In this article we will discuss about the anatomy of an orchid root.
Wanda – Root (Family – Orchidaeeae):
It is almost circular in outline and reveals following tissues from outside with-in:
1. It is the outermost, multiseriate covering of the root which represents the epidermis.
2. It is composed of compactly arranged elongated, dead cells which possess fibrous thickenings on their walls.
3. Single layered exodermis follows velamen. It consists of thick walled suberised cells.
4. Some of the cells of exodermis are thin walled called passage cells. Water passes through these cells from velamen to cortex.
5. It is several layers thick parenchymatous and rounded or oval in shape with many intercellular spaces.
6. Few outer layers of cortex contain chloroplast.
7. Endodermis is the innermost cortical layer consisting of barrel shaped cells with their inner and radial walls thickened.
8. Few endodermal cells opposite to protoxylem are thin walled and called passage cells.
9. Single-layered pericycle follows endodermis and consists of thin walled cells.
10. Many vascular bundles (more than six) are radial in organization.
11. Vascular bundles are exarch and polyarch.
12. Xylem elements alternate with the small patches of phloem.
13. Protoxylem vessels are narrow with spiral and annular thickenings on their walls.
14. Metaxylem vessels are larger with reticulate and pitted thickenings on their walls.
15. Phloem consists of sieve tubes, companion cells and phloem parenchyma
16. Some of the cells of xylem and phloem parenchyma become thick walled and lignified, around and in between phloem and xylem bundles. This represents conjunctive tissue.
17. It is well developed and composed of thin walled, round, parenchymatous cells with many intercellular spaces.
(a) 1. Presence of vessels in the xylem.
2. Vessels have perforated end walls with scalariform or regularly arranged holes. (Angiosperms)
(b) 1. Radial vascular bundles.
2. Protoxylem is exarch. (Root)
(c) 1. Absence of cambium.
2. Well developed pith. (Monocot)
(d) 1. Presence of velamen.
2. Passage cells are present in the exodermis. (Epiphyte)