Read the section in the text which deals with roots. That part
of a plant that absorbs water and mineral from the soil and transports
it to the
stem is called the root. Most roots of a plant are located under
the soil and anchor the plant in this
location. However some plants such as orchids have aerial roots
which absorb water from the air.
Roots are also used by plants to store food (starch) for future
use by the plant. Roots may
classified according to morphology and site of origin.
Root Types Involving Morphology
Tap Roots : one large main root and several
smaller secondary roots; examples –
carrots, beets
Fibrous Roots: many roots of the same size;
example grass
Root Types Involving Origin:
true roots : roots which develop from
seed
adventitious roots : roots which develop
from other plant parts: stem, leaves,
buds
Prop roots are a type adventitious root
To learn the structures in a root you must be able to identify the varies tissues located in a cross section and a longitudinal section of a root. By examining the longitudinal section of a root you can see the relationship between the three areas of embryonic tissues (primary meristem) and the mature tissue located in a mature stem.
Examine Figures Handed Out In Lecture
Longitudinal Section of Root.
Apical meristem - cell division occurs - All cells located
in the root are formed by
the process of mitosis which occurs in the apical meristem
Root cap - The root cap protects these delicate cells from being
crushed as the root
tip grows through the soil.
Zone of cell elongation - The newly formed cells will undergo elongation
above the
zone of cell division thus pushing the root apex through the
soil.
Zone of maturation - The cells above the zone of elongation will
mature and
differentiate into specific tissues depending on their location
in the root.
Three primary meristems (protoderm, ground meristem and procambium)
Protoderm develops into the epidermis
Ground Meristem develops into the cortex
Procambium develops into vascular tissue (xylem and phloem)
Examine Figure Handed Out In Lecture
Epidermis has extensions called root hairs which absorb water and minerals
Root hairs are unicellular epidermal cells
located only at the tips (beginning of
the region of maturation) of all roots.
They increase the surface area of the root
tip for water absorption. Most of
the water absorbed by roots enters through the
root hairs. Root hairs are found in an
area of approximately four inches above
the tip of the root.
Cross Section Of Root:
Examine Figure Handed Out In Lecture
Epidermis - prevents water loss
Cortex - made up of parenchyma cells - function to store starch
Endodermis (single ring of cells around
the vascular tissue) - provides selective
absorption of mineral from the exterior
to the xylem. It performs this task by
forcing water (minerals) to
pass through the living cell membrane and not
through cell wall by diffusion. Because
of the involvement of the cell membrane
mineral must pass through by active
transport. Active transport allows for
selective absorption. Thus, for minerals
to move from the cortex to the vascular
tissue they are screened by the membrane
of the endodermis. The way the
endodermis forces water to move through
the living part is that there is a layer
of wax (Casparian Strip) around the
top, side, and bottom edges with the front
and back open. Water can not move
through this wax.
Examine Figure Handed Out In Lecture
Apoplast tissue of the root- water and minerals will move through root through nonliving parts of cell until the endodermis is encountered.
Symplast tissue of the root – water and minerals may move through living parts of the cell (slow process). Movement through endodermis must be symplastic. This allows for active transport and selectivity.
Pericycle - retains the potential to become meristematic. In the older part of the root the pericycle tissue forms secondary or lateral roots. These roots develop at a forty five degree angle from the root it originates.
Phloem - Conducts photosynthates, located in the arms of the xylem
tissue
Xylem - Conducts water, center of root
Protophloem/ Metaphloem
Protoxylem/ Metaxylem
Review differences between monocots and dicots as to examples
and cotyledons.
Transverse section of monocot roots
Strands of xylem and phloem are distributed in ground tissue.
Pith present
Mycorrhizae Symbiotic relationship of soil fungi and roots.
Do not move beyond the endodermis
Root Nodules for Nitrogen Fixation
Refer to figures in text.
Rhizobium
bacteria that infects cortex with an infection thread and
stimulates the cortex cells to undergo mitosis to form root
nodule
Bacteria cells fill plant cells forming bacteriods
Nitrogen Fixation N2 converted
into NH4
Haustoria
parasitic plants form these modified roots
Refer to figures in text.
Indicate from which primary meristem each of specific tissues listed
above are formed (epidermis,
root hairs, cortex, vascular tissue - xylem and phloem, endodermis,
pericycle and vascular
cambium).
| Primary Meristems | Tissues |
| protoderm | |
| ground meristem | |
| procambium |
Click to view apical meristem http://www.mancol.edu/science/biology/plants_new/anatomy/rootder1.html
Click to view root dermal tissue http://www.mancol.edu/science/biology/plants_new/anatomy/rootder2.html
Click to view ground tissue http://www.mancol.edu/science/biology/plants_new/anatomy/rootgrnd.html
In your notes draw a cross section of a root at the region of
differentiation or maturation and label
the following tissues: epidermis, cortex, endodermis, pericycle, primary
xylem, primary phloem.
Using the text write a function for the tissues listed above.
| Tissues | Function |
| epidermis | |
| cortex | |
| endodermis | |
| pericycle | |
| primary xylem | |
| primary phloem |
Can you identify the various cell types which makeup the various tissues
listed above?
| Tissues | Cell Types - parenchyma, sclerenchyma, collenchyma, epidermal, sieve tube, vessel element |
| epidermis | |
| cortex | |
| endodermis | |
| pericycle | |
| primary xylem | |
| primary phloem |