CELLS AND TISSUES OF PLANTS
**********
In plants, there are several types of tissue:
GROUND TISSUE: the most basic type of tissue. This is the basic "flesh" of the plant.
three basic types of cells found in ground tissue:
There are several different types of parenchyma cells/tissue, categorized by function. For example, a CHLORENCHYMA cell is a parenchyma cell loaded with chloroplasts and specialized to perform photosynthesis.
- parenchyma cells are very generalized, shoebox shaped with thin cell walls.
These cells look basically like parenchyma cells, except that their walls are much thicker and more cellulose-rich, especially at the corners.
Collenchyma tissue is most often located under the epidermis, where it offers flexible support to plant stems.
These cells are dead and hollow at maturity. They offer compressional strength, and are composed of tremendous amounts of sclerenchyma along with an important structural molecule called lignin.
Two basic types of sclerenchyma cell:
1. fiber - elongate and tapered, such as what you'll find in cotton fibers and interspersed in wood.
2. sclerid - a little, stone-shaped crumble. Typical one is found in the flesh of pears (the little crunchy things that get stuck in your teeth.)
A major component of sclerenchyma cell walls is LIGNIN - a large, complex molecule composed of 6-carbon rings linked together in a sheet. These rings often have other substances stuck onto them, such as sulfur, etc.
Note: during paper manufacture, sodium bisulfite is used to dissolve lignin. This produces nasty, stinky toxic compounds which get washed out into the river. (Paper mills are not good neighbors.)
However, lignin can be used to manufacture useful things such as
vanillin (artificial vanilla flavor), plastics and other stuff requiring
aromatic compounds as a base.
VASCULAR TISSUE - the transport system of the plant.
Location: in all organs of the plant.(root, stem and leaf)
Function: xylem transports water and dissolved minerals from the roots to the rest of the plant.
phloem transports dissolved sugars, amino acids and other organic compounds from source to sink.
*************
Recall...A PRIMITIVE characteristic is one that has changed very little from an ancestral characteristic.
A DERIVED characteristic is one that has changed significantly from an ancestral form.
********************
Xylem is composed mostly of one or more of the following:
Phloem is composed mostly of one or more of the following:
Both types of cells may be found in any given species, but in general, vessel elements are usually found in only the more highly derived types of plants (such as flowering plants).
NOTE: tracheids and vessel elements are dead and hollow at maturity
Phloem consists of more primitive SIEVE CELLS and more derived SIEVE TUBE ELEMENTS.
These are alive and contain a protoplast (cytoplasm) at maturity, but they do not have a nucleus and organelles. How do they survive?
Sieve cells have albuminous cells which feed them.
Sieve tube elements have companion cells which feed them.
The albuminous cells are ANALOGOUS, not HOMOLOGOUS:
Sieve cell and albuminous cell have different parent cells.
Sieve tube elements and companion cell have same parent cell!
DERMAL TISSUE - The covering of the plant
MERISTEMATIC TISSUE: the source of all other tissues.
APICAL MERISTEM: totipotent tissue located at the tip (apex, hence the name) of every shoot and root.
(A "totipotent" cell can develop into any other kind of cell. Animals have them, too, and they are of great interest to medicine.)The Apical meristem differentiates into the three PRIMARY MERISTEMS:
From inside to outside:
PROTODERM --> EPIDERMIS
PROCAMBIUM --> VASCULAR TISSUE (xylem, phloem)
GROUND MERISTEM --> GROUND TISSUES (parenchyma, collenchyma and sclerenchyma)
(note: meristems located in the "elbows" where leaf meets stem are located in the "axillary buds")
the primary MERISTEMS result in primary growth, that is, increase in height.
A quick look at some of the tissues:
1. Vascular Tissue:
xylem - located on the innermost ring of the stem/root; it carries water and minerals from the roots to the rest of the plant.
phloem - located on the outermost ring (of vascular tissue) in the stem/root; it carries dissolved proteins, amino acids, sugars and other complex compounds in any direction (from a "source" to a "sink" as we will learn later)
Located between the two types of vascular tissue is the VASCULAR CAMBIUM. This is a SECONDARY MERISTEM which remains meristematic, although the rest of the procambium has matured into xylem or phloem.
The vascular cambium produces secondary growth: it increases the girth of the plant by turning into XYLEM towards the inside of the stem/root and into PHLOEM towards the outside of the stem/root.
As the xylem increases in girth, the phloem and bark eventually get "squeezed" so tightly at the outside of the stem/root that they split and slough off.
* secondary xylem (i.e., that produced by the vascular cambium) is permanent
* secondary phloem (produced by vascular cambium) is not permanent; it sloughs off as the plant grows.
Meristems that increase the height of the plant are called primary meristems.
Meristems that increase the girth of the plant (and are themselves generated by the primary meristems) are called secondary meristems.
Many plants LACK secondary meristems. These have only primary growth and never produce true wood (which is only grown by secondary meristem--vascular cambium). They are said to be HERBACEOUS.
Plants that have woody growth from a vascular cambium are said to be WOODY.
*******************