the leaf initiation rate, being higher in sun plants than in shade plants, together with a constant relative growth rate of the primordia. The higher growth rate is due to the higher number of cells in the primordia and this higher number of cells is determined during the phase of initiation: the larger the apical dome, the larger the number of cells initiating a primordium (Chapter 5.5 and appendix).<p/>In young plants mature leaf length increases steadily with leaf number until a stationary state is reached. Shade plants reach this state soon, plants from intermediate light intensities somewhat later and sun plants last. It appears that the rate of increase of total leaf area in the stationary state of growth is linearly related to the irradiation level. The causes of this fact are discussed (Chapter 5.6).<p/>The time interval between initiation and maturation of a leaf is constant at constant temperature, irrespective of leaf size. This has consequences for the relation between leaf initiation rate and leaf appearance rate. It is suggested to define the plastochron as the time interval between the appearance of successive leaves at equal developmental stages (Chapter 5.7).<p/>Homoblastic leaf growth is defined as growth, based on a steadily increasing size of the apical dome, combined with a selfdetermined growth of the leaf primordia, once initiated at the apex. In the internode this growth pattern is also recognized: the growth in height of the stem is proportional to the diameter of the apical dome. Because also leaf initiation is proportional to the size of the apical dome, the mature length of the internodes is more or less constant (Chapter 6).<p/>Leaf thickness may be the result of competition for substrates of the mesophyll and other tissues. The higher the irradiation level or the lower the temperature, the thicker the leaves are and the higher also the rate of light- and CO <sub><font size="-1">2</font></sub> -saturated photosynthesis per unit leaf area. The photosynthetic capacity of the mesophyll cells, composing the thickness of the leaf, is mutually equal (Chapter 7).<p/>The various observations are discussed and compared with the literature. The mathematical aspects of the observations are discussed in the Appendix.