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1 = emerging lateral root, 2 = lateral root primordium, 3 = root hair nearly fully grown, 4 = mature vessel element, 5 = level at which root hairs appear, 6 = endodermal cells differentiate, 7 = level at which elongation ceases, 8 = first vessel element begins to differentiate, 9 = region of maximum cell elongation, 10 = first sieve tube matures, 11 = cell division ceases in most cell layers, 12 = first sieve element begins to differentiate, 13 = pattern of future vascular cylinder becomes recognizable at this level, 14 = zone of maximum rate of cell division, 15 = apical initials, 16 = root hair zone, 17 = growth in length, 18 = elongation zone, 19 = meristem, 20 = rootcap
Primary Root Growth
Endodermis• Stage I – Endodermal cells have a Casparian
Strip• Stage II – Endodermal cells develop suberin in
all wall areas except for plasmodesmata – especially inner tangential wall and radial walls
• Stage III – Endodermal cell wall becomes thickened with added layers of cellulose and lignin
• Passage cells – Endodermal cells that remain in Stage I condition while neighboring cells are in Stage II or III (often near xylem poles)
• Exodermis with similar properties occurs in some species just beneath the epidermis
Orchid roots are modified by development of a velamen – a multiple epidermis consisting of dead cells – an exodermis separates the velamen from the cortex
Velamen
Exodermis
Dicot root drawing at end of primary growth
1 = epidermis 2 = root hair, 3 = cortex, 4 = intercellular spaces, 5 = pericycle (one layer colored dark green), 6 = endodermis, 7 = primary phloem, 8 = protoxylem, 9 = metaxylem, 10 = initial vascular cambium, 11 = sieve plate
Secondary Growth in Roots
1 = primary xylem, 2 = endodermis, 3 = primary phloem, 4 = pericycle, 5 = vascular cambium, 6 = secondary phloem, 7 = secondary xylem
Carrot roots have more secondary phloem than xylem
1 = secondary phloem,
2 = vascular cambium,
3 = parenchymatous woody secondary xylem
Types of Mycorrhizae
Ectotrophic mycorrhizae– Sheathing mycorrhizae
Endotrophic mycorrhizae– VA (vesicular-arbuscule) mycorrhizae
Ectotrophic Mycorrhizae
• Sheathing mycorrhizae of forest trees
• Roots have club-like appearance
• 5000 species of mushrooms form these
Endotrophic Mycorrhizae
• Common on herbaceous plants and trees
• Mycelium branches in cortex of root forming arbuscles and vesicles