Selaginella:habit,stemanatomy,strobilus,gametophyte
Classification:
Division : pteridophyta
Class : lepidopsida
Order : selaginellales
Family : selaginellaceae
Genus : selaginella
Distribution and habitat
- World-wide in distribution.
- Most species are tropical, damp forests.
- Grow in moist, shaddy, damp habitats.
- Some species are xerophytic; can grow on dry rocky cliffs.
Habit
- Foliage leaves are small, simple, and symmetrical or asymmetrical in outline.
- Size: varies from species to species. It can be of mosses size to size of 20 meters (as in case of slender vine ).
- Homophyllum (subgenus) : the stem may be erect, with all leaves of same size, spirally arranged, densely clothing the stem.
- Heterophyllum(subgenus): the stem may be prostrate, with short erect branches.
- Dimorphic leaves are seen. Leaves borne in pairs and with the two pair markedly different in size .
- The smaller leaf of heterphyllum is inserted on dorsal side and larger leaf on ventral side of stem . hence, large leaf alternates the small leaf.
- The prostrate axis bears elongate, downwardly growing colorless, leafless, cylindrical appendages = rhizophores.
- Each rhizophores develop tufts of adventitious roots.
- Along leaf, on the adaxial side of the leaf and near its base, a membranous outgrowth is present = ligule.
Stem anatomy
- Mature portion of stem differ from other pteridophytes in having the vascular tissue set off from the cortex by radially elongate endodermal cells:trabeculae,with intercellular spaces between them
- cortex contain angular cells without intercellular spaces.
- All cells of cortex are thin walled;or those towards apex are sclerified.
- Epidermis is one celled thickness,without stomata.
- Stele organization can be from simple protostele or polycyclic siphonostele.
- Stem can also be polystelic and have 2,3,4,…..16 seprate stele running through the intervening region.
- Root is monostelic
- Leaf vascular bundle is very simple
- Leaf contain disc shaped chloroplast with pyrenoid like bodies present
- Epidermis of leaf may have stomata on adaxial or abaxial or on both.
- Significance of ligule:protection of young leaf-growing apex.
Strobilus
- Sporophylls are similar to foliage leaves.
- Sporophyll bears a single stalked sporangium that lies on adaxial side between the base and fase of ligule
- Sporangia are of two types
- 1)microsporangia
- 2)macrosporangia
- Macrosporangia:microsporophyll
- Microsporangia:microsporophyll
- Both thes are always in definite strobilli
- Strobili grow on apex,sporophylls are spirally arranged;
- Many species have macro and micro sporophyll borne in the same strobilus/on different strobilus.
Adult macrosporangia
- Four lobed
- Drying out of sporogonial jacket initiates hygroscopic movement that splits the sporangium wall along thin walled jacket cells.
- It germinates to produce female gametophyte
Adult microsporangia
- All cells are potential sporocytes
- .
- Male gametophyte develops from these spores while,attached to sporophyte
Gametophyte of selaginella
- Most superficial apical cell are potential archegonial initials;several develop into archegonia
- Fertilization may takes place while gametophyte is still attached to sporangium.
- It also produce new sporophyte; out of this fertilization of male and female reproductive organs.
