primary internal structure & normal secondary growth in...
TRANSCRIPT
Dr. (Miss) Kalpana R. DatarAssistant Professor
DEPARTMENT OF BOTANYWillingdon College, Sangli.
Primary Internal structure & Normal Secondary growth in Sunflower stem
B. Sc. II - Botany
Primary Internal structure of Sunflower stem
Transverse section passing through the
internodal region shows three distinct regions –
1. Epidermal region -
2. Cortical region-
3. Stelar region-
1) Epidermal region-1. It shows epidermis, cuticle and
trichomes.
2. The epidermis –
i. Single layered, uniseriate.
ii. Thin walled, living, barrel shaped or
rectangular, compactly arranged cells.
iii. Thin cuticle on the outer walls. 3. They are covered with trichomes or multicellular hairs
on the outer surface.Function- Protection
b) Cortical region- Extra stelar Ground Tissue-It is divided into three regions1.Hypodermis –i. Just below the epidermis, ii.3-5 layers, made up of
collenchyma.iii. Some patches of collenchyma
possess chloroplast. iv. intercellular spaces are absent.
Function- Gives mechanical support to young stem.
2. Cortex
i. Present below the hypodermis.
ii. Few layers of parenchyma
cells.
iii. Oval or spherical, thin walled cells.
iv. Intercellular spaces are present.
v. Few resin ducts are present.
Function- food storage &
mechanical strength.
3.Endodermis –
i. Innermost layer of cortex.
ii. Single layered, barrel shaped
parenchymatous cells.
iii. Starch grains are abundantly present. so it is
also called starch sheath.
iv. Casparian strips are present in these cells.
c) Stele region (Vascular Region)–Tissues internal to the endodermis is called stele. Stele region contains-1. Pericycle -i. Outermost layer.
ii. multilayered, consists of patches of parenchyma alternating with patches of sclerenchyma.
iii. In young stem, sclerenchyma patch is present at the cap of vascular bundle near the phloem patch. It is called ‘Hard-bast’. (Hard tissue-Sclerenchyma & Bast - Phloem tissue. Hard tissue present near to the phloem).It gives mechanical strength to the phloem.
Vascular bundles: The vascular bundles are
Conjoint (xylem & phloem on the same radius),
Collateral(Xylem &phloem lie side by side.),
Open(Cambium is present
in between xylem &phloem).
ii. Vascular bundles are arranged in a ring.iii. Each vascular Bundle contains xylem, phloem and cambium.Stele is Eustele typeXylem- i. contains Protoxylemand Metaxylem elements and xylem parenchyma.ii. Xylem is endarch- Protoxylemtowards the centre and metaxylemtowards the peripheryPhloem: Contains thin walled tissue
Phloem: Contains thin walled tissues.Cambium:i. It is a thin strip of actively dividing meristematic
cells.ii. It is present in between xylem and phloem.iii. It is made up of 2-3 layer of meristematic cells.iv.The cambium in the vascular bundle is knownas Stelar or fascicular cambium or intrastelarcambium or intrafascicular cambium.v. The division of cambium cells adds phloemtowards periphery and xylem towards centre.
Medullary rays-
i. These are thin walled, fairly big, polygonal
or radially elongated cells.
ii. These are present in between the vascular
bundles.
Pith-
i. It is present in the centre.
ii. It is made up of parenchyma cells, with
intercellular spaces.
Secondary growth in Dicot stem-Sunflower stem
In intrastelar region – Due to Vascular cambium.In extrastelar region- Due to cork cambium or phellogen.
In intrastelar region-1.When the secondary growth begins
fascicular/ intrafascicular cambium becomes active.
2.The permanent cells present in between the vascular bundles become meristematic and form cambium strip- Interfascicular Cambium.
3.Intrafascicular and interfascicular cambium grow laterally and forms cambium ring.
4. Activity of cambium ring begins. The cambium is more active on the inner side than the outer side.
5.Cambium adds more secondary xylem towards inner side and relatively less amount of secondary phloem towards outer side.
6.The cambium forms radial narrow bands of parenchyma cells running across the secondary xylem and secondary phloem is called as Secondary medullary rays.
7. Primary xylem, Primary phloem and pith gets crushed due to formation of more and more secondary tissues.
The extra-stelar secondary growth does not
occur in Sunflower stem.
Sunflower is annual herbaceous plant ,only
one xylem ring is formed in it.
No initiation of cork cambium hence periderm
formation does not occur.
Thus the secondary growth in Sunflower
stem is restricted to the stelar region only
T.S of old Sunflower stem
Phloem
Cambium
Xylem
A
B
Cambium Ring
C
References:http://www.biologydiscussion.com/shoot-system/stems/internal-structure-of-stem-with-diagram/13943
http://cdn.biologydiscussion.com/wpcontent/uploads/2015/10/clip_image00479.jpg
http://cdn.biologydiscussion.com/wpcontent/uploads/2014/12/image44.png
http://www.biologydiscussion.com/plants/anatomical-structure-of-plantswithdiagram/6450
