Psilotum
Psilotum belongs to the family Psilotaceae,
class Psilophytinae, division Psilophyta of pteridophyte (Smith, 1957).
Habitat: Psilotum with two species ie. Psilotum
nudum and Psilotum flaccidium is widely distributed in tropical and
subtropical regions. The plant is xerophytic in nature but can grow in various
habitat. Psilotum nudum is the only member found in India.
Habit: Psilotum is a slender often densely tufted
shrubby plant with a height of 20-100 cm.
Both the species are generally epiphytic in habit and grow upon tree ferns and
palms, they may also grow terrestrially in the soil or in crevices of rocks.
Structure of the sporophyte
Morphology or external structure:
The sporophyte ie. plant body is
differentiated into a slender root less subterranean rhizome and an aerial
shoot. The rhizome contains a mycorrhizal fungus and it is covered with hair
like absorbing structures called rhizoids. The rhizome branches dichotomously. On
the rhizome erect or aerial green (photosynthetic) perennial branches are
developed. Aerial branches are either pendent (epiphytic) or erect
(terrestrial). Aerial shoots are also dichotomously branched. The basal parts
of the shoots are cylindrical but the distal parts are ribbed or flattened.
The aerial branches on the upper part bear
small scale like appendages are known as leaves. These leaves are arranged irregularly
or more or less definitely in two or three rows.
Anatomy or internal structure:
Rhizome and aerial branch:
Epidermis: Epidermis of the rhizome is indistinct and
thin walled (without cuticle), stomata absent in epidermis.
Epidermis of the aerial branches is one
celled thick with heavily cutinized outer walls. Stomata present (mainly at the
grooves).
Cortex: In the rhizome, cortex is composed of only thin
walled parenchymatous cells containing mycorrhizal fungus, the cells of the cortex
are without any intercellular spaces.
Cortex of the aerial branches is massive and
differential into three regions ie. a) hypodermal outer cortex is 2-5 layered,
composed of chlorenchymatous cells with intercellular spaces, b) middle cortex lies
internally next to outer cortex and is composed of 4-5 layers of elongated sclerenchymatous cells
with little or no intercellular spaces. c) Inner cortex lying internal to
middle cortex, this zone is composed of thin walled parenchymatous cells
without intercellular spaces but containing abundant starch grains. Inner
cortex is internally delimited by an endodermis with distinct casparian strips.
Stele: Stele of aerial branches is actinostelic
protostele with exarch xylem. Phloem lies between endodermis and xylem. The
stele of rhizome is haplostelic or actinostelic protostele.
Leaf: The leaf has single layered epidermis
without stomata. The interior of leaf is composed of parenchyma cells without a
vein.
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