Studies on the Leaf Formation in Rice Plants. : I. Observation on the successive development of the leaf.

Abstract

In rice plants, when a leaf begins to emerge from the adjacent lower leaf sheath, the shoot always contains four immature leaves (including leaf primordia) at different developmental stages, irrespective of the change in environmental conditions or of the difference in growth stages of the plant. These immature leaves or leaf primordia show successive stages of leaf development and are termed from the youngest as P1, P2, P3, P4 respectively (Fig. 2). Thus the emerging leaf corresponds to P5, and the lower one just adjacent to it, to P6. The morphological and anatomical characters of these were examined with special reference to the distribution of meristems, the differentiation of vascular system and the differentiation as well as the maturation of epidermal tissues. Several kinds of meristem (Fig. 1) are involved in the formation of a leaf. First of all, by the activity of the apical meristem, a minute protuberance (P1) appears at the foot of the shoot apex. While it grows sidewards by the activity of the marginal meristem as well as upwards by the activity of the apical one, it encircles and covers the shoot apex like a hood (P2 in Fig. 4A). P3 is large enough to envelope all inner portions near the shoot apex (Fig. 4B). The thickness of midrib at this and later stages is attained by the ventral meristem (Fig. 2:V). P4 shows a typical leaf form with leaf blade and leaf sheath demarcated by the newly formed ligule (Fig. 4C). There is conspicuous expansion of leaf blade at this stage by the activity of the ground meristem, and the form of the immature leaf at this stage (Fig. 7) resembles that of the mature leaves of other monocotyledonous plants or some of the dicotyledonous ones. From the stage P5, the activity of almost all meristems becomes more or less indistinct, and instead of these meristems the intercalary meristem situated in the basal portion of the leaf plays an important role in the growth of the leaf. The distribution of this meristem is determined by stomatal differentiation. By the activity of this meristem a conspicuous elongation occurs in the leaf blade (P4-P5), and then, one stage later, in the leaf sheath (P5-P6). At the stage P6 both leaf blade and leaf sheath attain their full length and the final form of the leaf is achieved. Meanwhile, large vascular bundles differentiate at the stage P2, and small one, at the stage P3 (Fig. 5). They appear first at the base of midrib and later towards the margin of the leaf. The differentiation of each vascular bundle advances acropetally into the leaf. The connecting strands of these vascular bundles differentiate at the tip of P4 and it advances basipetally. The differentiation of epidermal tissues occurs at the tip of P3 (in the case of hairs) and P4 (in the case of stomata) respectively, and advances basipetally in accordance with the basipetal extinction of the activity of the intercalary meristem.