Selaginella species are creeping or ascendant plants with simple, scale-like leaves (microphylls) on branching stems from which roots also arise. The stems are aerial, horizontally creeping on the substratum (as in Selaginella kraussiana), sub-erect (Selaginella trachyphylla) or erect (as in Selaginella erythropus).
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[tex]\mathfrak{Internal \ Structure \ of \ Selaginella : }[/tex]
[tex]\mathfrak{1. Stem:}[/tex]
A Tranverse section (T.S.) of the stem of Selaginella is
somewhat circular in outline and shows the following
structures:
(i) Epidermis:
It is the outer most covering layer comprising of a single cell in
thickness. The cells of the epidermis are without hairs and stomata.
The epidermis is surrounded on all sides by a thick coating of cuticle.
(ii) Cortex:
Inner to the epidermis is present a well-defined zone of cortex. The
cortex may or may not be differentiated into inner and outer cortex. In
case of S. selaginoides, the whole of the cortex is made up of
parenchymatous cells while in S. kraussiana, it is differentiated into
sclerenchymatous outer cortex and parenchymatous inner cortex.
The parenchymatous cortex is usually made up of angular cells i.e.,
without intercellular spaces but in some cases the cells are rounded
and provided with a few inter-cellular spaces.
(iii) Stele:
The central portion of the stem is occupied by a well-developed stele.
The stele is of protostelic type i.e., xylem is present in the centre and
surrounded by phloem on all sides. Phloem, in turn, is surrounded by
a single layered pericycle. Pith is absent.
The stele remains suspended in the centre by radially elongated
tubular, unicellular structures known as trabeculae. These are formed
by the radial elongation of the endodermal cells. Trabeculae are
provided with conspicuous casparian strips. In between the trabeculae
are present large spaces known as air spaces.
The number of stele is variable in different species of Selaginella. It is 1
(monostelic e.g., S. spinulosa), 2 (distelic e.g., S. kraussiana) or 12-16
(polystelic e.g., S. laevigata). The organization of the stele is also
variable in different species. It may be protostele (e.g., S. spinulosa) to
siphonostele (e.g., S. laevigata, var. lyalli).
The stele is surrounded by a single layered pericycle made of
parenchymatous cells. The xylem is usually monarch (e.g., S.
kraussiana), or diarch (e.g., S. oregana) or multiarch (e.g., S.
spinulosa).
It is usually exarch but sometimes it may be mesarch (e.g., S.
selaginoides). Xylem is usually made of tracheids. Vessels are
completely absent. Xylem is surrounded on all sides by phloem which consists of sieve cells and phloem parenchyma. Companion cells are absent in phloem
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Selaginella species are creeping or ascendant plants with simple, scale-like leaves (microphylls) on branching stems from which roots also arise. The stems are aerial, horizontally creeping on the substratum (as in Selaginella kraussiana), sub-erect (Selaginella trachyphylla) or erect (as in Selaginella erythropus).
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[tex]\mathfrak{Internal \ Structure \ of \ Selaginella : }[/tex]
[tex]\mathfrak{1. Stem:}[/tex]
A Tranverse section (T.S.) of the stem of Selaginella is
somewhat circular in outline and shows the following
structures:
(i) Epidermis:
It is the outer most covering layer comprising of a single cell in
thickness. The cells of the epidermis are without hairs and stomata.
The epidermis is surrounded on all sides by a thick coating of cuticle.
(ii) Cortex:
Inner to the epidermis is present a well-defined zone of cortex. The
cortex may or may not be differentiated into inner and outer cortex. In
case of S. selaginoides, the whole of the cortex is made up of
parenchymatous cells while in S. kraussiana, it is differentiated into
sclerenchymatous outer cortex and parenchymatous inner cortex.
The parenchymatous cortex is usually made up of angular cells i.e.,
without intercellular spaces but in some cases the cells are rounded
and provided with a few inter-cellular spaces.
(iii) Stele:
The central portion of the stem is occupied by a well-developed stele.
The stele is of protostelic type i.e., xylem is present in the centre and
surrounded by phloem on all sides. Phloem, in turn, is surrounded by
a single layered pericycle. Pith is absent.
The stele remains suspended in the centre by radially elongated
tubular, unicellular structures known as trabeculae. These are formed
by the radial elongation of the endodermal cells. Trabeculae are
provided with conspicuous casparian strips. In between the trabeculae
are present large spaces known as air spaces.
The number of stele is variable in different species of Selaginella. It is 1
(monostelic e.g., S. spinulosa), 2 (distelic e.g., S. kraussiana) or 12-16
(polystelic e.g., S. laevigata). The organization of the stele is also
variable in different species. It may be protostele (e.g., S. spinulosa) to
siphonostele (e.g., S. laevigata, var. lyalli).
The stele is surrounded by a single layered pericycle made of
parenchymatous cells. The xylem is usually monarch (e.g., S.
kraussiana), or diarch (e.g., S. oregana) or multiarch (e.g., S.
spinulosa).
It is usually exarch but sometimes it may be mesarch (e.g., S.
selaginoides). Xylem is usually made of tracheids. Vessels are
completely absent. Xylem is surrounded on all sides by phloem which consists of sieve cells and phloem parenchyma. Companion cells are absent in phloem