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GROWTH The Onion Root Tip As a Model for Growth. ZONE C ZONE B ZONE A Root Cap Click on the red spots to find out what is happening in each zone. l.s.

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Presentation on theme: "GROWTH The Onion Root Tip As a Model for Growth. ZONE C ZONE B ZONE A Root Cap Click on the red spots to find out what is happening in each zone. l.s."— Presentation transcript:

1 GROWTH The Onion Root Tip As a Model for Growth

2 ZONE C ZONE B ZONE A Root Cap Click on the red spots to find out what is happening in each zone. l.s. root tip ZONE D

3 Zone of Cell Division Information: New cells are produced by MITOSIS Information: New cells are produced by MITOSIS Information: Note New cells are CUBOIDAL in shape Have thin cellulose cell walls Have dense cytoplasm with no vacuole Information: Note New cells are CUBOIDAL in shape Have thin cellulose cell walls Have dense cytoplasm with no vacuole

4 Zone of Expansion Information: Cells in this region have Thin cellulose cell walls (primary) Small vacuoles cytoplasm with low (water potential) Information: Cells in this region have Thin cellulose cell walls (primary) Small vacuoles cytoplasm with low (water potential) Information: Of surrounding solution is HIGH so water moves into vacuole of cell Information: Of surrounding solution is HIGH so water moves into vacuole of cell As a consequence of water movement the volume of the cells increases and pushes out the thin cell walls

5 Zone of elongation Information: The arrangement of cellulose fibres in the cell wall results in the expansion of cells only in a lengthways direction Information: The arrangement of cellulose fibres in the cell wall results in the expansion of cells only in a lengthways direction Information: Cells in this region are typified by Large vacuoles Column shape Thin (primary)cell walls Information: Cells in this region are typified by Large vacuoles Column shape Thin (primary)cell walls Water continues to enter the cells down a water potential gradient – as a consequence the cells elongate

6 As the cells mature, it lays down more cellulose inside to form a secondary cell wall. This prevents further growth but allows for DIFFERENTIATION e.g. Zone of Differentiation If the secondary cell wall contains LIGNIN, then the cells will develop into XYLEM or SCLERENCHYMA The end cell walls may become perforated as the cells develop into PHLOEM SIEVE TUBE ELEMENTS

7 A simple experiment – A bean root can be marked at 1mm intervals. The bean can be left to grow and the root observed every 12 hours. Note – the region of greatest elongation is just behind the tip


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