1. Mature (Permanent) tissues 113 C- Vascular system 113
- There are two types of collateral vascular bundles:
a- Open collateral vascular bundles:
- In Dicots. And Gymnosperms.
- Each vascular bundle posse-
sses cambium between xylem
& phloem, which can produce
secondary xylem and second-
ary phloem during secondary
growth.
- The addition of secondary
xylem & phloem opens the
bundle by separate the 1ry
xylem & 1ry phloem from each
other;

2. Mature (Permanent) tissues 114 C- Vascular system 114
b- Closed collateral vascular bundles :
- In many Monocots and the lower herbaceous Dicots.
- No cambium between xylem & phloem tissues, so
no secondary vascular tissues are formed.
- Each vascular
bundle is ensh-
eathed completely
with sclerenchyma
fibres (or bundle
sheath) which tight-
en the closureof
the bundles.

3. Mature (Permanent) tissues 114 C- Vascular system 114
II- Bicollateral vascular bundles:
- Similar to open collateral
vascular bundles, but with
two phloem regions one is
located internally (near the
pith) & the other is located
externally to the xylem (near
the cortex).
- The cambium exists only
betweenthe outer phloem
& xylem.
- All vascular tissues phloem
(outer & inner) & xylem are on
the same radius.

4. Mature (Permanent) tissues 114 C- Vascular system 114
- This type of vascular bundles occur in certain ferns and is
common in stems of members of the family Cucurbitaceae,
(e.g. cucumber, squash,..), and some other families.
III- Concentric vascular bundles:
- This type is characterised by the location of either phloem
or xylem centrally, where it is encircled by the other type.
- There are two types of concentric vascular bundles:
(a) Amphivasal (or Amphixylic) vascular bundles :
- In which the xylem surrounds the phloem.
- Occur in some Monocot plants and certain Dicot stems and
many rhizomes (e.g. Dracaena stem).

5. Mature (Permanent) tissues 114 C- Vascular system 114
(b) Amphicribral (or Amphiphloic) vascular bundles:
- In which the phloem surrounds the central xylem.
- Occur in ferns and small bundles of flowers, ovules and
certain leaves and in pith bundles of some families (e.g.
Polypodium rhizome).

6. Mature (Permanent) tissues 115 C- Vascular system 115
IV- Radial arrangement of xylem & phloem strands:
- In roots the phloem and xylem are present as separate
strands which alternate and lie on different radii, separated from each other by parenchymatous cells.
- No definite bundles are formed.
- In dicot roots the number of xylem (or phloem) strands is
between 2-8, while in Monocot roots it is generally more
than 8.

7. Mature (Permanent) tissues 116 C- Vascular system 116