Chapter 31, pages 668-691
Plant Anatomy-2 10:29-14:44
I. Secondary Growth in Roots
A. Formation of vascular cambium
1. Forms from the residual procambium between the xylem and phloem
2. Forms from the pericycle at the tips of the protoxylem poles.
3. Finally all parts link up into a single wavy ring
B. Activity of vascular cambium
1. When the ring is complete
the vascular cambium of the root acts just like the vascular
cambium
of the stem
2. The vascular cambium
in the region of the primary phloem is more active than the
other vascular
cambium until the ring of vascular cambium become circular and
not wavy
3. Old root has annual rings just like a stem
4. Not possible to tell an old root from an old stem
C. Cork cambium
1. Also forms a cork cambium as does the stem
2. Originates in the pericycle in the root (originates in the cortex in the stem)
3. In the old root it will
form in the outer regions of the living
phloem
as is the case for
the stem
D. Lateral root formation
1. Lateral roots originate from cells in the pericycle at the tip of a xylem pole
2. Grows out through the
cortex
and
epidermis
by crowding and dissolving the cells
ahead of the
root
cap
3. Lateral roots emerge
with a root cap and continue to grow and elongate just like any
other root
II. Mycorrhizae
A. Roots of plants in symbiosis with a fungus
B. Two types of mycorrhizal roots
1. Ectotrophic
a. Fungus external or on surface of root or cortical cells
b. Occurs in pines, birches, willows, and oaks
c. Drastic change occurs in root shape
(1) 0.5 cm long
(2) No root cap
(3) Produces a simple monarch root
(4) Fungus squeezes in between the cell walls of the
cortex
and forms a covering
sheath or
mantle
around the root
2. Endotrophic
a. Fungus enters and actually penetrates the cortical cells
b. No mantle forms over the root
3. More efficient in mineral absorption
4. Not absolutely essential for the growth of the plant
5. Mycorrhizae may be beneficial in addition by:
a. Secreting hormones
b. Secreting antibacterial agents
III. Bacterial Nodules
A. Fix nitrogen by changing N2 (molecular nitrogen) to NH4 (ammonium ion)
B. NH4+ is one of the forms of nitrogen that plants like
C. Rhizobium is the bacterium responsible for the fixing of nitrogen in the roots of legumes
D. Infection process:
1. Root hairs become infected with Rhizobium
2. Thread of infection passes through the root hair to the epidermis to the cortex
3. Bacteria divide and stimulate cortex to grow into a nodule
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