Secondary growth

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 Diagram Stem And Root

Figure 6.7 Diagram of stem and root showing areas of differentiation.

Secondary growth results in the thickening of stems and roots and, in many cases, the production of wood.

Cambium Plum

Lenticel Xylem

Annual ring

Cork

Cork cambium

Medullary ray

Phloem

Pith

Figure 6.8 Cross-section of lime (Tilia europea) stem showing tissues produced in secondary growth

Lenticel Xylem

Annual ring

Cork

Cork cambium

Medullary ray

Phloem

Pith

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, is 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.

If bark 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 (or 'girdled'), 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, Acer davidii and many pines and rhododendrons have attractive bark and are particularly valued for winter interest (see Figure 6.9).

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 (see p92). The success of a graft depends very much on the rapid callus growth derived from the cambium, from

Betula Nigra Lenticels

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 (Salixalba) (j) Prunus serula tibetica (k) Date Plum (Diospyros lotus) (l) Black Walnut (Juglans nigra)

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 (Salixalba) (j) Prunus serula tibetica (k) Date Plum (Diospyros lotus) (l) Black Walnut (Juglans nigra)

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 rays, 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.

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Responses

  • Nibs
    Is bark a growth tissue of the stem?
    8 years ago

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