Stelar evolution and alternation of generation

Stelar evolution and alternation of generation

1. Stelar evolution and alternation of generation in Pteridophytes Sankrita Gaonkar Assistant Professor in Botany [email protected]

2. Contents Stelar evolution Types of sele References

3. Stelar evolution Stele - central or core of the axis includes vascular system and pith (if present) L eaf gap – space in the stem of a plant through which the leaf grows. L eaf is connected to stem by the leaf trace , which grows through the leaf gap. Large leaf traces play important role in determining vascular organization of stem No leaf gaps at young leaf stage As we proceed towards mature parts of stem, small leaf gaps begin to appear at the point of leaf trace insertion on stem stele These small gaps enlarge in size with enlargement of leaf trace bundle Since leaves are arranged spirally on stem, stele of stem becomes broken Stellar theory – Van Tiegham and Douliot (1886)

4. Stelar evolution Highlights of stelar theory: Stele is real entity and present universally in all higher plants Cortex and stele are two fundamental parts of shoot system Stele & cortex are separated by endodermis Main components of stele – xylem and phloem Pericycle, medullary rays & pith are also components of stele

5. Types 1. Protostele Simplest, primitive Vascular cylinder consists of solid core of xylem surrounded by phloem, pericycle and endodermis No pith Protostele name – Jeffrey (1902) Fundamental stelar type – from this other types evolved Haplostele Actinostele

6. Haplostele Smooth central core of xylem – surrounded by uniform phloem layer Found in extinct psilophytales – Rhynia and Horneophyton Found in living species – Selaginella In some Selaginella – 2 haplosteles ( Distelic ) Can be tristelic or tetrastelic In S. selaginoides – monostelic haplostele has metaxylem completely surrounding protoxylem – mesarch ii) Actinostele Xylem core stellate or starshaped E.g Lycopodium , upper stem portion in S. selaginoides , Psilotum In some, xylem occurs as parallel plates alternating with phloem plates – Plectostele Mixed protostele – actinostele with irregular xylem groups embedded in phloem mass

7. 2. Siphonostele Modification of protostele Stele in which the protostele is medullated – siphonostele Contains a tubular vascular region and a parenchymatous central region i ) Ectophloic siphonostele – pith surrounded by xylem strand. E.g. Osmunda ii) Amphiphloic siphonostele – pith surrounded by endodermis, pericycle, phloem and xylem. E.g. Adiantum

8. 3) Solenostele Pith with one leaf gap i ) Ectophloic – Pith present. Xylem is surrounded by only on the outer side by phloem. E.g. in Osmunda ii) Amphiphloic – P ith present . Xylem is surrounded on both sides by phloem. E.g. Marsilea rhizome

9. 4) Dictyostele Solenostele that is broken into a network of separate vascular strands. B reaking up of stelar core is due presence of large number of leaf gaps. Each such separate vascular strands is called meristele . Each meristele is of protostelic type .

10. 5) Polycyclic stele Most complex amongst all vascular cryptogams Such type of steles are always siphonostelic in structure P ossesses two or more concentric rings of vascular tissue This may be a solenostele or a dictyostele 2 concentric ring – e.g. Pteridium B. Polycyclic dictyostele

11. 6) Eustele M odification of the siphonostele V ascular system consists of a ring of collateral or bicollateral vascular bundles situated on the periphery of the pith. E.g. Equisetum I s modification of ectophloic siphonostele Splitting is because of overlapping leaf gaps

12. 7) Polystele More than one stele in the axis D erived from protostele – because each stele shows protostelic condition E.g. – Selaginella laevigata – 16 steles are present Polystele

13. Stelar evolution Protostele – primitive type of stele Present in earliest vascular plants and now retained by some living vascular cryptogams like Psilotum, Tmesipteris , Selaginella, Lycopodium etc. Extinct psilophytes – Rhynia – had protostelic vascular organization Later xylem became irregular and attained star like shape – actinostele Further, xylem splits into parallel plates alternating with phloem plates – plectostele Haplostele to actinostele to plectostele – considered as one line of evolution – Lycopsid line of evolution Important evolutionary change occurred in protostele is appearance of central pith Several modifications – led to complex stelar types

14. References Pandey B.P. (2009). Botany for degree students: B.Sc. First Year. S. Chand & Company Ltd. New Delhi. Vashishta BR. 2010. Botany for degree students: Pteridophytes. S. Chand & company ltd.