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Internal Structure... Stem, Root & Leaf

The internal structure of stem consists of the epidermis, hypodermis, cortex, endodermis, pericycle, pith, medullary ray and the vascular tissue system. The stem also shows secondary growth.


Internal structures to know of herbaceous stems:

Pith: Large central area for storage & support.

Cambium: Found as a circle around inner stem & outer surface. Forms woody secondary tissue for support.

Cortex: Storage area between cambium and epidermis.

Epidermis: Thin layer of skin cells. Protection.

Xylem: Water conduction up.

Phloem: Sap (organic molecules) conduction, down to roots.


Internal Anatomy of Woody Stems


Pith: Original stem at very center of stem.

Xylem: Water conduction tubes connect leaf to roots. Inner most xylem dies & forms wood.

Phloem: Outer tubes just inside bark to carry food from leaves to roots.

Heartwood: Dead wood (xylem) in center of stem. It is either dry or filled with tars. Frequently darker than live wood.

Sapwood: Live outer wood conducting water and sap.

Cambium: Special cells that make new wood (xylem) & new phloem & bark to make tree trunk thicker. Found between xylem & phloem.

Bark: Outer protective (from insects, fire, and injury) and waterproof layer of stem. Made by cambium. Outer barks is dead, inner is alive.

Springwood: Light colored rings of xylem in wood made when growing season is good (spring & early summer).

Summerwood: Darker colored tree rings made when growing season is poor (late summer, fall, & winter).


The Internal Structure of Root


The internal structure of root consists of epidermis, cortex, endodermis, pericycle, vascular system and pith. The anatomy of the dicot root and monocot root is different from each other.


Root structure:


a) Epidermis: Outer layer of cells ("skin"). Protection.

b) Root Hair: An extension of specialized root epidermal cells increasing surface area for absorption of water & minerals.

c) Cortex: Region between epidermis & vascular cylinder. Supports plant parts & stores food.

d) Endodermis: Layer of cells just outside vascular cylinder.

e)Pericycle: Cylindrical layer of cells inside endodermis. Origin of cork & secondary (side) roots.

f) Vascular Cylinder: Arrangement of vascular tissues as a central cylinder in roots.

g) Xylem: Living (outer) vascular system carrying water & minerals throughout plant.

h) Phloem: Living (inner) vascular system carrying dissolved sugars and organic compounds throughout plant.


Internal Structure of a Leaf

The internal structure of a leaf consists of the epidermis; mesophyll containing palisade and spongy parenchyma; and the vascular system. The monocot leaf has certain anatomical difference from the dicot leaf.


Vascular Tissue


This is to do with the xylem and the phloem. Remember that the xylem is always at the top of the leaf, and the phloem at the bottom. This is because the xylem transports water(needed for photosynthesis) to the leaf cells, and diffuses into the chloroplasts.
The phloem transports food from the mesophyll cells to the rest of the plant. The xylem and phloem are vascular tissue that are situated in the leaf vein. The vein is supported by fibres(sclerenchyma), which keep the shape of the leaf, flat.


Mesophyll


There are two main types of Mesophyll cells, the palisade and spongy cells.

Palisade - contain the most chloroplast, and are at the top of the leaf. Are closely packed - allowing more cells and therefore more chloroplasts to be near the surface. There are small intercellular spaces inbetween them which allows gaseous exchange to occur.

Spongy - have less chloroplasts, and larger intercellular spaces. They also store carbohydrates made by photosynthesis. Dissolved carbohydrates diffuse into the phloem, to be transported to the rest of the plant.


Chloroplasts


Structures which contain chlorophyll and this is where the photosynthesis actually happens! They have a double membrane, which is selectively permiable. This allows H2O and CO2 to diffuse into it, and O2 to diffuse out of it.

These are the structures which contain chlorophyll, and this is where the photosynthesis actually happens! They have a double membrane, which is selectively permiable. This allows H2O and CO2 to diffuse into it, and O2 to diffuse out of it.

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