Vascular Architecture in Plants
Above: primary plant stem vascular tissue in cross-section. Primary stems are generally green and photosynthetic and
have no or little secondary growth and so are non-woody and occur in young shoots and in herbaceous plants. This
diagram illustrates the structure of the vascular cylinder or
stele (surrounding stem tissues are not shown). White
indicates
phloem, xylem is shown in black and central parenchymatous pith is dotted. Protosteles occur in many
fossil plants, in psilopsids and club mosses and in some roots. In these steles there is a central solid cylinder of xylem
ensheathed by phloem. For example, the psilopsid
Psilotum nudum has an actinostele (in which the xylem cylinder is
ridged) and the club moss
Lycopodium clavatum has a plectostele, in which plates of phloem infiltrate the xylem, which
is nevertheless still continuous.

Xylem has to be rigid so that it can draw up sap under suction pressure without collapsing. This makes xylem good for
plant support and in woody plants it is the main supporting or skeletal tissue. To provide greater support without
manufacturing more expensive xylem, the plant stem can contain a hollow cylinder of xylem. Just as in the hollow long
bones of mammals, it is possible to have a bone which contains less bony material but which is nevertheless stronger,
as in having greater flexural stiffness - a wide cylinder is much harder to bend than a narrow cylinder, though we have
to maintain a minimum thickness of the walls of the hollow cylinder to prevent buckling. This gives rise to a
siphonostele, in which the vascular tissue forms a hollow cylinder or siphon, although the central cavity may be filled
with parenchymatous pith. In the ectophloic siphonostele in which the phloem remains on the outside surface of the
xylem or an amphiphloic siphonostele (also called a
solenostele) in which phloem coats both the inner and outer
surface of the xylem cylinder. Siphonosteles occur in ferns, and in some gymnosperms and flowering plants.

The vascular cylinder is not generally a complete cylinder, because at the leaf-bearing nodes of the stem xylem and
phloem vessels must arc away from the stele to enter the leaf as a
leaf trace. If the leaf is a small scale-like leaf or
microphyll, the vascular cylinder may remain more-or-less intact, otherwise, if a larger trace enters a large leaf or
macrophyll a gap may result in the xylem cylinder, called a leaf gap. This gap only extends so far up the cylinder before
the cylinder closses over at the internode above to become a complete circle again in cross-scetion.
leaf traces and leaf gaps
circular vessels in a tree
stele types
Vascularisation is essential for any large land plant. Water, carrying mineral nutrients from the soil need to travel from
the roots to other parts of the plant in the xylem vessels, whilst sugary sap must, for example, translocate from
photosynthesising leaves and storage organs to growing parts and ripening fruit in phloem vessels. Plants are often
divided into vascular and non-vascular, which roughly parallels evolution with large terrestrial forms appearing later in
evolution as vessels evolved sufficiently to supply their needs. However, this division is a simplification. Seaweeds
might not require xylem, since they can absorb water across their entire body surface when submerged, but they do
have a well-developed network of phloem vessels, called trumpet hyphae, interconnecting the various parts of the
seaweed body. Mosses are tiny and although terrestrial, many require damp conditions and so it is not surprising that
they do not require advanced vascular systems. However, even small mosses may have specialised conducting tissue,
functioning as a prototype vascular system.
In the dictyostelium we essentially have a modified siphonostele in which the stem has very short internodes, so that
leaf gaps of the nodes above and below overlap to completely and in cross-section the stele appears to be broken up
into a series of strands. This type of stele occurs in some ferns.

In the
eustele (literally 'true stele') the vascular tissue is broken up into discrete longitudinal strands or bundles. This
type of stele occurs in the internodes of the
horsetail Equisetum and in some gymnosperms and flowering plants. It is
the 'typical dicotyledonous' primary stem illustrated in text books, e.g.
Helianthus.

Finally, in the
atactostele we have the discrete bundles of vascular tissue scattered throughout the stem. This type
of stem is characteristic of monocotyledonous flowering plants.

Secondary Growth

After attaining their initial width and length, the parts of many plants undergo secondary growth, increasing in
thickness and strength. New secondary xylem and phloem will be produced to supply the growing plant. The covering
of the epidermis may become replaced with
periderm bearing 'breathing pores' or lenticels. In a eustele, a layer of
undifferentiated cells between the phloem and xylem of each vascular bundle (fascicle) my form a growth zone or
meristem where new cells are produced. This is called the fascicular cambium and is continuous with a layer of
similar cells inbetween the vascular bundles, the
interfascicular cambium. Thus there is a complete cylinder of
meristematic tissue only one or a few cell layers thick.As these cells divide by mitosis, they both replace themselves to
maintain the meristem and produce cells which differentiate into new xylem vessels on the inside of the stele and new
phloem on the outside.

This type of secondary growth is extensive in woody plants and indefinite in many trees, with the secondary xylem
forming the wood and the secondary phloem the innermost layer of the bark (or just beneath the bark proper).
Herbaceous plants may also undergo some secondary growth, depending on species. In  
Pelargonium (Geraniacea)
the vascular bundles are so close together that secondary growth readily produces an entire fused cylinder of
vascular tissue. In
Helianthus (sunflower, Asteraceae) a similar continuous cylinder may form at the base of the stem,
but higher up there is no secondary growth, but the interfascicular parenchyma (parenchyma inbetween the vascular
bundles) forms sclerenchyma to toughen the stem and give it extra strength. A periderm may not form in
Helianthus,
but the epidermis continues to produce new cells and expand.

As has been discussed elsewhere (see
wood) trees often grow by adding annual rings of wood such that the stem
increases in girth. If we picture a tree stem as consisting of a cone of wood then essentially a new cone is added over
the top of this each year or growing season.

     Review the detailed structure and function of wood and
xylem vessels.

In this article we will look at some additional features of xylem architecture in trees. A single xylem vessel does not
generally extend the whole height of the tree but may be a meter or so in length and communicates with neighbouring
vessels so that the sap can flow the whole height of the tree from vessel to vessel. The xylem sap is drawn up the tree
from the roots by a suction pressure generated by water loss, chiefly through stomata in the leaves of the tree
canopy. This loss of water to the atmosphere is
transpiration and the stream of water flowing through the xylem,
carrying valuable mineral salts from the soil, is called the
transpiration stream. This does mean, however, that the
lower branches are nearer the source of the flowing sap and may tap more than their fair share, leaving insufficient
sap for the upper canopy which potentially needs it more. To combat this trees have a special architectural feature to
slow the movement of xylem sap into the lower branches -
concentric circular vessels.
Above: left, the complete vascular cylinder (siphonostele) of an internode. Middle, a small
vascular trace arcs away from the stele to enter a microphyll, leaving a notch in the vascular
cylinder which extends part-way up. Right, a leaf trace entering a macrophyll results in a gap,
or elongated slit, in the vascular cylinder, called a leaf gap.
Above: the grain in a tree with its bark removed. Note the concentric circular or elliptical
vessels at the bases of the lower branches.
An important part of a tree's insurance policy is the production of dormant buds. Most of these buds will never open
or develop, but should the canopy become damaged some of them may become active and replace the damaged
canopy. Each dormant bud has its own vascular supply which must elongate if the bud is to remain at the surface of
the stem as an
epicormic bud. As new layers of wood are added to the trunk, the vascular traces to the buds
elongate to maintain the buds at the surface (some buds may fail to keep up and become buried in the wood).
Above: a longitudinal section of a tree trunk showing the vascular supply to the dormant
epicormic buds. (Based on Busgen and Munch, 1929, in Thomas,2000; trees: their natural
history, Cambridge University Press).
Epicormic buds originally form as normal buds in the axils of leaves on young sheets but which remain dormant. In
some trees the majority of such buds remain dormant and some may abort. Trees can also form new buds de nova
when they are damaged, from any parenchyma tissue (adventitious buds).  Branches also have vascular traces
which can be traced to the centre of the trunk as a narrowing cone (a 'spike knot'). As new wood is added the
growth of the branch keeps pace and the knot consists entirely of wood firmly anchored to the surrounding wood of
the trunk. However, if the branch dies then wood added to the trunk will simply grow over it, encasing the dead
branch complete with its bark. The bark around the dead wood does not integrate well and such encased knots
easily fall out of a plank of cut wood.

Transport in plants


Article last updated: 27/2/15