As the stem length increases, so width also increases to support the bigger
plant and supply the greater amount of water and minerals required.
The process in dicotyledons is called secondary growth
(see Figure 6.8).
Additional phloem and xylem are produced on either side of the
cambium tissue, which now forms a complete ring. As these tissues
increase towards the centre of the stem, so the circumference of the
stem must also increase. Therefore a secondary ring of cambium (cork
cambium) is formed, just to the inside of the epidermis, the cells of
which divide to produce a layer of corky cells on the outside of the stem.
This layer will increase with the growth of the tissue inside the stem,
and will prevent loss of water if cracks should occur. As more secondary
growth takes place, so more phloem and xylem tissue is produced but
the phloem tubes, being soft, are squashed as the more numerous and
very hard xylem vessels occupy more and more of the cross-section
of the stem. Eventually, the majority of the stem consists of secondary
xylem that forms the wood
|Figure 6.8 Cross-section of lime (Tilia europea) stem showing
tissues produced in secondary growth
The central region of xylem sometimes becomes darkly stained with
gums and resins (heartwood
) and performs the long-term function of
support for a heavy trunk or branch. The outer xylem, the sapwood
still functional in transporting water and nutrients, and is often lighter
in colour. The xylem tissue produced in the spring has larger diameter
vessels than autumn-produced xylem, due to the greater volume of water
that must be transported; a distinct ring is therefore produced where the
two types of tissue meet. As these rings will be formed each season,
their number can indicate the age of the branch or trunk; they are called annual rings
. The phloem tissue is pushed against the cork layers by the
increasing volume of xylem so that a woody stem appears to have two
distinct layers, the wood in the centre and the bark
on the outside.
is removed, the phloem also will be lost, leaving the vascular
cambium exposed. The stem’s food transport system from leaves to the
roots is thus removed and, if a trunk is completely ringed
the plant will die. Rabbits or deer in an orchard may cause this sort of
damage. 'Partial ringing', i.e. removing the bark from almost the whole
of the circumference, can achieve a deliberate reduction in growth
rate of vigorous tree fruit cultivars and woody ornamental species.
Initially, the bark
is smooth and shiny, but with age it thickens and the
outer layers accumulate chemicals (including suberin) that make it an
effective protection against water loss and pest attack. This part of the
bark (called cork
) starts to peel or flake off. This is replaced from below
and the cork gradually takes on its characteristic colours and textures.
Many trees such as silver birch, London Plane, Prunus serrula
and many pines and rhododendrons have attractive bark and are
particularly valued for winter interest (see Figure 6.9).
|Figure 6.9 Examples of the decorative effects of tree bark (a) Myrtus luma (b) Euonymus alatus (c) Eucalytus
parvifolia (d) Quercus agrifolia (e) Caucasian Wing-nut (Pterocarya fraxinifolia) (f) Eucalyptus urnigera (g) Pinus nigra ssp. Salzmannii (h) Betula utilis jacquemontii (i) White Willow (Salix alba) (j) Prunus serula tibetica (k) Date Plum (Diospyros lotus) (l) Black Walnut (Juglans nigra)
|Figure 6.10 Bark of
silver birch showing
Since the division of cells in the cambium produces secondary growth, it
is important that when grafting a scion
(the material to be grafted) to a stock
, the vascular cambium tissues of both components be positioned as
close to each other as possible. The success of a graft depends
very much on the rapid callus
growth derived from the cambium, from
which new cambial cells form and subsequently from which the new
xylem and phloem vessels form to complete the union. The two parts
then grow as one to carry out the functions of the plant stem.
A further feature of a woody stem is the mass of lines radiating outwards
from the centre, most obvious in the xylem tissues. These are medullary
, consisting of parenchyma tissue linking up with small areas on the
bark where the corky cells are less tightly packed together (lenticels
These allow air to move into the stem and across the stem from cell
to cell in the medullary rays. The oxygen in the air is needed for the
process of respiration
, but the openings can be a means of entry of
some diseases, e.g. Fireblight. Other external features of woody stems
include the leaf scars
which mark the point of attachment of leaves
fallen at the end of a growing season, and can be a point of entry of
fungal spores such as apple canker.
This has the same functions as those of a dicotyledon; therefore the cell
types and tissues are similar. However, the arrangement of the tissues
does differ because increase in diameter by secondary growth
not take place. The stem relies on extensive sclerenchyma tissue for
support that, in the maize stem shown in Figure 6.4, is found as a sheath
around each of the scattered vascular bundles. Monocotyledonous stem
structures are seen at their most complex in the palm family. From the
outside, the trunk would appear to be made of wood, but an internal
investigation shows that the stem is a mass of sclerified vascular
bundles. The absence of secondary growth in the vascular bundles
makes the presence of cambium tissue unnecessary.
Secondary thickening is found not only in trees and shrubs, but also
in many herbaceous perennials and annuals that have woody stems.
However, trees and shrubs do exhibit this feature to the greatest extent.
Tissues of the root
|Figure 6.11 Cross-section of Ranunculus root showing
thickened outer region, large area of cortex and central
vascular region or stele enclosed in an endodermis.
The layer with the root hairs, the epidermis
, is comparable with
the epidermis of the stem; it is a single layer of cells which has a
protective as well as an absorptive function. Inside the epidermis is
the parenchymatous cortex
layer. The main function of this tissue
is respiration to produce energy for growth of the root and for the
absorption of mineral nutrients. The cortex can also be used for the
storage of food where the root is an overwintering organ.
The cortex is often quite extensive and water must move across it in
order to reach the transporting tissue that is in the centre of the root. This
central region, called the stele
, is separated from the cortex by a single
layer of cells, the endodermis
, which has the function of controlling
the passage of water into the stele. A waxy strip forming part of the cell
wall of many of the endodermal cells (the Casparian strip) prevents
water from moving into the cell by all except the cells outside it, called passage cells
Water passes through the endodermis to the xylem
transports the water and dissolved minerals up to the stem and leaves.
The arrangement of the xylem tissue varies between species, but often
appears in transverse section as a star with varying numbers of
tissue is responsible for transporting carbohydrates from
the leaves as a food supply for the production of energy in the cortex.
A distinct area in the root inside the endodermis, the pericycle
cell division and produces lateral roots, which push through to the main root
surface from deep within the structure. Roots age and become thickened
with waxy substances, and the uptake rate of water becomes restricted.