The Cell

Plant Cell

Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:

• A large central vacuole, a water-filled volume enclosed by a membrane known as the tonoplast maintains the cell’s turgor, controls movement of molecules between the cytosol and sap, stores useful material and digests waste proteins and organelles.

• A cell wall composed of cellulose and hemicellulose, pectin and in many cases lignin, are secreted by the protoplast on the outside of the cell membrane. This contrasts with the cell walls of fungi (which are made of chitin), and of bacteria, which are made of peptidoglycan.

• Specialised cell-cell communication pathways known as plasmodesmata, pores in the primary cell wall through which the plasmalemma and endoplasmic reticulum of adjacent cells are continuous.

• Plastids, notably the chloroplasts which contain chlorophyll and the biochemical systems for light harvesting and photosynthesis, but also amyloplasts specialized for starch storage, elaioplasts specialized for fat storage and chromoplasts specialized for synthesis and storage of pigments. As in mitochondria, which have a genome encoding 37 genes plastids have their own genomes of about 100-120 unique genes and probably arose as prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.

• Unlike animal cells, plant cells are stationary.

• Cell division by construction of a phragmoplast as a template for building a cell plate late in cytokinesis is characteristic of land plants and a few groups of algae, notably the Charophytes and the Order Trentepohliales

• The sperm of bryophytes have flagellae similar to those in animals, but higher plants, (including Gymnosperms and flowering plants) lack the flagellae and centrioles that are present in animal cells.

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What else is there? Here are the three most important components of the Plant cells (and of ALL eukaryotic cells!)

Mitochondria: membrane-bound organelle found in the cytoplasm of almost all eukaryotic cells (cells with clearly defined nuclei), the primary function of which is to generate large quantities of energy in the form of adenosine triphosphate (ATP).
Mitochondria are typically round to oval in shape and range in size from 0.5 to10 μm. In addition to producing energy, mitochondria store calcium for cell signaling activities, generate heat, and mediate cell growth and death.

Golgi Apparati: membrane-bound organelle of eukaryotic cells (cells with clearly defined nuclei) that is made up of a series of flattened, stacked pouches called cisternae.
The Golgi apparatus is responsible for transporting, modifying, and packaging proteins and lipids into vesicles for delivery to targeted destinations. It is located in the cytoplasm next to the endoplasmic reticulum and near the cell nucleus.

And, last but not least, the Nucleus: it controls and regulates the activities of the cell (e.g., growth and metabolism) and carries the genes, structures that contain the hereditary information (D.N.A.)

 

 

Secondary thickening- Lignification

 

In many vascular plants, secondary growth is the result of the activity of the vascular cambium, a meristem that divides to produce secondary xylem cells on the inside of the meristem (the adaxial side) and secondary phloem cells on the outside (the abaxial side).
This growth increases the girth of the plant root or stem, rather than its length, hence the phrase “secondary thickening”. As long as the vascular cambium continues to produce new cells, the stem or root will continue to grow in diameter. In woody plants, this process produces wood.

Phloem

Phloem

In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), particularly sucrose, a sugar, to all parts of the plant where needed.

The main conducting elements of the phloem are the sieve elements, of which there are two different types: sieve cells and sieve-tube members.
Sieve cells have narrow pores, their sieve areas are quite uniform in structure, and they are distributed evenly on all walls. One of the principal differences between sieve cells and sieve-tube members is the presence of sieve plates in sieve-tube members, that are absent in sieve cells.
Sieve cells are the only type of food-conducting cells in most seedless vascular plants and gymnosperms, whereas in angiosperms only sieve-tube members are present. Sieve-tube members occur end-on-end in longitudinal series called sieve tubes. They are in contact via plasmodesmata.
Typically, the final walls are interspersed with primary pit areas (groups of plasmodesmata), that later on develop into sieve plates. Sieve tubes in the phloem of angiosperms are flanked by one or several plasma-rich, nucleated companion cells, that do not occur in gymnosperms.

 

Xylem

Xylem and phloem

The xylem is part of the vascular system that conveys water and dissolved minerals from the roots to the rest of the plant and may also furnish mechanical support.
It consists of specialized water-conducting tissues made up mostly of narrow, elongated, hollow cells. These cells may be of several types, including tracheids (the basic cell type), vessel members, fibres, and parenchyma.
Xylem constitutes the major part of a mature woody stem or root: the wood of a tree is composed of xylem. (see “Secondary thickening – Lignification” post for further details)
Xylem formation begins when the actively dividing cells of growing root and shoot tips (apical meristems) give rise to primary xylem. As the growing part of the plant builds past the xylem thus formed, the vascular cambium produces secondary xylem tissues that cover the primary xylem.
When this happens the primary xylem cells become dead and empty, losing their conducting function and forming a hard skeleton that serves only to support the plant. Thus, in the trunk and older branches of a large tree only the outer part of the wood (secondary xylem) serves in water conduction, while the inner part (heartwood) is composed of dead but structurally strong primary xylem.

Juniperus

Juniperus ‘Blue Carpet’

Juniperus (berries)

Juniperus thurifera

General description: Junipers vary in size and shape from tall trees, 20–40 m tall, to columnar or low spreading shrubs with long trailing branches. They are evergreen with needle-like and/or scale-like leaves.
They can be either monoecious or dioecious.

Common name: Juniper

Family: Cupressaceae

Height: 0.5 m. to 40  m

Aspect: sunny or partial shade

Soil requirements:

Why we use in horticulture: the low-spreading species are excellent evergreen ground cover, with green or silver foliage, depending on the species (e.g. Juniperus squamata

Notes: Juniper berries are a spice used in a wide variety of culinary dishes and best known for the primary flavoring in gin

Pieris

Pieris ‘Forest Flame’

Pieris ‘Forest Flame’ (flowers)

Pieris japonica

General description: evergreen shrub. The leaves are spirally arranged, often appearing to be in whorls at the end of each shoot with bare stretches of shoot below; they are lanceolate-ovate, 2–10 cm long and 1-3.5 cm broad, leathery textured, and with an entire or serrated margin. The young leaves in spring are typically brightly coloured. The flowers are bell-shaped, 5–15 mm long, white or pink, and arranged in racemes 5–12 cm long. The fruit is a woody capsule which splits into five sections to release the numerous small seeds.

Common name: Pieris

Family: Ericaceae

Height: 1.5  m.

Aspect: partial shade; sheltered

Soil requirements: acid

Why we use in horticulture: valued for year-round interest due to bright red new growth in early spring, chains of small, white flowers in mid-spring, and buds that remain on the plant through the winter.

Notes:

Euonymus

Euonymus europaeus

Euonymus alatus

Euonymus fortunei ‘Emerald ‘n’ Gold’

General description: Euonymus is a genus of flowering plants in the staff vine family, Celastraceae. The flowers occur in small groups, inconspicuous and of green or yellow shades. The leaves are opposite (rarely alternate) and simple ovoid, typically 2–15 cm long, and usually with a finely serrated margin.
The fruit is a pink-red four- or five-valved pod-like berry, which splits open to reveal the fleshy-coated orange seeds. The seeds are eaten by frugivorous birds, which digest the fleshy seed coat and disperse the seeds in their droppings

Common name: Euonymus;  Spindle;  Spindle Tree

Family: Celastraceae

Height: 2.5  m.

Aspect: sunny or partial shade

Soil requirements:

Why we use in horticulture: for the attractive foliage (red in autumn as in E. alatus; many species have variegated leaves, as in E. fortunei) and berries

Notes: the wood of some species was traditionally used for the making of spindles for spinning wool; this use is the origin of the English name of the shrubs.

Camellia

Camellia japonica ‘Adolphe Audusson’

Camellia japonica (leaves)

Camellia sinensis (Tea plant)

General description: they are evergreen shrubs and small trees up to 20 meters tall. Their leaves are alternately arranged, simple, thick, serrated, usually glossy, and usually three to 17 cm long. Their flowers are usually large and conspicuous, one to 12 cm in diameter, with five to nine petals in naturally occurring species of camellias. The colors of the flowers vary from white through pink colors to red, but yellow flowers are found in just a few species of camellias.

Common name: Camellia

Family: Theaceae

Height: 2 up to 20  m.

Aspect: sunny or partial shade

Soil requirements: acid; moist

Why we use in horticulture: for the flowers (white, pink, red)

Notes: Camellia sinensis, the tea plant, is of major commercial importance because tea is made from its leaves

 

Mahonia

Mahonia in flower

Mahonia x media ‘Winter Sun’

More Mahonias

General description: evergreen shrub closely related to Berberis

Common name: Mahonia

Family: Berberidaceae

Height: 1.5  m.

Aspect: sunny or partial shade

Soil requirements:

Why we use in horticulture: for the yellow flowers and the remarkable foliage

Notes:

Clematis

Clematis tangutica

Clematis virginiana

Clematis armandii

Clematis sp.

Clematis alpina

Clematis viticella

General description: the genus is composed of mostly vigorous, woody, climbing vines / lianas. The woody stems are quite fragile until several years old. Leaves are opposite and divided into leaflets and leafstalks that twist and curl around supporting structures to anchor the plant as it climbs

Common name: Clematis

Family: Ranunculaceae

Height:

Aspect: sunny

Soil requirements: high pH

Why we use in horticulture: a climber plant with a great choice of colours

Notes: