2) Vascular Organization

Question 1

1) Define stele

Answer 1

• Vascular tissue of axes & closely associated tissues.

Question 2

2) List three features of Protosteles

Answer 2

1. Earliest vascular plants had stems with a central core of vascular tissue.
2. This consisted of a cylindrical strand of xylem surrounded by a region of phloem.
3. Around the vascular tissue there might have been an endodermis that regulated the flow of water into & out of the vascular system.

Question 3

3) List four features of the Haplostele (Protostele).

Answer 3

1. Consists of a cylindrical core of xylem surrounded by a ring of phloem.
2. Endodermis surrounds the stele.
3. Centrarch = protoxylem in the center of a metaxylem cylinder.
4. Prevalent in members of the rhyniophyte grade, such as Rhynia.

Question 4

4) List four features of the Actinostele (Protostele).

Answer 4

1. The core is lobed or fluted.
2. Found in many species of club moss (*Lycopodium *and related genera).
3. **Exarch **(protoxylem external to the metaxylem) + consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem.
4. Exarch protosteles are a defining characteristic of the lycophyte lineage.

Question 5

5) List four features of the Plectostele (Protostele).

Answer 5

1. May be derived from actinostele.
2. Plate-like regions of xylem appear in transverse section surrounded by phloem tissue.
3. In fact. these discrete plates are interconnected in longitudinal section.
4. Some modern club mosses have plectosteles in their stems.

Question 6

6) List three features of the Siphonostele.

Answer 6

1. Have a region of ground tissue called the pith internal to xylem.
2. The vascular strand comprises a cylinder surrounding the pith.
3. Have interruptions in the vascular strand where leaves (typically megaphylls) originate (called leaf gaps).

Question 7

7) Siphonosteles can be:

Answer 7

1. Ectophloic: Phloem present only external to the xylem.
2. Amphiphloic: Phloem both external and internal to the xylem. Among living plants, many ferns & some Asterid flowering plants have an amphiphloic stele.

Question 8

8) Amphiphloic siphonostele can be called a:

Answer 8

• Solenostele:
  1. If the cylinder of vascular tissue contains no more than one leaf gap in any transverse section (i.e. has non-overlapping leaf gaps).
  2. Primarily found in fern stems today.
• Dictyostele:
  1. If multiple gaps in the vascular cylinder exist in any one transverse section.
  2. The numerous leaf gaps and leaf traces give a dictyostele the appearance of many isolated islands of xylem surrounded by phloem.
  3. Each of the apparently isolated units of a dictyostele can be called a meristele.
  4. Among living plants, this type of stele is found only in the stems of ferns.

Question 9

9) Most seed plants stems possess a……which has been interpreted as a……, and is called a:

Answer 9

• vascular arrangement
• derived siphonostele

Eustele:

1. Primary vascular tissue consists of vascular bundles, usually in one or two rings around the pith.
2. Found in stems & roots of monocot flowering plants.
3. Vascular bundles can be collateral (with the phloem on only one side of the xylem) or bicollateral (with phloem on both sides of the xylem, as in some Solanaceae).

Atactostele:

1. Found in monocots.
2. Vascular tissue in the stem exists as scattered bundles