article : PLANT CELLS AND TISSUES

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Recall that cells are the fundamental units of life. Cells are arranged into tissues and, in plants, tissues are arranged into tissue systems

I. Meristems

A. Following development of the embryo the formation of new cells, tissues and organs becomes restricted almost entirely to meristems (undifferentiated tissue from which new cells arise)

B. Within the meristems certain cells are able to divide repeatedly into two sister cells:

1. Initial - the self perpetuating sister cell that remains in the meristem

2. Derivative - the other sister cell which moves into the plant body. These may also continue to divide until they become differentiated into specific types of cells

C. There are two types of meristems:

1. Apical - they occur at the tips of the roots and shoots and they are responsible for extension of the plant body

a. Apical meristems give rise to the primary tissues of the plant, forming the primary plant body

b. Since there are three tissue systems in plants (see below), the apical meristem gives rise to three primary meristems:

1) Protoderm - gives rise to the outer, protective coating of the plant

2) Procambium - gives rise to the vascular tissue

3) Ground meristem - gives rise to ground tissue

2. Lateral - they are responsible for lateral growth and they produce the secondary tissues which constitute the secondary plant body

a. Lateral meristems are called cambia (cambium). There are two types:

1) Vascular cambium - gives rise to secondary vascular tissue, xylem to the inside and phloem to the outside

2) Cork cambium - gives rise to mostly cork

D. Growth of the plant body involves both cell division and enlargement

E. Eventually cells may undergo differentiation, i.e. they become different from each other and from the original meristematic cells from which they originated. At maturity differentiated cells may be living or dead

F. The basic plant body tissue patterns are established by early meristematic activity. Cell division and enlargement largely affects the plant shape

II. Tissues

A. Tissues are groups of cells that are structurally and/or functionally distinct. Tissues are further arranged to form tissue systems, which are initiated during the development of the embryo by the ground meristem, procambium and protoderm

B. Three tissue systems in plants:

1. Ground � contains three types of tissues: parenchyma, collenchyma and sclerenchyma

2. Vascular - consists of xylem and phloem tissues

3. Dermal - consists of epidermis which may later be replaced by periderm

C. Tissue types:

1. Parenchyma - is the progenitor of all other tissues and the most common component of ground tissues. It is composed of parenchyma cells which perform virtually all the metabolic activities of plant cells

a. Parenchyma cells are living at maturity, polyhedral (many sided) shaped and capable of cell division

b. In the primary plant body they occur as continuous masses in the cortex of roots, stems, piths of stems and roots, and leaf mesophyll. They may also occur as vertical strands of cells in vascular tissues and also as horizontal strands (rays) in secondary vascular tissues

c. Some may have secondary walls

d. Those with only primary walls play an important role in regeneration and wound healing. They may be lignified, suberized or cutinized

2. Collenchyma - composed of collenchyma cells which are living at maturity

a. They usually occur as discrete strands or as continuous cylinders beneath the epidermis in stems and petioles (e.g. celery). They also border veins in dicots

b. Collenchyma cells are relatively long, with nonlignified primary walls which allows them to stretch

c. Collenchyma cells commonly have unevenly thickened cell walls

d. They provide support, largely for the primary plant body

3. Sclerenchyma - composed of sclerenchyma cells which are non-living and lack protoplasts at maturity

a. Sclerenchyma cells may be found in all parts of both the primary and secondary plant body

b. They have thick, lignified secondary walls. They provide strength and support in parts that have ceased elongating

c. There are two type of sclerenchyma cells:

1) Fibers - long slender cells which occur in strands or bundles, e.g. hemp, flax and jute. They provide support and some storage

2) Sclerids - variable in shape, often branched, may occur singly or in groups in ground tissues throughout the plant. They make up the seed coat of seeds, shells of nuts, stones of drupes, and give the pear its gritty texture. Their function is primarily for protection

4. Xylem - principle water conducting tissues in vascular plants. May be primary or secondary in origin

a. The principal conducting cells are called tracheary elements

b. They are elongated cells which are dead at maturity

c. Two types of tracheary elements:

1) Tracheids - generally long and thin, may have pits in their walls but they have imperforate end walls. They occur in the seedless vascular plants, gymnosperms and some primitive angiosperms

2) Vessel members - elongated shape but not as long or thin as tracheids. They have perforate end walls. Vessel members are joined together into long tubes called vessels. Occur primarily in the angiosperms

5. Phloem - collective term for the principle food conducting tissue in vascular plants. The individual cells are called sieve elements. Sieve refers to clusters of pores called sieve areas, through which the protoplasts of adjacent sieve elements are interconnected

a. Sieve elements are living at maturity but they either completely lack nuclei or they contain only remnants of them.

b. At maturity the tonoplast of the vacuole disappears so that there is no differentiation between the cytoplasmic and vacuolar contents. The remaining elements of the protoplast (plasma membrane, plastids, smooth ER, mitochondria) are distributed along the cell walls. There are no ribosomes, dictyosomes, or nuclei

c. Sieve tube members are usually associated with parenchyma cells called companion cells which are complete living cells connected to them via numerous plasmodesmata. The companion cells perform many of the metabolic activities for the sieve tube members

d. There are two types of sieve elements:

1) Sieve cells - have narrow pores, no sieve plates. They occur in most seedless vascular plants and in gymnosperms

2) Sieve tube members - have large pores on sieve plates, usually on end walls. Sieve tube members are stacked end to end to form sieve tubes. They occur only in angiosperms

6. Epidermis - the outermost layer of the primary plant body covering leaves, floral parts, fruits, seeds, stems and roots (until they undergo secondary growth).

a. The epidermis is generally only one layer thick

b. The epidermis is composed mostly of unspecialized cells, either parenchyma and/or sclerenchyma

c. The epidermis of leaves often contain trichomes, various types of hairs

d. Guard cells which form stomata, pores for gas exchange, are also present on one or both surfaces

7. Periderm - replaces the epidermis in stems and roots with secondary growth

a. Commonly composed of parenchyma and sclerenchyma cells, as well as cork and phelloderm, which will be discussed under secondary growth

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