Cell

Cell [ Biology ]

Dictionary of botanic terminology - index of names


	A cell is the smallest structural self replicating unit of an organism that is capable of independent functioning. All living organisms are composed of one or more cells.

A cell is a living structure consisting of one or more nuclei, cytoplasm, and various organelles, all surrounded by a semipermeable cell membrane, cells within an organism may be more or less specialised and differentiated for particular functions.
Animal cells and plant cells differ fundamentally.

Plant Cell anatomy [ Biology ]

Plant cells share many structures with animal cells: Plasma membrane, Nucleus and nucleolus, Mitochondria, Ribosomes, Endoplasmic reticulum, Golgi apparatus, Peroxisomes, Microtubules.
Plant cells differ from animal cells in lacking some structure: centrioles and intermediate filaments.
Plant cell have some distinctive organels and structure: Chloroplast, a cell wall and large vacuoles

PLANT CELL TYPES
1.	PARENCHYMA - most numerous cells in young plants. These cells are living when they perform their function (although may still serve intended purpose when dead, e.g. phellem).
	Shape: commonly polyhedral – variable or shaped like soap bubbles with a large central vacuole. Cell Wall: Usually have thin walls consisting of a primary cell wall and shared middle lamella or primary and secondary, may be lignified, suberized or cutinized Location: can be found in many regions of the plant. Parenchyma tissue occurs in the cortex, pith, pith rays, xylem and phloem. Function: Parenchyma performs many of the living functions of the plant, including respiration, digestion, photosynthesis – essentially all metabolic functions. Storage & conduction tissues frequently contain parenchyma. Most store substances such as starch and lipids: Leaves have important parenchyma cells that are specialized for photosynthesis
2.	COLLENCHYMA - an elongated supporting cell with thick cell walls that remains alive. Primary cell wall is thickened, no secondary cell wall.
	Shape: elongated Cell Wall: primary only, highly hemicellulosic and pectic, not lignified. Function: Living at maturity. Provides more flexible support to tissues than sclerenchyma cells. The primary distinguishing character is that it is extensible. Since it depends on turgor, it is sensitive to wilting. Location: Collenchyma may be distributed peripherally as a cylinder in young stems (just beneath the epidermis) or only in patches in ribs also supports petioles, nonwoody shoots, growing organs.. Collenchyma is also found along the veins in leaves of dicots.
3.	SCLERENCHYMA - Often dead at functional maturity, but may remain living. generally very long. Have heavily thickened secondary walls because their main function is support, must be distinguished in fibres and sclereids:
	I	FIBRES - provide for rigid support in wood (e.g., bark) and other parts and are typically nonliving cells.
		Shape: very long and organized into bundles. Cell wall: primary and thick secondary walls – often lignified. Location: Sometimes in cortex of stems, most often associated with xylem and phloem. Also found particularly in the leaves of monocots.
	II	SCLEREIDS - variously shaped and densely packed as in nut shell or other seed coats.
		Shape: Often isodiametric; usually shorter than fibres. Cell wall: primary and thick secondary walls - generally lignified. May be living or dead at functional maturity.
4	TRACHEARY ELEMENTS - Elongated tapering,
	I	TRACHEIDS - Elongated tapering, dead at functional maturity
		Shape: very long Cell wall: : primary and secondary, lignified Contains pits but no perforations. Function: Xylem Chief water-conducting element in gymnosperms and lower vascular plants, also found in Angiosperm xylem.
	II	VESSEL MEMBER - Elongated tapering, Dead at functional maturity.
		Shape: Vessels are composed of vessel members in series connected by perforation plates. Cell wall: primary and secondary, lignified. Function: Xylem Chief water-conducting element in Angiosperm xylem.
5	SIEVE ELEMENTS - elongated, tapering. Cells are living in their functional state. Higher plants’ sieve cells lack a nucleus at maturity, but some lower vascular plants are exceptional. Tonoplast often not discerned at functional maturity.
	I	SIEVE CELL - very long
	Shape: very long Cell wall: primary in most species, with sieve areas on the walls. Sieve areas are wall areas with pores through which the protoplasts of adjoining cells are connected. Callose is often associated with wall and pores. Function:: Phloem Chief food-conducting element in phloem of gymnosperm and lower vascular plants.
	II	ALBUMINOUS CELL -
		Cell Shape: generally elongated. Cell Wall: primary. Cells are living in their functional state and are associated with sieve cells, although they are generally not derived from same mother cell as sieve cell. Location: Phloem Function: Believed to play a role in movement of food into and out of the sieve cell

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Holdfast roots [ Botany ]
Dictionary of botanic terminology - index of names

Some species of climbing plants develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place.

Climbing plants, like the poison ivy (Toxicodendron radicans), Boston ivy (Parthenocissus tricuspidata), and trumpet creeper (Campsis radicans), develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place. Usually the Holdfast roots die at the end of the first season, but in some species they are perennial. In the tropics some of the large climbing plants have hold-fast roots by which they attach themselves, and long, cord-like roots that extend downward through the air and may lengthen and branch for several years until they strike the soil and become absorbent roots.

Major references and further lectures:
1) E. N. Transeau “General Botany” Discovery Publishing House, 1994