Leaf Structure

Needles persist on Picea abies shoots for 5-7 years. The shape of the needle in cross-section is diagnostic for the genus Picea (NESTEROVICH etal. 1986). The shape of needle in cross-section varies along the length of the needle. It is rhomboid in the middle, somewhat flattened near the base, and almost square near the needle apex. The needle is single veined and has a distinctly differentiated endodermis. A thick-walled epidermis with a heavy cuticle and stomata covers the needle (Fig. 6.9). Stomata occur in longitudinal rows on all four sides of the needle in three to four rows on the upper side and one or two on the lower side. The stomata of P. abies are deeply sunken and appear as though suspended from the subsidiary cells, which overarch them (Fig. 6.9). There are 40-50 stomata per mm2 of needle surface (COLLEAU 1968, CIZKOVA1988).

Sclerified hypodermis occurs beneath the epidermis except under the rows of stomata. Mesophyll lies between hypodermis and endodermis and occupies 92% of the cross-sectional area. The vascular cylinder occupies 6.7% of the needle cross-section. In spruce the vascular cylinder is four times smaller in diameter than in pine (CIZKOVA 1988), and is much larger in trees from higher than lower elevations (DRAXLER and RUPPERT1989). Two collateral bundles are located at the center of the vascular cylinder. The xylem is on the adaxial side (toward the shoot axis), and phloem on the abaxial side. Vascular bundles are comprised of proto- and meta-phloem cells and proto- and metaxylem elements. Protophloem cells are crushed very early in needle ontogeny.

Figure 6.9. Spruce needle in cross section (photo A. Hejnowicz)

A - young needle; early May; B - two-year-old needle; C - bud scale; early May; D - marginal part of the needle: ep - epicuticular waxes, s - stomata, e - endodermis, h - hypodermis, m - mesophyll; E - central part of a two-year-old needle: e - endodermis, f - fibers, x - xylem, ph - phloem, m - mesophyll, tt - transfusion tissue; F - marginal part of the needle: rd - resin duct

Epidermal cells are flat (Fig. 6.9D, F) and about 7┬┐am thick. The outer tangential wall is thicker than the inner wall. It is covered with a thick layer of cuticle and epicuticular waxes (Fig. 6.9). The thickness of an epidermal cell is about 9 am. There are five phases of epidermal cell wall differentiation: protodermal; steady; unsteady; lignification; and secondary modification (e.g. accumulation of polyphenols). The last phase occurs in the second to fifth year of needle growth (TENBERGE 1992). Hypodermal cells are 0.021 mm X 0.018 mm in cross-section. During needle maturation their walls became thicker and more lignified.

Mesophyll, the photosynthetic parenchyma, consists of cells with vertical ridges protruding into the cell lumen. The ridges develop as invaginations of the primary cell wall. Resin ducts are sparse and lie mainly under the hypoderm as elongated cysts, 2 mm long. They are lined with 5-7 epithelial cells and surrounded by a sheath of 8-10 thick-walled cells (Fig. 6.9F). The endodermis is a single-cell thick layer of parenchyma lacking chloroplasts (Fig. 6.9E). The number of cells comprising the endodermis in Picea species is relatively constant and is regarded as a diagnostic feature within the genus (Marco 1939).

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