Page 23 - LN-ANATOMY OF FLOWERING PLANT

Cork Cambium
The stem continues to increase in girth due to the activity of vascular cambium. Due
to this, the outer cortical and epidermis layers get broken and needs to be replaced
to provide new protective cell layers. Therefore, another meristematic tissue called
cork cambium or phellogen develops usually in the cortex region.
Phellogen is a couple of layers thick. It is made of narrow, thin-walled and nearly
rectangular cells. Phellogen cuts off cells on both sides. The outer cells differentiate
into cork or phellem while the inner cells differentiate into secondary cortex or
phelloderm.
The cork is impermeable to water due to suberin deposition in the cell wall. The cells
of secondary cortex are parenchymatous. The phellogen, phellem and phelloderm
are collectively known as periderm.
Bark
Bark is a non-technical term used to describe all tissues exterior to the vascular
cambium, therefore including secondary phloem. The bark refers to a number of
tissues, i.e., periderm and secondary phloem.
The bark that is formed early in the season is called early or soft bark. Towards the
end of the season, late or hard bark is formed.
Lenticels
At certain regions of stem, the phellogen cuts off closely arranged parenchymatous
cells on the outer side instead of cork cells. These parenchymatous cells soon
rupture the epidermis, forming a lens-shaped openings called lenticels. The lenticel
are mosdy found in woody trees.
The lenticels permit the exchange of gases between the outer atmosphere and the
internal tissue of the stem.

Secondary Growth in Roots
The secondary growth in the root is the thickness due to the formation of secondary
tissues by lateral meristems. With the exception of some annuals, most of the dicots
and gymnosperms show secondary growth in their roots. It occurs by the production
of two types of secondary tissues,
i.e., the secondary vascular tissues and periderm. These tissues are formed by
meristems are vascular cambium and cork cambium, respectively.
Formation of Vascular Cambium
The conjunctive parenchyma cells, on the lateral sides of the phloem bundles as well