Variations in Chromosome Number

 

 Variations in Chromosome Number

Somatic cells of higher plants and animals usually have chromosomes in pairs (2n), that is, two of each kind of chromosome are present in each cell. Mature germ cells, having undergone reduction division, normally have one member of each pair (n).

However, sometimes irregularities occur in normal diploid (2n) chromosome number.  Variation in chromosome number produces two types of individuals or cells : 1. Those whose somatic complements are exact multiples of the basic haploid number characteristic of the species and 2. Those whose somatic complements are irregular multiples of the basic number.

The individuals or cells having chromsomes that are multiples of basic number are called as euploids. They may be haploid, diploid, triploid, tetraploid, etc.

The individuals or cells having irregular chromosome numbers, in which the addition or loss of one or more chromosomes to the complete diploid chromosome complement of an organism are called as aneuploids.

 

Euploidy:

(Gr Eu: Even or true; ploid- unit) Individuals or cells having chromosomes that are multiples of basic number are called as euploids and the phenomenon is called as Euploidy.

The number of chromosomes, in a basic set is called monoploid (x) number. Individuals with two sets of chromosomes are known as diploids (@x), and with more than two sets are called polyploids. The polyploidy types are 3x(triploid), 4x(tetraploid), 5x(Pentaploid), 6x(hexaploid) and so on.

 

The haploid(n) refers to strictly to the number of chromosomes in gametes. In most animals and plants, the haploid number(n) and the monoploid number (x) are the same. However, in case of polyploids, the usage x is done instead of n, e.g., wheat has 42 chromosomes with 6x=42 and x =7. However, gametes contain 21 chromosomes, hence 2x= 42, n=21.

Monoploidy:

The monoploid organsims have one set of chromosomes or one genome(n) in the nuclei of their body cells. The monoploid individuals can arise spontaneously in natural population as rare abnormalities, but in animal kingdom many forms like ants, bees and wasps, the males are normally monoploids (being derived from unfertilized eggs).

Monoploids are characteristically sterile because in their germ cells meiosis is not found normally and chromomosomes lack pairing.

Monoploids plants are often weak and sterile.

Polyploidy:

An organism with more than two sets of chromosomes are called polyploidy. Ploidy levels higher than tetraploid are not commonly found in natural population, but some important crops and ornamental flowering plants are polyploids, e.g., Wheat (hexaploid, 6x), strawberries (octaploid, 8). About one third of all grasss are polyploids.

There are three different types of polyploids, namely i. autopolyploids, ii. Allopolyploids Autopolyploids:

Autopolyploids are those polyploids, which have the same basic set of chromosome multiplied. For instance, if a diploid species has two similar sets of chromosomes (AA), an autotriploid will have (AAA) and an autotetraploid will have (AAAA).

Autopolyploids arise in following ways:

•          By the union of diploid gametes produced in the absence of meiosis or due to abnormal meiosis (autotetraploid)

•          By somatic doubling of the chromosomes in a zygote due to abnormal mitosis

•          By the union of a haploid gamete with the diploid gamete (autotriploid)

Autotriploids are known in Watermelons, sugarbeet, tomato, grapes and banana. They are generally sterile and cannot produce seeds. Therefore, they have great commercial value in producing seedless varieties of fruits.

Autotetraploids(4n) are the most common forms of polyploids. They are usually larger, more succulent and have bigger pollen grains, fruits, etc. Autotetraploids are seen in rye (Secale cereal), corn (Zea mays), red clover (Trifolim pretense).

Cytology of Autopolyploids:

In an autopolyploid, there will be more than two sets of homologous chromosomes. This leads to formation of multivalents instead of bivalents at metaphase of Meiosis I.  An important difference exists between autotriploids and autotetraploids. In autotetraploids normal disjunction is possible giving rise to diploid gametes, while in autotriploids it is not possible.

In an autotriploid, there are three sets of homologous chromosomes. It these sets are normally paired, trivalents would result. Tirvalents cannot disjoin normally and will either disjoin 2:1 chromosomes to two poles or will disjoin 1:1 leaving one chromosome as laggard.

Number of chromosomes in gametes of triploid organisms, therefore, will vary from n to 2n. Most of these gametes are unbalanced leading to high degree of sterility.

In autotetraploids, since there are four sets of chromosomes, quadrivalents are formed,which disjoin in a normal 2:2 manner giving diploid gametes. Rarely, however, a quadrivalent may disjoin 3:1 or may leave a chromosome as a laggard at anaphase I. Therefore, autotetraploids also have a certain degree of sterility, although it will not be as high as autotriploids.

 

Allopolyploidy:

When the polyploidy results due to the doubling of chromosome number in a F₁ hybrid which is derived from two distinct species, then it is called allopolyploidy.

For example, let ‘A’ represent a set of chromosomes in species X and ‘B’ represent another genome in a species Y. The F₁ will then have one A genome and another B genome. The doubling of chromosomes in this F₁ hybrid(AB) will give rise to tetraploid with two A and two B genome. Such a polyploidy is called an allopolyploid or amphidiploid.

 

 

                             Species X            X         Species Y

                                  (AA)                                 (BB)

                                                           

                                                          AB – Diploid F₁ hybrid (sterile)

                                                                        Colchicine

                                                                     AABB – Amphidiploid tetraploid

If the genome are sufficiently dissimilar structurally, no synapsis will occur in the diploid hybrid and high sterility will ensue as the result of the random segregation of unpaired chromosomes. Doubling of the chromosome number to give the tetraploid AABB will, however, provide for regular synapsis and segregation. Genome A will pair with genome A, and genome B will pair with genome B.

Raphanobrassica is a classical example of allopolyploidy. In 1927, G.D Karpechenko, a Russian scientist, reported a cross between Raphanus sativus (2n=18) and Brassica oleraceae (2n=18) to produce F₁ hybrid which was complete sterile, because no pairing occurred in F₁ hybrid as there is no homology between the genomes of Raphanus and Brassica.

However, a number of unreduced (diploid) gametes were formed, and several tetraploid individuals were recovered. On cytological examination these fertile plants were found to have 2n=36 chromosomes, which showed normal pairing into 18 bivalents.

The polyploids occur frequently n plants but rarely in animals, because plants can propagate vegetatively through grafting, cuttings and by rooting. Thus, sterile triploids, pentaploids, etc., are maintained from generation to generation.

Animals usually reproduce sexually through gamete formation. The gamete formation in polyploids is irregular due to abnormal meiosis or failure of chromosome pairing.

The polyploidy is invariably related with gigantism. These plants have been found to contain large sized pollen grains, leaves, stomata, xylem vessels, etc. the polyploid plants show more vigorous vegetative growth.

Polyploidy reduces the fertility of plants in variable degrees.

Ascorbic acid content is higher in tetraploid cabbage and tomatoes than in corresponding diploids.

Polyploidy leads to evolution and is a means of origin of new species, especially in plants.

Aneuploidy:

It is a general, the two members of a pair of homologous chromosomes regularly segregate during meiosis in a normal diploid to give a haploid set of chromosomes in a gamete or a spore.  Whereas, in mitosis two cells of like chromosomal constitution are formed.

Exceptions occur, however, to give cells or organisms deficient or duplicated for a particular chromosome. Aneuploidy (aneu- uneven; ploidy- unit) is the addition or loos of one or more chromosomes to the complete diploid chromosome complement of an organism. Organism whose chromosome number is not an exact multiple of the basic number of the group is called as aneuploidy.

The aneuploidy may be of the following types:

Monosomics:

Diploid organisms which lack one chromosome of a homologous pair are called monosomics. Their genomic formula is 2n-1.

Since, monosomics lack one complete chromosome, they are genetically imbalanced. Therefore, they are either lethal or of reduced vitality. A monosomic individual produces gametes of two types n and n-1. The n-1 gametes do not survive in plants, but, in animals they cause genetic imbalance, which is manifested by high mortality or reduced fertility of resulted organisms.

Polyploids can tolerate the loss of a chromosome with greater ease than a diploid, so monosomics are much more common in polyploidy species. All the 24 monosomics in the tetraploid Nicotiana tobaccum and all 21 monosomics in the wheat – Triticum aestivum have been identified.

Monosomic codition for a particular chromosome may be associated with a characteristic morphology. Therefore, by looking at the morphology of monosomics and that of their progeny, genes can be located on specific chromosomes. For example, in wheat a newly arisen mutation can be located with little difficulty by crossing the mutant plant (as a male) to all of the 21 monosomics, and determing the patterns of segregation that deviate from the expected.

Double monosomics (2n-1-1) or triple monosomics (2n-1-1-1) could also be produced in polyploids like wheat. Double monosomics mean that the chromosome number is 2n-2, like in a nullisomic, but the missing chromosomes are non-homologous.

Nullisomics:

Organsims which lack a single pair of homologous chromosomes are called as Nullisomics. The genomic formula for nullisomics is 2n-2.

These may be occasionally in nature, but seldom survive long enough to be recognized and perpetuate. However, a nullisomic polyploidy (e.g., hexaploid wheat. 6x-2) may survive but exhibit reduced vigour and fertility.

E.R. Sears experimentally produced all the 21 possible nullisomics in wheat, Triticum aestivum. By association certain pheontypes with corresponding chromosome arrangements, nullisomics have been used effectively in locating several different genes in wheat.

  Trisomics:

Individuals having one chromosome extra to the diploid genome are called trisomics. Since, the extra chromosome may belong to any one of the different chromosomes of a haploid complement, the number of possible trisomics is an organism will be equal to its haploid chromosome number. For instance, haploid chromosome number in barley is n=7, consequently, seven trisomics are possible.

Trisomics were obtained for first time in Datura stramonium by A. F Blakelslee and Belling. Since, haploid chromosome number in the Datura is n=12, 12 different monosomics are possible. The morphological differences between the trisomics in Datura  include the shape, size, spine pattern of the capsule, indicating thereby that different genes for capsule types are located on 12 different chromosomes of Datura.

Production of Trisomics:

Trisomics may originate spontaneously due to production of n+1 type of gametes due to rare non-disjunction of a bivalent. However, trisomics are readily obtained by crossing a diploid as male with a triploid as a female or by selfing triploids. In either case, trisomics are obtained in large number and can be identified through phenotypic effects of individual chromsomes.

Cytologic of trisomics:

A trisomics has an extra chromosome which is homologous to one of the chromosomes of the complement. Therefore, it form a trivalent in Meiosis. This trivalent may take a variety of shapes such as an “frying pan’, configuration, or a bivalent and a univalent dough nut shape, etc.

Development of Male Gametophyte in Angiosperms

 

In the life history of angiospermic plant there are two stages, the sporophyte and gametophyte which alternate with each other.

Sporophytic stage:

This stage is derived from the diploid zygote, so it is a diploid stage in angiospermic plants. The plant body belongs to this stage. Reproductive organs or flowers develop on this plant.

Gametophytic stage:

This is the highly reduced haploid stage of angiospermic plant. This stage is derived from a spore which is a product of meiotic division of spore mother cells.  In angiosperms, spore mother cells are of two types : the microspore mother cells and megaspore mother cells.  Microspore mother cells develop in an anther while megaspore mother cell develops from the nucellus of the ovule. Each of these mother cells in turn gives rise to respective microspore and megaspores after undergoing meiotic divisions.  Consequently the microscope and megaspores which are haploid structures give rise to male and female gametophytes respectively.  The male gametophyte gives rise to male gametes and female gametophyte forms egg cell.  Fusion of male gamete and female gamete results in the formation of diploid zygote that later becomes embryo in the seed.

When the seed germinates the embryo develops into a sporophytic plant.

Structure of Anther and Microsporogenesis

Structure of Anther

Pollen grains which contribute the male gametes, are formed within an anther. A typical (dithecous) anther consists of two anther lobes, between which there is a column of sterile tissue called the connective. Each anther lobe in turn bears two pollen chambers or pollen sacs. Each pollen chamber represents a microsporangium. Thus, a typical dithecous anther comprises of four pollen sacs or microsporangia.

In some plants, such as Moringa and Wolffia, each anther lobe has only one microsporangium. Such anthers are called monothecous.

 A very young anther comprises of a homogenous mass of cells bound by a well-defined epidermis. During its development the anther assumes a four-lobed, and rows of hypodermal cells become differentiated in each lobe by their larger size, radial elongation, and more conspicuous nuclei. These form the archesporium.

The archesporial cells divide in a plane parallel to the outer walls of the anther lobe (periclinal division) to form a primary parietal layer toward the outside and a primary sporogenous layer toward the inside.

The cells of the parietal layer undergo a series of periclinal and anticlinal divisions to form 2-5 concentric layers composing the wall of the anther. The primary sporogenous cells either directly or after a few divisions, function as microspore mother cells.

Each anther at maturity consists of sporogenous tissue covered by an anther wall.

Anther Wall:

The anther wall consists of epidermis, endothecium, middle layers and tapetum

Epidermis:

It is the outermost layer of the anther, it is one celled thick and useful for protection. The epidermal cells undergo repeated anticlinal divisions in order to cope up with the enlargement of the anther.

The epidermal cells present between the pollensac are thin walled and constitute ‘stomium’ which is useful for the dehiscence of pollen sac. The stomium degenerates by the time the spore mother cells undergo divisions.

Endothecium:

The layer of cells lying immediately beneath the epidermis is the endothecium. Its maximum development is attained at the time when the pollen grains are about to be shed. The cells are radially elongated and from their inner tangential walls thickened fibrous bands develop. These thickenings are hygroscopic.

The presence of fibrous bands, differential expansion of the outer and inner tangential walls, and the hygrospic nature of the endothecial cells help in the dehiscence of anthers at maturity.

Middle Layers:

Next to the endothecium there are usually one to three middle layers. They are ephemeral in nature and are crushed during meiosis of pollen mother cells.

Tapetum:

The innermost layer of anther wall is called as tapetum which of considerable physiological significance, for all the food material entering into the sporogenous cells must pass through it. The cells of tapetum are large, with thin cell walls, abundant cytoplasm and prominent nuclei.

Based on behaviour, the tapetum is of two types:

1.     Amoeboid ( also called invasive or periplasmodial):

This type of tapetum is characterized by breakdown of the inner and radial walls of the tapetal cells. The protoplasm moves or protrude into the anther cavity, where they coalesce to form a continuous mass called the tapetal periplasmodium, closely investing the pollen mother cells or the microspores.

This type of tapetum is found in Alisma, Butomus, Tradescantia,Typha

2.     Secretory or Glandular :

In this type, the tapetal cells remain in their position throughout the microspore development. Subtances are contributed by secretion from the inner faces of the cells. Toward the close of the meiotic divisions in the microspore mother cells, the tapetal cells begin to lose contact with each other and finally cells are entirely absorbed at the time when the microspores begin to separate from one another.

This type of tapetum is common occurrence in angiosperms

A common feature of tapetum , amoeboid as well as secretory is the presence of more than one nucleus.

The tapetum plays a significant role in the development of pollen. Tapetum stores starch and protein during early stages of anther development and such tapetal reserves are used by pollen grains. During post-meiotic period, tapetum exhibits nutritional value in concern with the pollen wall formation.

During meiosis it transport the nutrients to the inside of the anther locule since tapetum is the only channel through which material can reach the meiocytes.

 

Sporogenous Tissue:

The tissue present inside the anther wall is called sporogenous tissue. It is diploid in nature. The sporogenous cells may directly function as microspore mother cells or pollen mother cells  or they may undergo a few mitosis to add up to their number before entering meiosis.

Each pollen mother cells by a meiotic division, give rise to a group of four haploid microspores or pollen tetrads. Cytokinesis during the meiotic division is of two types in the pollen mother cells, they are –

1.     Successive type : the nuclear divisions are accompanied by wall formation. After the first meiotic division a wall is formed separating the two nuclei. This leads to the formation of a dyad. After second division walls are again fomed resulting in a tetrad.

2.     Simultaneous type : in this type of cytokinesis, the first meiotic division is not followed by wall formation. Consequently, binucleate cell is formed after meiosis – I; there is no dyad stage. The two nuclei synchronously undergo the second meiotic division. The walls are formed after the second meiotic division, giving rise to a tetrad.

After the completion of the meiotic division in the microspore mother cell, different types of tetrads are formed. These are –

i.  Tetrahedral type: this type of arrangement is associated with simultaneous type of cytokinesis in the pollen mother cell. In the tetrad three microspores are seen in one surface while the fourth one appears on the backside.

ii.     Isobilateral type: four microspores are arranged in the four corners of the square. All the four spores are visible in the same surface. This is formed due to successive type of cytokinesis in the microspore mother cell. E.g., Zea mays.

iii.   Decussate type: microspores are arranged in two pairs. One pair of microspores is at right angles to the other. E.g., Magnolia, Atriplex

iv.    T Shaped type: two microspores are arranged side by side, while the other two appear one above the other. Thus, giving a T shaped appearance. E.g., Aristolochia.

v. Linear type: four microspores are arranged one above the other in a single row.

Usually, the microspores soon separate from one another but in some plants they adhere in tetrads to form the so called ‘compound‘ pollen grains. In the Mimosaceae there are larger units composed of 8 to 64 cells and in genera belonging to the Asclepiadaceae all the microspores in a sporangium remain together to form a single mass called the pollinium.

 Development of Male Gametophyte:

In angiosperms, the microspores represent the beginning of the male gametophytic generation. It is the first cell of male gametophyte.

They possess a haploid nucleus and dense cytoplasm. Soon after it’s release from the tetrad the microspore or pollen grain rapidly increases in volume and accompanying formation of a vacuole. The nucleus is displaced from the centre to a place adjacent to the wall.

The first division of the microspore gives rise to the vegetative and generative cells. The generative cell is small and lens shaped.

Pollen grains are usually dispersed from the anther in 2-celled condition. In certain plants the generative cell divides in the pollen grain while it is in the anther itself giving rise to two male gametes or sperm cells. The pollen grains are thus 3-celled condition in these plants.

The pollen grains or male gametophyte reach the stigma either in 2-celled or 3-celled condition, through various pollinating agents.

By absorbing water available in the exudates of stigma, the pollen germinates and the intine of the pollen grain comes out and forms a pollen tube. Generally only one pollen tube develops from a pollen grain (Monosiphonous). But in Malvaceae and Cucurbitaceae more number of pollen tubes may form from a single pollen grain (Polysiphonous).

The vegetative and generative nuclei enter into the pollen tube along with cytoplasm. The vegetative and generative cells change their position during their travel in the pollen tube. The generative cell undergoes mitotic division and two haploid male gametes are formed. This is called as spermatogenesis as the male gametes are equivalent to the ciliated sperms of the lower groups of plants.

 Nemec Phenomenon or Embryo sac like pollen grains:

Normally, the number of nuclei in a mature pollen grain is two or three. Occasionally, grains with more nuclei resulting in abnormal type of the male gametophyte occurs. In 1898 Nemec noted that in anther of Hyacinthus orientalis the pollen grains some time form large eight nucleate structures showing a resemblance to embryo sacs.

He believed that they arose as the result of degeneration of the generative nucleus and three divisions of the vegetative nucleus.

The microspores increase in size to form large sac-like bodies. Later the nucleus undergoes three successive divisions to form 8 daughter nuclei. Of these, 3 lie at the end where the exine is still intact, 3 at the opposite end and 2 in the middle. Cells found at the exine end represent egg and synergids, the opposite end nuclei form the antipodals. The tow central nuclei represent the polar nuclei.

 

Androgenic Embryoids (Haploid plants)

Guha and Maheshwari working with anther cultures of Datura innoxia reported that embryo like structures called embryoids develop from the pollen grains. The embryoids give rise to haploid plants.

The anther is excised from the flower bud of Nicotiana and cultured on a suitable medium. Pollen grains inside the anther by repeated divisions form a multicellular tissue within the parent wall. Ultimately the pollen wall bursts, releasing the tissue mass. This tissue mass is capable to grow directly into an ambryoid and germinates to from a haploid plant.

Secondary Growth in Plants

 

Growth in plants occur by the activity of the apical meristems present in the apices or tips of stem, branches and roots. Apical meristems divide and form new cells. In due course such tissues become permanent and lays down the basic or fundamental body of plants. Thus the tissues which derive their origin from the apical meristem are called primary permanent tissues and the plant body made of primary tissues is called primary body.  Thus, the apical meristem is responsible for the formation of the primary growth of the plants.

Thus, apical meristems cause linear growth.

In monocotyledons and pteridophytesthis primary structure remain as such throughout the life of the plants. It is structurally and functionally both self-sufficient.

In Dicotyledons and Gymnosperms the primary growth is not able to bear the load of plant – branches, flowers, fruits. Therefore, there is required additional strength to the stem and branches.                    

Thus, growth in thickness is called secondary growth. The increase in thickness due to the addition of secondary tissues cut off by the vascular cambium and the cork cambium in the stellar and extra-stelar regions respectively, is called secondary growth.

Secondary growth is common in gymnosperms and dicotyledons, but is normally absent in monocotyledonous plants. However, in some exceptional genera of monocotyledons such as Dracaena and Yucca, anomalous secondary growth can be observed.

Secondary Growth in Dicot Stems

Secondary growth in dicot stems is initiated in the intrastelar region with the activities of cambium (fascicular or interfascicular cambium). In the extrastelar region cork cambium gives rise to the periderm.

Intrastelar Secondary Growth:

The secondary growth occurring in the stellar region is called intrastelar secondary growth. It includes the following:-

1.     Formation of Cambial Ring:

In young dicotyledonous stem limited number of vascular bundles are arranged in form of a ring. Each vascular bundle is conjoint, collateral, open and endarch. They are called as open because a single layer of cambium cells called the intra fascicular cambium (fascicle – bundle) is present between the xylem and phloem.

In between the vascular bundles there are medullary rays, the cells of which become mertistematic and form a cambium strip called the interfascicular cambium, i.e., the cambium in between the two vascular bundles.

The fascicular and inter fascicular cambium join laterally to form a complete ring of cambium.

Activity of Cambium:-The vascular cambium consists of two types of initiating cells – the fusiform initials and ray initials. The fusiform initials are elongated and spindle shaped. They produce the elements of secondary xylem and secondary phloem. The ray initials are small and isodiametric. They produce phloem rays to the outside and the xylem rays to the inside. These rays are called the vascular rays.

 Formation of Secondary Tissues:- The cells of vascular cambium divide repeatedly by periclinal method i.e., in a direction parallel with the epidermis. Each time a cambial cell divides into two, one of the daughter cells remainmeristematic, while the other is differentiated into a permanent tissue.

 Those cells which are produced outside the cambial ring differentiate into secondary phloem and those produced to the inner side of the cambial ring differentiate into secondary xylem.

The cambium cells divide continuously in this manner producing secondary tissues on both sides of it. In this way, new cells are added to the xylem and phloem, and the vascular tissues increase in size.

Normally the cambium produces more amount of secondary xylem to the innerside and less amount of secondary phloem to the outerside.

The cell formed from the ray initials of cambium in the region between the vascular bundles become the secondary medullary rays. They extend from pith to the secondary xylem and phloem. The portion of the ray present in the xylem region is called xylem ray or wood ray and the portion of the ray in the phloem is called phloem ray. These rays help in radial conduction of water, salts and food materials.

 The formation of new cells from the cambium results in an enlargement of the stem that is known as secondary thickening. As the stem increases in thickness, the primary phloem and primary xylem become obliterated and replaced by secondary phloem and secondary xylem.

 

Extra Stelar Secondary Growth:

Due to the formation of secondary vascular tissues in the stellar region, an outward pressure is exerted on the epidermis. Due to this, epidermisgets stretched and ultimately tends to rupture exposing the living cells.

At this stage, a new protective layer called the periderm is produced in the cortical region.

Periderm is formed by the activity of a secondary lateral meristem called phellogen or cork cambium.

Secondary growth in cortex begins with the appearance of a meristematic layer either sub-epidermal or epidermal (e.g., Teak, Azadirachta) or in the cortex (e.g., Aristolochia).

In contrast to the vascular cambium, the phellogen is relatively simple in structure and composed of one type of cells. They are rectangular and have vacuolated protoplasts and may contain tannins and chloroplasts.

The cells of phellogen divide vertically and cut off many cells toward the outside and toward the inside. The cells formed towards the innerside develop into secondary cortex or phelloderm and those cells formed towards the outer side develop into phellem or cork. Usually more amount of cork is produced than the secondary cortex. The phellogen (cork cambium), phellem (cork) and phelloderm (secondary cortex) together constitute periderm.

 Phellem or Cork:

Phellem arises towards the outerside of the phellogen. They are polygonal and uniform in shape. The cells are closely arranged without intercellular spaces and with thin cellulose cell walls. The cells later become dead by losing their protoplasts and their walls become thicker due to the deposition of suberin. The cells are impervious to water and gases. They give protection to inner parts of the organ.

Commercial Cork:

The phellem of Quercussuber (oak tree) is the source of commercial cork. In this plant, the phellogen arises in the epidermis, which forms extended masses of cork tissues. At the age of twenty years, when the tree is about 40 cm in circumference, this outer layer, known as virgin cork.this cork is stripped off for the quick formation of commercial cork.

The exposed tissue dries out to about 1/8 inch in depth. A new phellogen is established beneath the dry layer and rapidly produces a massive cork of a better quality than the first. After 9 or 10 years the new cork layer of formed with sufficient thickness to be commercially valuable and is in turn removed.

The stripping of the cork take place at intervals of about nine years until the tree is 150 or more years old. The commercial cork cells have thin walls and cells are filled with air. Due to suberin, it is impervious to water and resistant to oil. Because of air filled lacuna, the cork is light in weight, and has thermal insulator qualities. The important properties of the commercial cork are its imperviousness, its lightness, toughness and elasticity.

Phelloderm:

The phellogen cuts off the phelloderm cells towards inner side. The phelloderm cells are living cells with cellulose walls. The cells contain vacuolated cytoplasm and shows a conspicuous nucleus. In most plants, they resemble cortical cells but they are arranged in radial rows because they arise from the tangentially dividing phellogen.

In some species, they act as photosynthetic tissue and aid in starch storage.

Bark:

All the tissues outside the vascular cambium of the stem is called as bark. Thus, it includes the secondary phloem and periderm.

As the periderm develops, it becomes separated by a non-living layer of cork cells from the living tissues. The tissue layers thus separated become dead.

When the cork cambium arises from the inner layers of the cortex, the bark is thick; e.g., Thuja. If it is formed from the outer layers, the bark is thin; e.g., Psidium guajava.

When the cork cambium is organized in the form of a complete ring, the bark that is produced also develops in the form of a ring, which can be stripped easily. This is known as ring bark. E.g., Betula, Clematis. Whereas in Psidium, Eucalyptus, the cork cambium originates in strips, the bark is in form of overlapping strips. The bark is removed as strips or scales. Such bark is known as scale bark.

Bark in Cinchona (yields quinoine) and Cinnamomum (source of Dalchini) are commercially important.

Lenticels:

Due to secondary growth, the periderm develops in place of the epidermis. Since the cork tissue is composed of closely arranged, dead, suberin coated cells, gaseous exchange between the internal tissues and the external atmosphere is obstructed.

So, to carry out the gaseous exchange, small openings composed of mature cells develop. These openings are called as lenticels. They are located opposite to stomata and carry out their function in the secondary body of the plants.

The lenticels originate beneath the stomata, either just before, or simultaneously with the initiation of the first layer of the periderm. As the lenticel formation begins, the parenchyma cells found near the sub-stomatal cavity lose their chlorophyll and divide irregularly in different planes giving rise to a mass of colourless, rounded, thin walled, loose cells called complementary cells.

As the complementary cells increase in number, pressure is caused against the epidermis and it ruptures. The thin walled loose complementary cells alternate with masses of more dense and compact cells called the closing cells. These cells together form a layer called closing layer.

Complementary cells are thin-walled, rounded and loose with sufficiently developed intercellular spaces among them. Their cell walls are not suberized. Due to the presence of profuse intercellular spaces, the lenticels perform the function of exchange of gases between the atmosphere and internal tissues of the plant.

 Annual Rings or Growth Rings:

The secondary xylem in the stems of perennial plants commonly consists of concentric layers each one of which represents a seasonal increment. In transverse section of the axis, these layers appear as rings, and called annual rings or growth rings.

They are commonly termed as annual rings because in woody plants of temperate regions and in those of tropical regions where there is an annual alternation of growing and dormant period, each layer represents the growth of one year.

The cambium exhibits its activity as periodical or seasonal due to climatic variation. During spring season, the plant has to translocate more water and mineral because they develop new buds, leaves and flowers. Therefore, the cambium becomes more active in this season and forms xylem vessel with wider cavities. The xylem formed during spring season is called early wood or spring wood.

On the other hand, during winter the rate of assimilation is decreased and there is less need of vessels for sap transport, the cambium is less active and gives rise to narrow vessels, tracheids and wood fibres. The xylem formed during winter is called late wood or autumn wood.

Thus spring wood with wider vessels and autum wood with narrow vessels formed during one year together make an annual ring or growth ring. Thus, the periods of active growth alternate with the periods of slow growth.

Generally, the late wood is more denser and harder than the early wood.

By counting the total number of annual rings, the age of the plant can approximately be determined. Thus, determination of age of a tree by counting the annual rings is known as dendrochronology.

Porous wood and Non Porous wood:

Gymnosperms (conifer and cycads) lack vessels and their wood is made of only tracheids. Therefore, their wood is called non-porous and soft wood. On the other hand angiosperm wood is made of tracheids and vessles both. Therefore, their wood is called porous and hard wood. Hard wood or soft wood have no relation with physical hardness of wood.

In porous woods, when large vessels of unequal diameter are arranged more or less in a ring, the wood is called ring porous wood. E.g., Castanea ring porous vessels conduct more water. On the other hand, when vessels of equal dimensions are found uniformly distributed, the wood is called diffuse porous wood, e.g., Acer, Betula.

 Heart Wood and Sap Wood:

The outer region of the old trees consisting of recently formed xylem elements is sapwood or alburnum. This is of light colour and contains some living cells in association with vessels and fibres. This part of the stem performs the physiological activities, such as conduction of water and minerals, storage of food, etc.

The central region of the old trees, which was formed earlier is whose cells are inactive, non-fucntional without any living cells is called as heart wood or duramen. The secondary xylem in this region is filled up with tannins, resins, gums and other substances which make it hard and durable and it is dark in colour. Their vessels are plugged with tyloses. 

The function of heartwood is no longer conduction, it gives only mechanical support to the stem.

The sapwood changes into heartwood very gradually. During the transformation a number of changes occur – all living cells lose protoplasts, water content of cell walls are reduced, food material are withdrawn from the living cells, tyloses are formed.

From economic point of view, heartwood is more useful than sapwood. Heartwood, as timber is more durable than sapwood, because the reduction of food materials available for pathogens by the absence of protoplasm and starch.

The haemotoxylin is obtained from the heartwood of Haematoxylon campechianum.

Because of the absence of resin, gums and colouring substances, sapwood is preferred for pulpwood, and for wood to be impregnated with preservatives.

 

Tyloses:

In many plants, axial and ray parenchyma cells located next to the vessels form ballon-like outgrowth through pit cavities into the lumen of the vessels. These outgrowths are called as tyloses.

The parenchyma cells, adjoining the half-bordered pits of vessels, penetrate into the vessel in the form of short protuberances. These protuberances gradually enlarge to form ballon-like structuresThe  nucleus and part of the cytoplasm of the parenchyma cell commonly migrate into the tyloses. The tyloses are filled with starch, resins, gums and other substances. 

Usually, they are sufficiently large and the lumen of the vessel is almost blocked. They add to the durability of the wood. Tyloses prevent rapid entrance of water, air and fungus by blocking the lumen of the vessel. In many plants the development of tyloses takes place by means of wounding.