11.2: Secondary Growth

In secondary growth, primary tissues and residual meristematic tissues produce secondary meristems, which then produce secondary tissues. Whereas primary tissues allow for vertical growth, secondary tissues allow for lateral growth: they allow stems and roots to become wider by producing wood. In addition to growing wider, secondary growth exchanges the living epidermis for a thick layer of dead, waterproofed cells called cork. The cork and a few other layers of tissue comprise something called the periderm, or perhaps more familiarly called bark.

This type of growth first evolved in gymnosperms. It is also present in many angiosperms. As a general rule, monocots do not undergo secondary growth (though some, like bamboo, have an analogous process).

Gymnosperms

Figure \(\PageIndex{1}\): A cross section of a pine root in the early stages of primary growth. The first layer of periderm is forming, causing the epidermis to slough off. The periderm is currently the outermost layer, compose of a few layers of suberized cork cells, the cork cambium, and large cells called the phelloderm. The cortex is just inside the phelloderm and is getting smaller as the root grows. Ranks of cells just inside the cortex are secondary phloem cells (stained a blue-green color) which are being produced by the vascular cambium (a ring of cells stained light blue between the xylem and phloem). Just within the vascular cambium, making up the center of the root, is the secondary xylem (stained red, due to the secondary wall). There are large holes in the secondary xylem where resin canals are traveling through. Strings of parenchyma cells traverse laterally through the vascular tissue. These are called xylem rays and phloem rays, depending on their location within the vascular tissue. Image from Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons.

Angiosperms

Figure \(\PageIndex{2}\): Older Quercus root, 40x. "In young roots, the vascular cylinder is surrounded by two rings of cells, the pericycle and endodermis, and above these layers the cortex and epidermis. In older roots underling activity of the cork cambium replaces the epidermal and cortical tissues with a protective zone of cork rich periderm. The outermost layer of periderm consists of layers of cork cells, the phellem, which produce the waterproofing substance suberin. Cork cells are dead at maturity. Deep to the phellem is a layer of living cork cambium or phellogen and just beneath that layers of cork parenchyma or phelloderm. Many cells in the periderm contain dark staining tannins. The vascular cylinder consists of an outer narrow ring of phloem,and deep to this, the vascular cambium. The vascular cambium remains active, producing annual growth of secondary phloem towards the outside of the root and secondary xylem towards center of the root. Because of greater production of xylem, the bulk of the vascular cylinder is dominated by radially arranged rays of secondary xylem interspaced with medullary rays of parenchyma cells. Annual growth rings of spring and summer wood is difficult to distinguish in roots. Both the pericycle and endodermis, which wrap vascular cylinder in younger roots, are lost due to seasonal growth of the vascular cylinder. The center of the root is made up of primary xylem." Caption text and image from Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons.

Figure \(\PageIndex{3}\): A labeled cross section through an older Quercus root, 100x. A=Periderms, B=Secondary Phloem, C=Vascular Cambium, D=Secondary Xylem, E=Primary xylem. Image from Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons. Labels added by Maria Morrow.

Figure \(\PageIndex{4}\): A labeled cross section through the outer tissues of an older Quercus root, 400x. A=Cork cells, B=Cork cambium, C=Phelloderm (A, B, and C=Periderm), D=Secondary phloem fibers, E=Secondary phloem, F=Vascular cambium, G=Secondary xylem. Image from Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons. Labels added by Maria Morrow.

Figure \(\PageIndex{5}\): A cross section through the secondary xylem of an older Quercus root, 400x. The rows of darker cells (indicated by arrows) are parenchyma cells that traverse the secondary xylem and phloem. In the xylem, they are called xylem rays. In the phloem, they are called phloem rays. Image from Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons. Labels added by Maria Morrow.