Xylem and phloem

• Xylem used to TRANSPORT WATER + MINERALS from roots up to leaves and other parts of the plant. 
• Xylem tissue consists of TUBES to carry water + dissolved minerals, fibres to help SUPPORT the plant. 

Xylem VESSELS: 

• In DICOTYLEDONOUS plants, most obvious features of xylem are xylem vessels elements. 
• LONG cells with THICK walls that have been impregnated with LIGNIN.
• Lignin WATERPROOFS walls of the cells. As a result, the cells die, their end walls and contents DECAY. 
• This leaves a LONG COLUMN of DEAD cells with no contents (a xylem vessel). 
• Lignin STRENGTHENS the vessel walls + prevents vessel from collapsing. This keeps the vessel OPEN even at times when water may be in short supply. 
• Ligning THICKENING forms PATTERNS in the cell wall which prevent the vessel from being TOO RIGID and allows FLEXIBILITY of the stem/branch.
• Some places, LIGNIFICATION is INCOMPLETE. Leaves PORES in wall of vessel called PITS which allow water to LEAVE 1 vessel and pass into an adjacent one or living parts of the plant.

Xylem tissue can carry WATER and MINERALS from ROOTS to TOP of plants, because:

• Made from DEAD CELLS aligned end to end to form CONTINOUS COLUMN. 
• Tubes are very NARROW so water column does not break easily + capillary action can be effective. 
• PITS in lignified walls allow water to move SIDEWAYS from 1 vessel to another. 
• Lignin DEPOSITED in walls in spiral, annular or reticuate patterns allows xylem to STRETCH as plant grows + enables stem/branch to BEND. 

Flow of water is not impeded because:

• There are NO END WALLS,
• There are NO CELL CONTENTS,
• There is NO NUCLEUS/CYTOPLASM,
• LIGNIN THICKENING PREVENTS walls from COLLAPSING. 

Xylem is a plant TRANSPORT TISSUE that carries WATER from the ROOTS to the rest of the plant. It consists of HOLLOW COLUMNS of DEAD CELLS lined end-to-end and reinforced with LIGNIN. It provides important SUPPORT for the plant.

• Function of phlom is to TRANSPORT SUGARS from 1 part of plant to another up OR down the stem. 
• Phloem TISSUE consists of 2 types of cell: SIEVE TUBE ELEMENTS and COMPANION CELLS. 

Sieve tubes:

• Not true cells as they contain very LITTLE CYTOPLASM and NO NUCLEUS. 
• Lined up end-to-end to form a TUBE, in which the plant transports sugars (usually SUCROSE)
• Sucrose is DISSOLVED in WATER to form SAP.
• Unlike xylem vessels, this tube contains CROSS-WALLS at INTERVALS. Cross-walls are PERFORATED by PORES to allow sap to FLOW. 
• Sieve tubes have very THIN WALLS and are usually 5 or 6 sided.

Companion cells: 

• SMALL cells, each with a LARGE NUCLEUS and DENSE CYTOPLASM. 
• Have numerous MITOCHONDRIA to produce ATP and carry out METABOLIC PROCESSES needed by SIEVE TUBE ELEMENTS. 

• Includes using ATP as a source of energy to load SUCROSE into the SIEVE TUBES. 
• CYTOPLASM of companion cells and sieve tube elements are LINKED through many PLASMODESMATA. 
• Plasmodesmata are GAPS in the cell WALLS allowing COMMUNICATION and FLOW of substances between the cells. 

Phloem is a plant TRANSPORT TISSUE that carries the PRODUCTS OF PHOTOSYNTHESIS to the rest of the plant. It consists of SIEVE TUBE ELEMENTS and COMPANION CELLS.

Transport system in plants moves water in VASCULAR TISSUE.

• XYLEM transport WATER and soluble MINERALS UP the plant.
• PHLOEM transports SUGARS and other assimilates UP and DOWN the plant. 
• Xylem and phloem are found TOGETHER in VASCULAR BUNDLES. 
• Around the vascular bundle is a sheath of cells called the ENDODERMIS which has a key role in getting water into the xylem vessels. 
• Just inside the endodermis is a LAYER of MERISTEM CELLS (that can DIVIDE) called the PERICYCLE. 

• Vascular bundle is found at the CENTRE of a young root. 
• There is a large central core of xylem, often in the shape of an X.
• Phloem is found in between the arms of the X-shaped xylem. 
• This arrangement provides STRENGTH to WITHSTAND PULLING forces to which roots are exposed to. 

Xylem and phloem in the stem:

• Vascular bundles are found near the OUTER EDGE of the stem. 
• In NON-WOODY plants, the bundles are SEPARATE and DISCRETE. 
• In WOODY plants, the bundles are SEPARATE in YOUNG stems, but become CONTINOUS in OLDER stems. 
• This means there is a complete RING of vascular TISSUE just under the BARK of a tree. 
• This arrangement provides STRENGTH and FLEXIBILITY to withstand the BENDING forces to which stems and branches are exposed.

Xylem is found towards the INSIDE of each vascular bundle. Phloem is found towards the OUTSIDE of each bundle. 

• In between the xylem and phloem is a LAYER of CAMBIUM. 
• The cambium is a LAYER of MERISTEM CELLS that DIVIDE to produce NEW xylem and phloem. 

Xylem and phloem in the leaf:

• Vascular bundles form the MIDRIB and VEINS of a leaf.
• There are 2 major groups of FLOWERING plants: DICOTYLEDONS + MONOCOTYLEDONS.
• These 2 groups have DIFFERENT patterns of veins.