1. The Vascular Cambium
Definitions Cell division related to cambial activity
Axial: Along the axis of the organ, or organism
Radial: At right angles to the axis, i.e., along a radius
Tangential: At right angles to a radius.
Ray Initial: Meristematic cambial cell. Forms a file of cells (one or more wide) that is composed of parenchyma. Orientated ALONG a RADIUS.
Contributes to the RADIAL transport system
Fusiform Initial: Meristematic cambial cell. Forms new secondary xylem and secondary phloem and associated cells.
Contributes to the AXIAL transport system.
The vascular cambium occurs in all vascular plants, but is not prominent in monocotyledons, where secondary growth does not involve the production of secondary vascular tissue. It is described as a lateral meristem, which produces new secondary vascular tissues, through the division of unique meristematic cells called fusiform, and ray initials. FUSIFORM initials add to the axial component (the tubular system; xylem elements and phloem elements, which are responsible for the transport and movement of materials through the water conducting xylem and the assimilate conducting phloem); whilst the RAY initials contribute to the radial growth of roots stems, and branches. Ray cells form tracheids and parenchymatous elements. These function in radial as opposed to axial transport.
The cambium is not equally active in all plants. Indeed, some plants produce very little secondary vascular tissue, others produce secondary vascular tissues throughout the life of the plant.
This series of PowerPoint slides forms the basis of an overview lecture, but can be expanded to includ additional material, as appropriate. We have used it in both formats, the shorter overview presentation is provided here.
Notes

2. THE VASCULAR CAMBIUM
The vascular cambium is unlike the primary meristems (root and shoot apex) of the plant, in that it produces new cells and tissues which add to the axial system (i.e. the conducting system) as well as to the radial system (i.e. the lateral transport pathway). In contrast, apical meristems of the shoot and root add only to the axial system. The cells of the vascular cambium do not fit the regular concept of meristematic cells (i.e. small, isodiametric shaped cells, with a dense cytoplasm and containing large nuclei). Cambial cells are usually highly vacuolate and occur in two forms, namely fusiform cells and ray cells. Fusiform cells are prism-shaped with a distinct wedge-shape at both ends. Ray cells are short and squat. Tangentially, both cell types may be wider than they appear in radial section or longitudinal view. The slides that follow will assist to orientate you with respect to the planes of cell division within the cambium.
The two cell types (fusiform and ray cells) have unique functions. Fusiform cells usually only produce cells associated with the axial system -- that is, they produce either new elements of the xylem, or elements of the phloem. Fusiform cells thus add new cells to the AXIAL conducting system. Ray cells on the other hand, produce ONLY ray cells and thus add to the RADIAL system of the plant

3. AXIAL TANGENTIAL RADIAL
Axial: Longitudinal translocation, xylem & phloem elements.
TANGENTIAL
RADIAL
Radial: Lateral translocation. Carbohydrate from phloem, to parenchymatic (living) tissue, water from xylem to living tissues as well.

4. Fusiform vs. ray initials
radius
Fusiform and ray cells form FILES of cells – each file contains a number of differentiating elements. Both can divide radially OR tangentially
tangent
endarch
TANGENTIAL face
exarch
TANGENTIAL face
exarch
TANGENTIAL face
RADIAL face
RADIAL face
plane of cell division
normal plane of cell division

5. Development of secondary vascular tissues in stems
During primary growth, the vascular bundles produce PRIMARY vascular tissue.
These are the primary phloem (proto + meta) and primary xylem (proto and meta).
The fascicular cambium separates the two tissues.
Remember: a fascicle is a vascular bundle

6. Development commences within the fascicular cambium (between the primary phloem and primary xylem)1.

7. 2. First activity is in the vascular bundle Secondary xylem
FCZ = fascicular cambial zone
Secondary xylem
Secondary phloem FC
Secondary xylem and phloem are produced by the fascicular cambium (FC).

8. 3
The interfascicular regions begins to differentiate and a cambium originates here.

9. The interfascicular cambial area in herbaceous stems is not usually active, thus does not produce new phloem or xylem tissues 1P 2P CZ 2X MX PX
PPF 3b 3a
A widening band of secondary vascular tissue results from the cambium’s activity. CZ 4

10. The ring of secondary tissue is CZ
The ring of secondary tissue is
Complete. The interfascicular and fascicular cambia together form a vascular cambium

11. Cambial activity in the root
Phloem
Protoxylem
Metaxylem
Typical Dicot Root, end of primary growth
(cambial activity in Gymnospermous roots is very similar).
endodermis
pericycle

12. Formation of the cambium. Stage 1.
Typical Dicot Root
endodermis
pericycle
Formation of the cambium. Stage 1.
cambium appears between the metaxylem and the metaphloem

13. Formation of the cambium. Stage 2.
Typical Dicot Root
endodermis
pericycle
Formation of the cambium. Stage 2.

14. filling out 2X 2P
Cambial cells differentiate more rapidly at the interface between the metaxylem and the metaphloem.
secondary phloem is produced centrifugally (outwards) as is secondary xylem. In a centripetal (inward) direction. Outward pressure begins to be applied to the primary phloem strands. These strands are gradually forced outwards, to make way for the newly-added secondary vascular tissues.

15. 2. Primary phloem strands become crushed, loose functionality 2X
filling out
E = endodermis
P = pericycle
2X = secondary xylem
2P = secondary phloem
P – pericycle
3. Primary phloem strands have lost their functionality E P 2X 2P
1. Outward pressure caused by the addition of new cells continues – effectively ‘rounding out’ the ring of secondary vascular tissue

16. Remnant of primary phloem strands – (located by orange arrows) become completely crushed, and are non-functional E 2P 2X P
E = endodermis
P = pericycle
2X = secondary xylem
2P = secondary phloem
P – pericycle
The root will retain its primary xylem, which will be visible and may be functional