Sclereids (Stone Cells)
Sclereids are dead cells, usually these are isodiametric but some are elongated too. The cell wall is very thick due to lignification. Lumen is very much reduced. The pits may simple or branched. Sclereids are mechanical in function. They give hard texture to the seed coats, endosperms etc., Sclereids are classified into the folowing types.
Types of Sclereids
(a) Brachysclereids or Stone cells:
Isodiametric sclereids, with hard cell wall. It is found in bark, pith cortex, hard endosperm and fleshy portion of some fruits. Example: Pulp of Pyrus.

(b) Macrosclereids:
Elongated and rod shaped cells, found in the outer seed coat of leguminous plants. Example: Crotalaria and Pisum sativum.

(c) Osteosclereids (Bone cells):
Rod shaped with dilated ends. They occur in leaves and seed coats. Example: seed coat of Pisum and Hakea

(d) Astrosclereids:
Star cells with lobes or arms diverging form a central body. They occur in petioles and leaves. Ex: Tea, Nymphae and Trochodendron.

(e) Trichosclereids:
Hair like thin walled sclereids. Numerous small angular crystals are embedded in the wall of these sclereids, present in stems and leaves of hydrophytes. Example: Nymphaea leaf and Aerial roots of Monstera.

(i) Sieve tubes are long tube like conducting elements in the phloem.
(ii) These are formed from a series of cells called sieve tube elements.
(iii) The sieve tube elements are arranged one above the other and form vertical sieve tube.
(iv) The end wall contains a number of pores and it looks like a sieve hence called as sieve plate.
(v) These elements show nacreous thickenings on their lateral walls with simple or compound sieve plates.
(vi) The function of sieve tubes are believed to be controlled by companion cells.
(vii) In mature sieve tube, Nucleus is absent and it contains a lining layer of cytoplasm with special protein called slime body in it.
(viii) In mature sieve tubes, the pores in the sieve plate are blocked by a substance called callose.
(ix) The conduction of food material takes place through cytoplasmic strands.
(x) Sieve tubes occur only in Angiosperms.

S.No.	Characters	Dicot root	Monocot root
1.	Pericyle	Gives rise to lateral roots, phellogen and a part of vascular cambium.	Gives rise to lateral roots only.
2.	Vascular tissue	Usually limited number of xylem and phloem strips.	Usually more number of xylem and phloem strips.
3.	Conjunctive tissue	Parenchymatous; Its cells are differentiated into vascular cambium.	Mostly sclerenchymatous but sometimes parenchymatous. It is never differentiated in to vascular cambium.
4.	Cambium	It appears as a secondary meristem at the time of secondary growth.	It is altogether absent.
5.	Xylem	Usually tetrach	Usually polyarch

S.No. Characters Dicot Stem Monocot Stem 1.  Hypodermis  Collenchymatous  Sclerenchymatous 2.  Ground tissue Differentiated into cortex, endodermis and pericycle and pith Not differentiated, but it is a continuous mass of parenchyma 3.  Starch Sheath  Present  Absent 4.  Medullary rays  Present  Absent 5. Vascular bundles  (a) Collateral and open  (a) Collateral and closed  (b) Arranged in a ring  (b) Scattered in ground tissue  (c) Secondary growth occurs  (c) Secondary growth usually does not occur.