Stele
1. The central part of the stem inner to the endodermis is known as stele. It consists of pericycle, vascular bundles and pith.
2. In dicot stem, vascular bundles are arranged in a ring around the pith. This type of stele is called eustele.
Pericycle
1. Pericycle is the layers of cells that occur between the endodermis and vascular bundles. In the stem of sunflower (Helianthus ), a few layers of sclerenchyma cell occur in patches outside the phloem in each vascular bundle. This patch of sclerenchyma cell is called Bundle cap or Hardbast.
2. The bundle caps and the parenchyma cells between them constitute the pericycle in the stem of a sunflower.

Vascular Bundles
1. The vascular bundles consist of xylem, phloem and cambium. Xylem and phloem in the stem occur together and form the vascular bundles.
2. These vascular bundles are Wedge shaped. They are arranged in the form of a ring. Each vascular bundle is conjoint, collateral, open and endarch.
Phloem
1. Primary phloem lies towards the periphery. It consists of protophloem and metaphloem. Phloem consists of sieve tubes, companion cells and phloem parenchyma.
2. Phloem fibres are absent in the primary phloem. .Phloem conducts organic food materials from the leaves to other parts of the plant body.
Cambium
Cambium consists of brick-shaped and thin-walled meristematic cells. It is one to four layers in thickness. These cells are capable of forming new cells during secondary growth.
Xylem
1. Xylem consists of xylem fibres, xylem parenchyma vessels and tracheids. Vessels are thick walled and arranged in a few rows.
2. Xylem conducts water and minerals from the root to the other parts of the plant body
Pith
1. The large central portion of the stem is called pith. It is composed of parenchyma cells with intercellular spaces. The pith is also known as medulla.
2. The pith extends between the vascular bundles. These extensions of the pith between the vascular bundles are called primary pith rays or primary medullary rays. Function of the pith is storage of food.

Vascular Bundles:
1. In a monocot stem vascular bundles are scattered (atactostele) in the parenchymatous ground tissue.
2. Each vascular bundle is surrounded by a sheath of sclerenchymatous fibres called bundle sheath. 3. The vascular bundles are conjoint, collateral, endarch and closed.
4. Vascular bundles are numerous, small and closely arranged in the peripheral portion. Towards the centre, the bundles are comparatively large in size and loosely arranged.
5. Vascular bundles are skull or oval shaped.
Phloem
The phloem in the monocot stem consists of sieve tubes and companion cells. Phloem parenchyma and phloem fibres are absent.
Xylem
Xylem vessels are arranged in the form of 'Y'. The two metaxylem vessels are located at the upper two arms and one or two protoxylem vessels at the base. In a mature bundle, the lowest protoxylem disintegrates and forms a cavity known as protoxylem lacuna.

Tracheids are dead, lignified and elongated cells with tapering ends. Its lumen is broader than that of fibres. In cross section, the tracheids are polygonal.

(i) There are different types of cell wall thickenings due to the deposition of secondary wall substances.
(ii) They are annular (ring like), spiral (spring like), scalariform (ladder like) reticulate (net like) and pitted (uniformly thick except at pits).
(iii) Tracheids are imperforated cells with bordered pits on their side walls. Only through this conduction takes place in Gymnosperms.
(iv) They are arranged one above the other Tracheids are chief water conducting elements in Gymnosperms and Pteridophytes.
(v) They also offer mechanical support to the plants.