In the article cited below, notes from studies on secondary stem growth of dicotyledonous plants are included.
Primary growth leads to an increase in length and development of lateral appendages. Secondary growth is the formation of secondary tissue from the lateral meristem. Increases the diameter of the stem. In woody plants, most of the tissue is secondary tissue. They help protect, support and conduct water and nutrients.
Secondary tissues form two types of lateral meristem: the vascular cambium and the corky cambium or phelogens. The vascular cambium produces the secondary vascular tissue while the phellogen forms the periderm.
Secondary growth occurs in perennial gymnosperms and dicots such as trees and shrubs. It is also found in the woody stems of some plants. In such cases, the secondary growth corresponds to the annual ring, e.g. of B. sunflower.
A.Formation of secondary vascular tissues:
They are formed by a vascular cambium. The vascular cambium consists of two types of meristems: the bundle cambium or the intrafascicular and interfascicular cambium. The intrafascicular cambium is a primary meristem that appears as a strip in the vascular bundles. The interfascicular cambium arises secondarily from the medullary ray cells, which occur at the level of the interfascicular bands.
These two types of meristematic tissues come together to form a ring of vascular cambium. The vascular cambium is actually single-layered, but appears to be only a few layers (2-5) thick due to the presence of its direct derivatives. The cells of the vascular cambium divide both extrinsically and internally perylinically (bipolar division) and form secondary permanent tissue.
There are two types of cells of the vascular cambium: elongated spindle-shaped initials and shorter isodiametric initials of the rays (Fig. 6.29). Both appear in T.S. rectangular. The initials of the rays lead to the vascular rays.
The spindle initials split to form a secondary phloem on the outside and a secondary xylem on the inside (Fig. 6.28 B). Forming a secondary xylem from the inside, the vascular cambium gradually migrates outward as new cells are added.
This phenomenon is called dilation. New air chambers have also been added. They create additional rays every year (Fig. 6.28 D). The vascular cambium undergoes two types of division: additive (periclinal division to produce secondary tissue) and multiplicative (anticlinal division to spread).
The ray initials form the radial pattern (=horizontal or transverse pattern), while the spindle-shaped initials form the axial pattern (=vertical pattern) of the secondary vascular tissues.
1. Vascular Rays:
Vascular rays or secondary medullary rays are rows of ray cells formed in secondary vascular tissue. They are several cells high.
Depending on the width, vascular rays are single-row (one cell wide) or multi-row (two or more cells wide). Vascular rays can be homocellular (have one cell type) or heterocellular (have more than one cell type). The cells of the guiding rays close the intercellular spaces.
The part of the vascular radius present in the secondary xylem is called the wood or xylem radius, while the part present in the secondary phloem is called the phloem radius. Vascular streams conduct water and organic food and allow radial diffusion of gases. In addition, their cells store food.
2. Secondary phloem (lichen):
It forms a narrow circle on the outside of the vascular cambium. The secondary phloem does not increase in thickness because the primary and older secondary phloem are externally crushed by the development of new functional phloem (Fig. 6.28D). Therefore, no pods (annual pods) are formed in the secondary phloem. However, fragmented or non-functional phloem may contain fibers and sclerides.
Secondary phloem consists of the same type of cells as primary phloem (metaphloem): sieve tubes, companion cells, phloem fibers, and phloem parenchyma.
There are two types of paired phloem enchyme: axial phloem parenchyma, which consists of longitudinally arranged cells, and phloem radius parenchyma, which consists of radially arranged parenchyma cells that form part of the vascular radius present in the phloem.
The secondary elements of the phloem show a more regular arrangement. Sieve tubes are relatively more numerous, but shorter and wider. Sclerenchyma fibers appear in sections or bands. Sclerides are present in many cases. In such cases, secondary phloem is divided into soft phloem (secondary phloem without fibres) and hard phloem (part of phloem with a lot of fibres).
3. Secondary xylem:
It makes up the largest part of the trunk and is usually called wood. The secondary xylem consists of vessels, trachea (both components of the trachea), wood fibers, and wood pulp.
In addition to food storage, wood pulp can also contain tannins and crystals. There are two types: axial parenchyma cells, which are arranged longitudinally, and radial parenchyma cells, which are arranged radially or horizontally. The latter is part of the vascular bundle in the secondary xylem.
Secondary xylem does not distinguish between protoxylem and meta-xylem elements. Therefore, there are no vessels and tracheae with annular and spiral thickenings. The tracheal elements of the secondary xylem resemble the meta-xylem elements of the primary xylem with minor differences. They are relatively shorter and have thicker walls. Scar thickening is more common. Fiber is plentiful.
The width of the secondary xylem increases with the age of the plant. The primary xylem exists as a conical projection on its inner side. The flesh may become narrow and eventually crushed. The annual growth of secondary xylem is different in areas where there are two seasons, one favorable (spring or rainy season) and one unfavorable (autumn, winter or dry summer).
In the favorable season, the temperature is optimal. There is good sun and humidity. At this point, the newly formed leaves produce hormones that stimulate cambial activity. Activity decreases and ceases as the unfavorable season approaches. Therefore, the annual or annual increase appears in the form of special rings called annuals (Fig. 6.30).
Annual plants follow a sequence of fast growth (favorable season, e.g. spring), slow growth (before the onset of unfavorable season, e.g. autumn) and no growth (unfavorable time of year, e.g. winter). In tropical areas, where there are no long dry periods, the seasons are not clearly marked.
Years (growth rings). This tree was created in one year. It consists of two types of wood, spring and autumn wood (Fig. 6.31). Spring or early wood is much wider than autumn or late wood. It is brighter and less dense. Feathers consist of larger and wider xylem elements.
Fall or late wood is darker in color and denser. It contains compactly arranged smaller and narrower elements that have relatively thicker walls. Tracheids and fibers are more common in autumn than in spring wood.
The transition from spring to fall wood in the annual ring is gradual, while the transition from fall to next year's spring wood is abrupt. Therefore, the growth from year to year is completely different. The number of years corresponds to the age of the corresponding part of the stem. (They can be counted with incremental exercises.)
In addition to the age of the plant, the years of growth also provide information about the climatic conditions the plant has experienced in the past. Dendrochronology is the science of counting and analyzing the years of trees.
Conifers and deciduous trees:
Coniferous is the technical name for gymnosperm wood because it has no veins. Some soft woods are very easy to work with (e.g. Cedrus and Pinus). However, not all of them are "soft". Softness depends on the content of fibers and vascular rays. 90-95% of wood consists of coils and fibers. Grain radii make up 5-10% of wood.
Hardwood is a dicotyledonous plant with many veins. Due to the presence of vessels, hardwood is also called porous wood. In the fistula Cassia and Dalbergia sisso, the veins are relatively very wide in spring wood, while they are quite narrow in autumn wood. Such secondary xylem or wood is called annular porous.
In others (e.g. Syzygium cumini), larger containers are placed in the spring and autumn wood. This type of secondary xylem or wood is called diffuse porous. Ring-porous wood is more developed than diffuse wood because it allows better movement when plant demands are high.
white and heart:
The wood of older trunks (dalbergia, acacia) differs in two zones, the outer, light and functional sapwood or alburnum and the inner, darker and non-functional core or duramen (Fig. 6.33). The tracheas and medullary vessels become blocked by the growth of adjacent parenchymal cells in their cavities through the pits. These growths are called tyloses (Fig. 6.32).
Eventually, the parenchyma cells lignify and die. Various plant substances such as oils, resins, gums, aromatic substances, essential oils and tannins are deposited in the core cells. These substances are collectively referred to as extractive substances. They add color to the heart. They also have an antiseptic effect. The heart is therefore stronger and more durable than white.
It is resistant to attacks by insects and microbes. Heartwood is a commercial source of hemlock (Acacia catechu), hematoxylin (Haematoxylon campechianum), brasilin (Caesalpinia sappan) and santalin (Pterocarpus santalinus). However, the heart is susceptible to attack by wood-rotting fungi. Tree trunks are hollow as a result of their activity.
B.Creating the periderm:
To increase volume and prevent damage to the outer stromal tissue by forming secondary vascular tissue, dicotyledonous strains produce corky cambium or phellogen in the outer cortical cells. In rare cases, it may originate from the epidermis (e.g. teak, oleander), subcutaneous tissue (e.g. pear) or phloem parenchyma.
Phallogen cells divide both extrinsically and intrinsically (bipolarly) to form secondary tissues. The secondary tissue formed on the inside of the phellogen is parenchymatous or collenchymatous. It is called the secondary cortex or ulcer. Its cells have a radial arrangement.
Phellogen externally produces cork or felem. It consists of dead and compact rectangular cells that have suberized cell walls. Cork cells contain tannins. Therefore, they look brown or dark brown. The cork cells of some plants are filled with air, e.g. B. Quercus suber (cork or cork oak). Pheloderm, phellogen and fellem together form the periderm (Fig. 6.34).
The cork prevents water loss through evaporation. In addition, it protects the interior against the penetration of harmful microorganisms, mechanical injuries and extreme temperatures. Cork is lightweight, compressible, non-reactive and relatively fire resistant.
It serves as a bottle cap, cushioning and insulation. In places, phellogen creates ventilation pores instead of cork. These pores are called spiracles. Each spiracle is filled with a mass of slightly loosely arranged suberized cells called complement cells.
The spiracle ventilates pores in the bark of plants. They appear on the cortical surface in the form of raised scars with oval, round or oblong depressions (Fig. 6.34A). They are found on trees, but not on vines. They usually form in areas under which there are nozzles that facilitate gas exchange. Lentils can be scattered or form longitudinal rows.
Lenticella usually forms under a former stoma or epidermal stoma. Its edge is raised and formed by the surrounding cork cells. The spiracle is filled with loosely arranged, thin-walled, rounded and suberized (eg Prunus) or non-suberized cells, the so-called complementary cells (Fig. 6.34 B).
They close the intercellular spaces that allow gas exchange. Complementary cells are formed from loosely spaced phellogen cells and cell division of the substomal parenchyma. The suberized nature of the complementary cells prevents excessive water evaporation.
In temperate climates, the spiracles close in winter, forming compact guard cells over the complement cells.
In common parlance and in economic botany, all the dead cells that lie outside of the phellogen are collectively called bark. Due to the formation of new secondary vascular tissues, the outer cortical layers are constantly exfoliated. The peeling of the bark can occur in layers (leaf or grain bark, e.g. eucalyptus) or in irregular bands (scaly bark).
Scaly bark occurs when the phellogen appears as strips rather than rings, e.g. B. acacia (vem. kikar). The bark that forms in early vegetation is early or soft bark. The bark that forms towards the end of the growing season is late or hard bark.
The bark is insect-proof, rot-proof, fire-resistant, and acts as a heat shield. In commercial use, it is used for tanning (e.g. acacia), as a medicine (e.g. quinine - quinine) or as a spice (e.g. cinnamon, vem. dalchini). Quercus suber cork is used in the manufacture of bottle caps, insulators, floats, soundproofing materials and linoleum.
Importance of secondary growth:
1. Secondary growth increases the volume of the plant. It helps to increase air mass.
2. Secondary growth forms a cortical bark around the trunk that protects the interior from abrasion, heat, cold and infection.
3. New conductive tissues are added to replace old, dysfunctional tissues and meet the increased demands for transporting juices and organic nutrients over long distances.
Abnormal secondary growth:
This is an abnormal type of secondary growth found in some arboreal monocots (e.g. dracaena, yucca, agave) and storage roots (e.g. beetroot, sweet potato). In the stems of a monocot tree, the secondary cambium grows in the subcutaneous region. The latter forms the connective tissue and patches of meristematic cells. Meristematic spots develop into secondary vascular bundles.
Abnormal vascular bundles are also found in the cortex (cortical bundles, eg Nyctanthes) and medulla (eg Boerhaavia). In storage roots (e.g. beets), additional cambium rings appear on the outer side of the endoderm. They produce less secondary xylem but more secondary phloem. Secondary phloem contains abundant storage flesh.
Importance of secondary growth:
1. It is a way to replace old, non-functional tissues with new, active tissues.
2. Secondary growth plants can grow and live longer compared to other plants.
3. Provides a fireproof, insect proof and insulating cover around the older parts of the plant.
4. Commercial cork is a secondary growth product. It is obtained from the cork oak (Quercussuber).
5. Wood is a very important secondary growth product. Represents the secondary xylem.
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