Secondary Growth

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Introduction: It is assumed that the student has achieved a proper understanding of the primary structure of the dicot stem, and specifically on an understanding of the organization of the primary tissues in the two stems we have studied (Medicago, and Coleus). It may be a good idea to review both "Cells and Tissues of the Plant Body", and "The Shoot" before proceeding. Link to our raw teaching images for secondary growth.


I. Definitions
IIa. Cross section of Tilia stem at end of primary growth
IIb. Cross section of Tilia stem after the first year
IIc. Cross section of Tilia stem after three years growth
IId. Gross structure of woody stems
III. Secondary Growth in Roots

I. Some Important Definitions:

Primary tissues: Tissues generated from the growth of an apical meristem.

Cambium: A lateral meristem constituting a sheet of cells. Growth of these cells increases the girdth of the plant organ involved.

Secondary tissues: Tissues generated from the growth of a cambium.

Vascular Cambium: A cambium that gives rise to secondary xylem to the inside, and to secondary phloem to the outside.

Periderm: A structure that consists of a cork cambium (phellogen), with cork tissue (phellem) to the outside, and in some cases a layer of cells derived from and to the inside of the cork cambium called phelloderm. Functions to limit dehydration and block pathogens after the epidermis is disrupted by the onset of secondary growth: Link to view of a periderm of Tilia.

Cork: (phellem) you need know only the term "cork": Tissue dead at maturity generated from a cork cambium. The cell walls of the tissue are impregnated with suberin. This water-proofs the tissue. The cork used to seal wine bottles is "cork" tissue harvested from a species of oak.The cell theory was first proposed by Robert Hooke in 1665 after microscopic exaination a slice of cork.

Cork Cambium: A cambial layer that functions to produce cork, and in some cases, phelloderm. In roots is derived initially from pericyle. In stems from the cortex. Unlike the vascuar cambium these cambial layers do not persist for the duration of the life of the plant organ. Over time one cork cambium will be supplanted by another generated from parenchyma cells further inside: Link to an illustration.

Phelloderm: In some periderms a layer of living secondary tissue is generated by the cork cambium to the inside. We will not consider thie phelloderm in the following exercise.

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II. Secondary Growth in Stems

Link to entire Woody Stem Teaching Collection

IIa. Cross Section of Tilia (basswood) Stem at the End of Primary Growth

This stem differs somewhat from that of Medicago or Coleus. The obvious difference is in the organization of the vascular tissue. The pith rays are only one cell layer wide and the primary vascular tissue appears as a continuous ring. As in the stems studied earlier, the ground tissue inside the vascular tissue is called the pith and that outside the cortex. Dermal tissue consists of an epidermis.

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IIb. Cross Sections of Tilia (basswood) Stem: 1, 2 and 3 Years Old:

Section at the end of the first year: By the end of the first year, the primary structure of the stem has been transformed by the growth of the vascular and cork cambiums. The pith in the midde is intact as is the primary xylem. The secondary xylem is continuous with the primary xylem and extends out to the vascular cambium. The boundaries of the secondary xylem can be determined by where the rays begin in the cylinder of xylem as rays are a characteristic of secondary vascular tissue (link to illustration). Beyond the vascular cambium is secondary phloem followed by primary phloem. The obvious fibers visible are in the primary phloem and have differentiated since the end of primary growth. Beyond the phloem is cortex bounded by a periderm. The cork cambium is the last living tissue layer in the stem. Note the epidermis being sloughed off.

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Section at the end of three years growth: The obvious changes visible here are the growth rings present in the secondary xylem, and the growth of certain rays in the phloem forming wedge-shaped regions in that tissue.

The sequence of tissues outlined before are the same from the center outward: pith, primary xylem, secondary xylem, vascular cambium, secondary phloem, primary phloem, cortex, and periderm.

Using higher magnification it can be seen that the growth increments are areas where smaller thick-walled vessel members border larger thin-walled vessel members. The smaller cells make up late summer's growth and the larger cells early spring growth. By observing this boundary you should be able to tell in which direction is the pith - think about it.

The rays in the xylem are continuous with those in the phloem. The enlargement of some of the phloem rays relieves the tension on the phloem created by the expanding cylinder of xylem. This stress tends to create longitudinal rips in the phloem which would destroy its integrity. The expansion of these rays (they are called dialated rays) prevents these tears. The phloem outside of this ray tissue consists of bands of fibers alternating with areas containing sieve-tube members and companion cells.

Tangential (face) view of vascular cambium: This is a view of a longitudinal section made just inside the secondary phloem perpendicular to the rays. It provides us with a face-view of the sheet of vascular cambium. In it we can clearly see the two types of cells that make of the tissue: ray initials and fusiform initials. The ray initials give rise to the rays in both the phloem and xylem. The fusiform initials have their long axes arranged vertically. These cells give rise to tracheary elements in the xylem as well as to sieve-tube members and companion cells in the phloem.

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IIc. Gross structure of woody stems:

Woody stems are mostly seconday xylem (wood) surrounded by bark. The xylem may include heart-wood and sap-wood. Heart-wood is dead and non-functional. The sap wood is functional and has living parenchyma cells. The boundary between the bark and wood is the vascular cambium. The bark is divided into two regions by the cork cambium: the living area inside the cork cambium is the inner bark, and the dead tissue outside is the outer bark. Evidence of earlier cork cambiums can be easily discerned in some woody stems.


III. Secondary Growth in Roots:

Cross Sections of a Woody Root: Secondary growth in the root transforms the primary structure of the organ through the formation of two cambial layers: the vascular cambium and the cork cambium. The vascular cambium arises from a combination of the procambium and pericycle cells. The cork cambium is formed entirely from pericycle cells. With the onset of secondary growth the entire cortex is sloughed off.

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Cross section of a carrot root

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