Lecture 12

Vascular Cambium

I. Meristems - Overview

A.  Meristem definition: a region of undifferentiated cells that divide and form new cells that then expand, differentiate and undergo other internal changes.

B. In general (see exceptions below), meristematic cells have a large central nucleus with distinct nucleolus, usually dense cytoplasm (i.e. with many organelles) and numerous vacuoles, plasmodesmata, and a thin primary cell wall.

C. Types of meristems

1. Apical and axillary meristems. Examples: Syringa, labeled photo of Coleus, Pinus. The axillary meristem is just an iteration of the apical one, located in the axil of a leaf. Located at the shoot tip, cell divisions increase the length of the shoot. Directly produces only primary tissue. These include the the following primary meristems
a. protoderm (gives rise to epidermis) Coleus.
b. procambium (gives rise primary vascular tissue). Coleus.
c. ground meristem (gives rise to the ground tissue).
Animation of shoot apical meristem growth.

2. Lateral meristem (secondary meristem). These meristems produce secondary tissue. They increases the girth or diameter of an organ; the older the stem, the wider it is. Not present in all plants (woody not herbaceous plants). In general, roots and stems produce secondary tissue, although there are some odd examples of leaves producing secondary tissue.
Two kinds of lateral meristems:
a. Vascular cambium - secondary xylem (wood), secondary phloem (inner bark). Diagram. Located initially between the vascular bundles of the primary stem (interfascicular cambium); Examples: Helianthus, Solanum lycopersicum (tomato), Medicago stem X.S. Later, as the vegetative stem develops into a woody stem, the vascular cambium forms at interface between the xylem and phloem and within the "bark" of the woody plant. Animation of transition from primary to secondary growth.
b. Cork cambium (phellogen) - gives rise to the outer bark, which is periderm and replaces the epidermis. In this photo of 1 9 year old Gingko stem, the phellogen is the layer of blue cells just below the outermost layer of stacked pink staining cells (phellem). We will discuss this lateral meristem later.
3. Marginal meristem - the border or edge of a leaf has meristematic cells
4. Generalized meristem - during the embryonic stage in the life cycle of a plant, cell divisions are not confined to any particular region but take place all over the plant body. Example: Capsella embryo. Animation of embryo growth.
5. Intercalary meristem - in between mature tissues. Examples: hornwort sporophyte, grasses, basal meristem of cactus spine (Morawetzia) shown in photo1, photo2.
6.  Meristemoid.  Not really meristems proper. Present on the shoot epidermis of grasses producing silica cells, cork cells, and trichomes. Meristemoids divide only a few times and then terminate division when the structure is complete. In this example of Nephrolepis (Boston fern), a multicellular trichome is formed.

Note: Mauseth uses the term "intercalary meristem" for what we are calling a lateral meristem. He uses the term "lateral meristem" for what we are calling a marginal meristem.  Confusing? Yes!


II. Vascular Cambium

A. The vascular cambium produces
1. Internally 2˚ xylem - wood
a. a complex tissue that contains vessels, tracheids
b. wood fibers (libriform tracheids and fiber tracheids)
c. parenchyma (rays and axial parenchyma)
2. Externally 2˚ phloem - inner bark, a complex tissue
a. phloem, composed of companion cells and sieve tube members or albuminous cells and sieve cells
b. fibers
c. parenchyma - (rays and axial parenchyma)
3. An animation of secondary growth through the seasons.

B. Organization (Figure 10.1)
1. Meristematic cells of the vascular cambium somewhat different from those seen in the apical meristem (above). Two types of initials in the vascular cambium: fusiform and ray initials

2. Fusiform initials
a. Elongated to isodiametric in shape, fusiform = spindle shaped, really prismatic, wedge-shaped ends. How they look in section: fusiform shape in tangential section, rectangular in radial section, smaller rectangle in transverse (cross) section.

b. Derivatives of fusiform initials constitute the axial and radial systems. Produce the axial elements, i.e. those oriented longitudinally. The cambium cell arrangement determines the organization of the secondary vascular tissues.
1) In this transverse section of Pinus wood, the tracheids form uniform pattern of radial rows, each tracheid being the daughter cell of the same fusiform initial. 
2) Another good example of this is the storied wood of Wisteria and the non-storied wood of Rhus (Figure 10.2).

c. Cell division.
1) Cambial initials are usually bifacial, which means they produce cells off of two sides. They divide periclinally to produce xylem to the inside and phloem to the outside, sometimes alternating.  Periclinal divisions that add to the diameter (width) of the stem are called Additive (also called Proliferative) Divisions. They give radial files of the cells as seen in XS of the stem. Animation of vascular cambium cell divisions.
2) If cells are on only one side, the vascular cambium is called unifacial. Unifacial vascular cambia make either 2˚ xylem or 2˚ phloem, as was the case with early fossil land plants, pteridophytes, lycophytes, and in leaf veins.

d. Initials may divide again before their derivative cell has matured, so a zone of undifferentiated cells accumulates.  Sometimes hard to distinguish between the fusiform initial and its derivative, which is why some call the entire cambial zone the "cambium".  But conifers at least have a single layer that constitutes the initials (Figure 10.3).

3. Ray initials
a. ray initials produce ray cells - ray parenchyma
b. ray initials will be in groups
c. ray initials are rectangular in tangential, radial and cross sections
d.  Ray initials form at a constant rate from the fusiform initials. Cell division patterns are complex and vary between different plant taxa. Transverse divisions of the fusiform initial results in several cells, only some of which may survive and become ray initials. Example of tangential section of Juglans (walnut) and closer view HERE.
e.  Rays begin as a group of only 1-2 cells but they increase in height through later transverse divisions and by fusion with other rays.  To become multiseriate, radial anticlinal divisions and fusions occur.

C. Developmental Changes
1.  As the stem increases in width, the vascular cambium must adjust.  How it does this is complex, involving anticlinal (not periclinal, as above) cell divisions, intrusive growth, elimination of initials, and conversion of fusiform initial to ray initials.
2. Anticlinal divisions = multiplicative divisions. These increase the number of cells in the vascular cambium.  Example for Pinus (at arrow). Several types (Figure 10.5):
a. Radial anticlinal. Cell divided longitudinally, new wall connects to radial walls (ends)
b. Lateral anticlinal. Cell divided longitudinally, new wall connects to opposite ends of same lateral wall.
c.  Oblique anticlinal (= pseudotransverse). Cell divided longitudinally, new wall connects in an inclined fashion between two different lateral walls. Example: Juglans (Figure 10.6A).
3. Radial anticlinal divisions gives more vascular cambium cells that are arranged in stacks, gives rise to storied wood (considered an advanced condition).  The oblique divisions result in non-storied wood, no tiers of cells.
4.  After the anticlinal cell divisions, the derivatives enlarge tangentially, elongate via apical intrusive growth (remember this type of growth with fibers).  Intrusive growth also causes forking and invasion of rays (Figure 10.5 H-L, Figure 10.6 C) which may result in the ray splitting.

D.  Seasonal Changes
1.  Fusiform initials, and their derivatives, are shorter at the end of the growing season compared with the cells formed at the beginning.
2. In conifers, spiral grain is caused by periodic changes in the orientations of the walls set up during the anticlinal cell divisions - forms panels or domains of cells. Figure 12.12 from Evert.
3. Cytokinesis in fusiform initials.  See Figure 10.7 which shows the phragmoplast and cell plate in these cells that must divide longitudinally. Figure 10.6 B shows periclinal divisions in the cambium of Cryoptocarya.

E.  Example photos

Gymnosperms
Photo.  Pinus, transverse section, vascular cambium producing secondary phloem (to the right) and secondary xylem (to the left).
Photo. Pinus, radial section, showing vascular cambium producing secondary phloem (to the right) and secondary xylem (to the left).
Photo.  Pinus, paradermal tangential section, showing ray initials, oblique anticlinal divisions in fusiform initials

Angiosperms
Photo. Pyrus, paradermal tangential section, showing ray initials, fusiform initials producing non-storied wood
Photo. Robinia paradermal tangential section, showing ray initials, fusiform initials producing storied wood
Last updated: 15-Oct-22 / dln