Lesson: Periderm

Lesson Developer: Dr Anupama Shukla Department/College: Acharya Narender Dev College

Table of Contents

Chapter: Periderm

• Introduction

Cell structure

• • Development

Position of phellogen

• • Rhytidome

Morphology of bark

• • Lenticel

Protective tissue in monocotyledons

• Summary

Glossary

• Exercise
• References

Introduction

In most plants that have secondary growth, a layer of periderm develops, which is also known as cork. It is protective in function and replaces the epidermis of roots and stems, which is shed.

Bark

Bark

Figure: Bark of a pine tree

Source:http://en.wikipedia.org/wiki/Bark#mediaviewer/File:Pine_bark_tecpan_guatemala.J PG,http://www4.uwsp.edu/biology/courses/botlab/Lab06b.htm

Periderm is a secondary tissue and consists of three layers, the phellogen, phellem and phelloderm. The phellogen/cork cambium consists of a layer of meristematic cells which produce the phellem/cork towards the outer side and the phelloderm towards the inner side.

B

A

Figure: A) T.S. of a stem showing secondary growth. B) Diagrammatic sketch

Source:http://botit.botany.wisc.edu/Resources/Botany/Secondary%20Growth/Stem/Second ary%20Growth/1%20year/Phloem%20cortex%20periderm%20labelled.jpg.html, http://www.apsnet.org/edcenter/illglossary/Article%20Images/Forms/DispForm.aspx?ID=6 20

Variations in pattern of development of periderm

In some trees, periderm never develops and the epidermis is retained throughout life. In such plants, the cell wall of the epidermis thickens and cells divide radially and elongate tangentially, e.g. Viscum. In some other trees, the formation of periderm starts much later than the formation of secondary vascular tissue for example Citrus, Eucalyptus and Acer. In some other plants like Quercus suber and Aristolochia sp. a thick layer of cork develops. Periderm also develops at the site of wounding and is called wound periderm. Formation of periderm may also be observed at the site of leaf or branch abscission.

Cell structure

Phellogen

The phellogen is a secondary meristem since it arises from differentiated cells. It functions as a lateral meristem leading to increase in the girth of plants. The phellogen consists of a single layer of meristematic cells which appear rectangular in transverse-section and rectangular or polygonal in longitudinal section. These cells are living, contain vacuoles, chloroplasts and starch grains and lacks intercellular spaces. The phellogen cells show alternating periods of activity and inactivity. The variation in the activity of phellogen is dependent on various factors including seasons and various environmental factors such as temperatures, floods, hormones etc. For example in Robinia, short day conditions and high temperature or long day conditions and low temperature favor the activity of phellogen. Gibberellic and Naphthalene Acetic Acid however, have a negative effect on the initiation of phellogen. In Eucalyptus, high humidity and continuous supply of oxygen can prematurely initiate phellogen formation.

Phellem /Cork

Phellem/Cork cells are formed on the outside by the phellogen and resemble them in shape. These cells appear radially compressed, without any intercellular spaces and are arranged in radial rows in cross-section. The cells possess plasmodesmatal connections with each other. The characteristic feature of the cork cells is that their cell walls are suberized and therefore they are protective in function. Also, due to suberin deposition they become dead on maturation. In some species the cork cells may differentiate into crystals and sclereids.

Figure: A L.S of stem showing the phellem layer

Source: http://nickrentlab.siu.edu/PLB400/images/Figure12_1.jpg

The phellem may consist of two types of cells. These are:

• Hollow, thin walled, wider cells.
• Thick walled, radially flattened cells which are filled with dark colored tannin/resin like substances.

These two types of cells can occur together as alternating layers or bands e.g. Betula, Arbutus.

The primary wall is mostly cellulosic and sometimes may be lignified. Internal to this wall, suberin and wax lamellae get deposited alternately to form a thick suberin layer. In some plants this layer is followed internally by another thin cellulosic layer. Such cells get a thick wall. The suberin layer makes the cells impermeable to water, gases and acids. The walls look yellow or brown in color.

Figure: A) Schematic and B) transmission electron microscopic views of Arabidopsis suberized root cells. Suberized peridermal cells present the typical lamellar deposition within the cell walls. PC: peridermal cell.

Source: http://lipidlibrary.aocs.org/plantbio/polyesters/index.htm

After these wall layers are formed the protoplasm is lost, and the cell becomes filled with air or pigmented substances like tannin or resin. In Betula partial suberization occurs and this makes the periderm permeable to water.

Figure: Suberin Deposition

Source : http://cropwatch.unl.edu/potato/wound_healing_process

BOTTLE CORK

The commercial bottle cork is the periderm of the plant Quercus suber. It is compressible, elastic, light in weight, resistant and impervious to water, oils, acids and gases. It also has thermal insulating properties. These properties make cork an important protective layer for the plant as well as being valuable commercially. The cells of the phellem have thin walls and lack the cellulosic layer internal to the suberin. The walls have ultra thin pores which develop from plasmodesmata and later become blocked by dense material. The cell lumina develop gas after the death of the cytoplasm, which makes it an extremely light tissue. The living tissue outside the periderm becomes dead and when a lot of tissue becomes enclosed by the sequent layers, it adds to the insulating property of the cork.

Commercial cork is obtained after removing the periderm of a 20 year old tree. The subsequent phellogen is allowed to develop for about 10 years and then the cork of

commercial value can be harvested after every 10 years. The cork is very thick (many cells deep) and with active lenticels. Hence the complementary cells form tunnels or cylinders of loose tissue running from the phellogen to the outer most layer of the phellem. These appear as the crevices or brown spots in the smooth cork.

Figure: A collection of commercial corks

Source: http://www.cedric-pollet.com/site/en/biologie.php

In some plants the cells of phellem are thin walled but groups of thick walled cells occur within them. These thick walled cells have a lignified primary cell wall, a thick secondary wall followed by a thin suberin layer. They may also have a lining of thin cellulosic layer which may be lignified for e.g. Haloxylon and Anabasis. In some species, scattered among the suberized cells, some non – suberized cells also occur. Since they are similar to suberized cells they are also called phelloids.

Phelloderm

The cells of phelloderm are living and resemble the parenchymatous cells of cortex and are usually single layered. These are chlorophyllous in nature and perform the function of photosynthesis, they store starch and sclereids may also be present. When the periderm develops in deep layers, the phelloderm may be multiseriate. Such a phelloderm has cells arranged in radial rows.

Polyderm

In roots and rhizomes of families such as Hypericaceae, Myrtaceae, Onagraceae and Rosaceae development of a special structure called polyderm is observed. Polyderm is formed of alternating layers of partly suberized cells with non-suberized cells and is several layers deep e.g. Fragaria (strawberry). Only the outermost layer is dead whereas the non- suberized cells are used for storage purposes.

Figure: T.S. showing polyderm

Source: http://nickrentlab.siu.edu/PLB400/LecturesDLN/Lecture14_Periderm.html

Development

Periderm may be formed all around the stem or start in small patches and then spread out. The formation of the phellogen which is the meristem for the periderm is the first step. These cells then divide to form the cells of phellem and phelloderm.

Figure: Sub epidermal cells dividing to form phelloderm and phellogen (a diagrammatic sketch).

Source: http://nickrentlab.siu.edu/PLB400/LecturesDLN/Lecture14_Periderm.html

The phellogen can be initiated in living epidermal cells, parenchyma or collenchyma in the cortex. The parenchymatous cell become meristematic, loses their vacuoles, increases the volume of protoplast and starts dividing periclinally. The starch and tannins are gradually lost as the divisions proceeds.

First periclinal division forms two cells, usually the inner cell functions as phelloderm and the outer layer divide again periclinaly. The second division also forms two cells, the outer functions as cork while, the inner retains the meristematic activity and functions as phellogen initial. However, if the first division forms cork and phellogen initial no phelloderm is formed. Anticlinal divisions also occur in the phellogen initials to increase the circumference of the cylinder. Phelloderm is generally single layered but it may vary from two to six layered or even absent in some plants. Number of phellem layers is always more than phelloderm.

Position of phellogen

Periderm is the secondary tissue which replaces the epidermis, The first periderm is also replaced by new layers over a period of time. Each new layer of periderm is formed deeper into the living tissue so, it may reach the cortex and may also reach xylem. Generally it remains upto or within the secondary phloem. In the stems, the periderm develops in the epidermis e.g. Solanum, Quercus, Malus or in subepidermal layer e.g. Populus, Ulmus or in cortical layers e.g. Aristolochia, Robinia and Pinus or near vascular cylinder (in phloem) e.g. Berberis, Camellia and Vitis. The most common site is the sub-epidermal region.

Figure: Different origin of the phellogen: A. Solanum- epidermal; B. Vitis- phloem Source:http://www.mhhe.com/biosci/pae/botany/crang/periderm/index.html

In the roots of gymnosperms and dicots, it is formed in the pericycle and in the roots of monocots it is generally formed in the outer cortical region.

It is initiated parallel to the surface of stem. In angled stems it occurs first beneath the angle or ridge and is also deepest there. When subsequent periderms develop (usually one phellogen in each growth season), they are formed in 2 patterns:

1. When first periderm develops in inner layer, the subsequent layer form continuous cylinders e.g. Vitis
2. When first is in the epidermis or outer layers, additional are shell like in shape e.g. Pinus.

In some plants e.g. Q.suber, Fagus etc, no additional periderm is formed. In plants like Punica, Prunus, etc the first periderm lasts for 20-40 years, in some plants however the second periderm also develops in the first year.

Rhytidome

With the formation of periderm, the tissue external to it gets cut off from the water supply and dies, these tissues dry up and become cracked forming a hard crust on the outer surface. This tissue deepens in thickness when the additional (sequent) cork layers form and cut off more living tissue. The cork layers along with the layers of tissue that they enclose are called rhytidome. In the common or non-technical terminology bark is the name given to all the tissue that is peeled off. Actually bark includes all tissue external to vascular cambium i.e. secondary phloem and secondary cortex along with the rhytidome. Then inner bark is the term given to the living tissue part of the bark and outer bark is the name for rhytidome. When phellem is differentiating close to the surface, it forms a thin layer and rhytidome does not develop.

Figure: C.S. showing remnants of primary phloem and secondary phloem

P = periderm (cork), RP = remnants of primary phloem, RS = remnants of secondary phloem, F = fibre, and D= druse

Source : http://www.mhhe.com/biosci/pae/botany/crang/periderm/rhyti4/1030-13n.html

Figure: Rhytidome (outer bark) of Rubina sp.

Source: http://www.mhhe.com/biosci/pae/botany/crang/periderm/rhyti4/1016n.html

Morphology of bark

It differs in different plants according to the manner of its formation:

• When phellem is formed in superficial region of the plant and is a thin layer, the surface of the bark is smooth.
• When it is a thick layer occurring in the deeper regions, the bark is cracked and ridged.
• When periderm is shell like and the outer layer are sloughed then the bark is called scaly e.g. Pinus and Pyrus.
• When additional periderm is ring like, bark is called ring bark e.g. Vitis and Clematis.
• Sometimes it peels off as large sheets e.g. Platanus, Arbutus and Eucalyptus.

Figure: Morphology of the bark

Source: http://www.cas.miamioh.edu/~meicenrd/ANATOMY/Ch7_Protective/protective.html

After some time, when tissue accumulation increases the bark ruptures due to pressure and is sloughed off. The sloughing off of the bark occurs at the following sites:

1. Parenchyma cells on periphery of phellem.
2. Phelloids
3. Thin walled phellem.

Lenticels

Lenticels are small regions of loosely organized cells on the surface of the plant body. This may be on the epidermis (of fruits) or in the periderm. They appear as small black dots or as rough patches rising above the surface.

Figure: Lenticels (arrow)

Source: http://www4.uwsp.edu/biology/courses/botlab/Lab06b.htm

Development

They are formed with the development of first periderm or in the first growing season and sometimes even before the organ is completely elongated. Cells in the substomatal area start dividing in all directions, loose chlorophyll and the mass become colorless. Later on divisions move inwards and the cells organized gradually till they become periclinal in position. This forms the phellogen of the lenticels.

Sometimes the phellogen of the periderm may develop the lenticel. The phellogen of the periderm undergoes more divisions at certain points. These areas have cells having larger size and numbers therefore they protrude above the periderm as lenticels. They also have intercellular spaces and are thus loosely arranged. The intercellular spaces in lenticels are continuous with those in the tissue beneath them.

The cells produced by the phellogen of lenticel towards outside are called complimentary cells or filling tissue. The complimentary cells may be suberized or non suberized and are almost spherical and thin walled. The phellogen produces phelloderm towards the inner side.

When a large number of complimentary cells have formed, they put pressure on the outer cells, cause rupturing of the epidermis and expose the outer cells. The exposed cells die off, wither and are blown away to be replaced by new ones from inside. The phellogen of the lenticel in some plants also forms closing layers which may be single layer or a few cells deep. They contain compactly arranged cells and are alternating with the complimentary cells.

Figure: T.S. of stem showing lenticel with ruptured epidermis. Source: http://www4.uwsp.edu/biology/courses/botlab/Lab06b.htm,

There are two types of complimentary cells; one which are not connected with other cells, such cells have a loose powdery appearance e.g. Pyrus, Prunus. The complementary cells are held by closing layers. In the other type the complimentary cells are connected with each other forming a tightly held layer e.g. Salix

In the dicots, 3 types of lenticels can be seen.

Type 1: The complimentary cells are suberized e.g. Malus, Pyrus, Salix etc. There are lots of intercellular spaces and thin walled cells alternating with thick walled cells.

Type 2: The complimentary cells are non-suberized, loosely arranged e.g. Fraxinus, Quercus, Tilia etc. However at the end of season a layer of suberized complimentary cells can also be formed.

Type 3: The suberized and non-suberized complimentary cells occur in regularly alternating layers along with the closing layers e.g. Betula, Fagus, Prunus etc.

The lenticels usually develop below stomata and have intercellular spaces, therefore the function may be connected with an exchange of gases similar to stomata.

Occurrence

Area, number and arrangement of lenticels varies with different species. They may be arranged in either longitudinal or horizontal rows, but generally they are scattered in position. In storage roots they occur in vertical rows in pairs.

Shape

Lenticels are generally lens shaped convex side inside and outside. The orientation of the lenticel depends on rupturing which may be horizontal or vertical.

Duration

In plants where the first periderm persists for a longer period of time the lenticels also remains active for a longer time. They elongate transversely as they keep up with increase in circumference of the organ. This occurs by anticlinal divisions in the phellogen. In some plants they don’t increase in size. In subsequent periderm new lenticels are formed in new areas by the new phellogen.

Figure: T.S of stem (Sambucus sp.) showing lenticels in sequent periderms (arrow)

Source: http://bugs.bio.usyd.edu.au/learning/resources/plant_form_function1/plant_form/s

econdary_growth.html (Botany Teaching and Learning Modules, School of Biological Sciences, The University of Sydney; Displayed with permission).

Protective tissue in monocots

In monocots, usually the epidermis continues to act as a protective tissue. However once it ruptures, the secondary protective tissue develops. Generally the exposed cortical cells become suberized.

Other than this-

1. In some palms a hard periderm like dicots develop e.g. Dracaena.
2. In some monocots like palms a special tissue develops from a secondary meristem in the outer cortex. This meristem is storied and after periclinal divisions form the storied cork which has cells arranged in radial rows. These cells become suberized. They form irregular bands and enclose living cells. As the band forms further inwards, they alternate with non-dividing suberized and non-suberized cells which become crushed. This results in a layer similar to rhytidome.

When an injury occurs and living tissue is exposed to air the outer dead tissue gets separated from inner intact part by suberized layer of cells. This is called scar tissue. Phellogen develops inside the scar tissue closing layer and produce what is called as Wound Cork or Wound Periderm.

Summary

Secondary growth in dicotelydenous plants begins as they mature, to give the plants support and protection. Two types of meristematic tissues are seen in the plant body: the vascular cambium and the phellogen. The vascular cambium gives rise to the secondary xylem (or wood) and secondary phloem which provide mechanical support and nutrition respectively to the plant. This activity leads to an increase in the girth of the plant body. The epidermis of the plant thus gets stretched and needs to be replaced. The phellogen becomes active usually in the first or second year of growth. It produces two layers by periclinal divisions an outer phellem (cork) and an inner phelloderm. Together these three

layers are called “Periderm”. The periderm is the secondary protective tissue, and forms the outermost layer.

The periderm most commonly arises in the sub-epidermal position but may arise in the epidermis, cortex, or, even in the secondary phloem. The amount of phellem produced is always more than the phelloderm. The phellem is made up of suberized cells which makes it a protective layer impermeable to water, acids, gases etc. These cells are dead and do not allow transport across themselves thereby killing living tissue outside it. Gradually, with further growth, a thick layer of dead tissue develops which is shed in the form of bark. Successive layers of periderm develop in deeper tissues. When fresh tissue gets enclosed in between these layers it is called Rhytidome.

At certain points in the periderm, the phellogen produces a large number of small round cells called complimentary cells instead of phellem. These cells have intercellular spaces in continuation with the internal intercellular spaces. Due to their large number they rupture the outermost layer and form small eruptive structures called lenticels. The lenticels appear as small rough patches or dots on the surface of the stem/root/fruit (e.g. apple).In some of the monocotyledonous plants also, sometimes a protective suberized layer develops on the surface simulating the periderm.

Glossary

Anticlinal: Perpendicular division to the longitudinal axis of the cell.

Cork: A product of cork cambium formed outside, protective in nature and contains suberin deposition in its wall.

Lenticels: Isolated region in the periderm, where cells have very much intercellular spaces among them.

Lignin: One of the constituent of secondary cell wall, provides toughness and rigidity to the cell structure.

Periclinal: Parallel division to the longitudinal axis of the cell.

Periderm: Phellem (cork cells), phellogen (cork cambium) and phelloderm together known as periderm.

Phelloderm: Product of cork cambium formed toward inner to it, parenchymatous in nature.

Phellogen/

cork cambium: a type of lateral cambium which helps in secondary growth by producing phellem and phelloderm.

Plasmodesmata: A protoplasmic connection between two cells.

Rhytidome: A thick layer made up of fresh living tissue enclosed by several layers of sequent periderm.

Sclereids: Highly lignified, sclerenchymatous tissue with many pits and different morphology.

Secondary

growth: Growth of the plant mainly concerned with the increase in girth or thickness occurring in the later part of plant life.

Suberin: It is polymer of hydroxyl/ epoxy fatty acids joined together by ester bond, important constituent of cell wall differs from cutin in having significant amount of phenolic compounds.

Exercises

• 1. Define the following:
     1. Periderm
     2. Suberin
     3. Rhytidome
     4. Lenticel
  2. Differentiate between:
     1. Bark and Rhytidome
     2. Primary and Secondary Growth
     3. Phellem and Phelloderm
     4. Cork and Wound Cork
  3. Explain the development and structure of the periderm.
  4. What are the characteristic features of the phellem?
  5. What are the various positions where the phellogen develops?
  6. What is Sequent Periderm?
  7. What is a Lenticel? How does it develop?
  8. Describe the secondary protective tissue seen in the Monocots.

References

Esau, K. 1977. Anatomy of Seed Plants. Wiley Publishers, New York, USA. Fahn, A. 1974. Plant Anatomy. Pergmon Press, USA

Eames, A. J. and MacDaniels, L.H. 1925. An Introduction to Plant Anatomy, New York, USA. Mauseth, J.D. 1988. Plant Anatomy. The Benjamin/ Cummings Press, USA.

Web links

http://www.mhhe.com/biosci/pae/botany/crang/periderm/index.html http://bugs.bio.usyd.edu.au/learning/resources/plant_form_function1/plant_form/secondar y_growth.html