The process of leaf development of chinese cabbage was observed morphologically and histogenetically to clarify the mechanism of head formation.
1) Seedlings of chinese cabbage (cultivar; Nozaki-Kohai No.3) were used in this study and were grown in a phytotron at temperatures of 23°C during the day and 18°C during the night.
For histological observations slides were prepared from different parts of the leaves collected at different growth stages of the seedlings by paraffin microtechnique.
2) The leaf shape changed with node positions. The ratio of length to width of the leaf decreased and the number of vascular strands in the midrib increased as node position number inrreased, counting from the basal point.
3) The leaf consists of two distinct parts; lamina and midrib. Midrib includes several parallel veins (with one or several vascular bundles) in the parenchyma massive on the adaxial side. Each vascular bundle is composed of cambium, xylem, phloem and some supporting tissues. Epidermis of midrib consists of one layer of cells.
4) Trichome, an outgrowth of an epidermal cell, is an unicellular hair situated over the veins. The initial cell of the trichome has dense cytoplasm, a larger nucleus and a larger nucleolus as compared with those of other epidermal cells.
5) The epidermis of lamina consists of one layer of cells. Palisade and spongy parenchymatous cells contain abundant chloroplasts.
Stomata are differentiated from an epidermal cell of lamina, and distributed at random. The type of stoma complex is anisocytic, having three subsidiary cells surrounding the stoma.
The lamina has fine-netted venation. Parenchymatous cells surrounding the thick vein run parallel with the axis. These parenchymatous cells appeared white in the absence of chlorophyll, similar to those in the midrib.
6) During the early stage of development the successive plastochrones in the shoot apex shortened from 1.3 to 0.5 days. In the leaf primordium, the procambium differentiates at the tip of the leaf buttress. With the development of the primordium, the procambium extends into the apical and marginal parts of the leaf before the initiation of vascular bundle differentiation.
The interfascicular cambium was observed up to the 11th or the 13th leaf primordium.
After the initiation of midrib-like tissue in the leaf primordium, the marginal meristem starts to differentiate leaf lamina, which continues to develop prominently until the attainment of full size.
7) The direction of leaf outgrowth changed with ages. At the lower nodes leaf primordia grew upright, whereas at the higher nodes they folded the shoot apex inward, forming “small head.”
8) A tiny projection composed of larger cells is present between two leaf primordia. It is about 0.5mm long and 1mm wide. Each cell has a large nucleus and nucleolus, having denser cytoplasm as compared with other meristematic cells.
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