Dr. (Miss) Kalpana R. DatarAssistant Professor

DEPARTMENT OF BOTANYWillingdon College, Sangli.

kalpana_datar@yahoo.com.

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