The spruce root tip has a single group of initials arranged in a transverse tier (Fig. 6.8). Consequently, the vascular cylinder, cortex, and root cap have common initials (the open type of apical organization). The root cap is a structure protecting the apex and assisting the growing root in soil penetration. It is comprised of vertical cell files building the columella with transversally dividing cells. The columella produces derivatives for lateral portions of the root cap. While new cells are produced, the cells on periphery of the root cap are sloughed off.
Ai - apical meristem center; D - dermatocalyptrogen; C - columella; Pe - periblem (the future root cortex); Pl - plerome (the future central vascular cylinder)
The root of Norway spruce is diarchic, possessing two primary vascular strands comprised of proto- and metaxylem, which alternate with the two proto- and metaphloem strands. The vascular cylinder occupies the center of the root. The external layers of vascular cylinder are called the pericycle. The innermost layer of the cortex differentiates into the endodermis. Casparian strips are present in the radial and transverse anticlinal walls of the endodermal cells (JORUS1987). These band-like structures develop within the primary walls and contain suberin and lignins. The casparian strips establish the barrier to apoplastic movement of solutes into the vascular cylinder. Endodermal cells differentiate a short distance from the apical center. Metakutis, a protective layer of suberized cells typical for other woody plants that covers the apical meristem is absent in spruce roots (PLAUT1910).
The secondary structure of spruce roots differs from the stem. The periderm is constructed of only two or three layers of cells with thick, lignified walls. The arrangement of the cells in the secondary phloem is not as regular as in the stem. Sclereids are absent, but gelatinous fibers, which are absent in stem, are present in roots (CUTLER et al. 1987). In woody roots the growth rings are narrower and the transition from early to late wood is less distinct than in the stem. Axial and ray resin ducts are similar to those of stem wood.
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