AGATHIS..............................pptx

Agathis

1. AGATHIS ARAUCARIACEAE MORPHOLOGY Tall trees – 40 metres Evergreen, columnar trunk with whorled horrizontal branches. Leaves spirally arranged on main stem & opposite on lateral shoots. Reddish when young & dark green when old. Long, petiolate , thick glabrous , broad, ovate- lanceolate with parallel venation. Cushion like scars left on leaf fall. Leaves remain for 15 to 20 years. Root – typical tap root with developed branches.

2. ANATOMY Root – Epidermis – single layered. Cortex – parenchymatous , 6-7 layered. Endodermis – single layered – tanniferous . Pericycle – 5 to 7 layered, has resin ducts in rings. Stele – diarch condition. Sec. Growth as usual with uniseriate rings. Stem – Thin cuticle & epidermis. Many layered parenchymatous cortex – 2 zones. Outer zone – 3 to 4 layers small tanniferous cells. Inner zone – large cells, some with tannin. 2 rings of resin ducts – one below eidermis . Other associated with vas. bndls . – outside phloem. Vas.bndls . Collateral & open. Leaf traces in pairs. Sec. thickening – bordered pits & uniseriate rays & sclerids

3. Leaf – dorsiventral . Upper epidermis – rectangular cells, highly cuticularised . Hypodermis – stone cells seen. Sub-epidermal layer – with tannin cells. Palisade – elongated, compactly arranged. Inner to spongy another layer of palisade seen. Through out mesophyll - sclerids present. Stomata on both upper & lower epidermis. Vas.bndls numerous, alternate with resin ducts. Bundle sheath – sclerenchymatous fibres. Xylem & phloem as usual.

4. REPRODUCTION Plants are dioecious . Male cones cylindrical, solitary or in clusters on the lower branches of trees. Cones – erect, subsessile & axillary . Microsporophylls – spirally arranged around axis. Each with small stalk, peltate head & 5 to 15 microsporangia in 2 rows. Long cone axis elongates when sporangia mature. By a longitudinal slit – sporangia dehise in April- March. Microsprorogenesis same as in others – innermost wall layer forms tapetum .

5. Female cones large, solitary, stalked, sub-globose – upper part of trees. Central axis – megasporophylls spirally arranged – each broader than long – thick in the centre with 2 membraneous lateral extensions & a short upturned tip – a single ovule develops on the adaxial surface of megasporophyll & is fused to sporophyll only at its base – ovuliferous scale & bract scale completely fused.

6. Ovule development same as in others – unitegmic , orthotropus – group of archesporial cells – one of them as megaspore mother cell – R/D – triad of megaspores – lowermost one functional.

7. Functional Megaspore Nuclear division Upto 500 nuclei Cellularization occurs Archegonial initial – large central cell & small neck initial Neck initial – 12 wedge shaped cells in one tier Central nucleus – Egg nucleus & ventral canal nucleus Egg nucleus surrounds cytoplasm – Egg cell

8. POLLINATION – by wind FERTILIZATION – as usual. EMBRYOGENY Zygote nucleus Nuclear divisions 64 proembryonal nuclei Free nuclear proembryo Nuclei arranged in 3 groups Upper – single row of large radially elongated nuclei Middle – few round nuclei of 2 rows Lower – elongated nuclei - fewer in number

9. Wall formation Suspensor develops – pushes embryo deep inside Only simple polyembryony Cotyledons – 2 to 4 in number Seed coat – thick – outer 4 to 5 layers & inner 5 to 6 layers Seed germination epigeal    

10. AGATHIS