Page 21 - LN-ANATOMY OF FLOWERING PLANT

The dicot stem, in its primary state of growth contains narrow layers of
intrafascicular cambium in between the xylem and phloem.
The tissues involved in the secondary growth are the two lateral meristems, i.e.,
vascular cambium and cork cambium.
1. Vascular Cambium
The meristematic layer that is responsible for cutting off vascular tissues such as
xylem and phloem is called vascular cambium. It is present in a patch of a single
layer in young stem which later on develops into a complete ring.
Formation of Cambium Ring
The parenchyma cells of the primary medullary rays adjacent to the intrafascicular
cambium undergo dedifferentiation and give rise to interfascicular cambium. This
joins the intrafascicular cambium of either side to form a complete ring of meristem
called the cambium ring.

Activity of Cambial Ring
The cambial ring becomes active and begins to form new cells, both towards and
inner and the outer sides. The cambial ring is made up of two types of cells ray
initials and fusiform initials. The cells added to the inner side of cambium ring by the
division of the fusiform initials gradually become the elements of the secondary
xylem. While, the cells added to the outer side of the cambium become elements of
the secondary phloem. While, the cells added by the division of ray initials to the
inside as well as outside become elements of the secondary medullary rays.
The cambium is generally more active on the inner side than the outer. As a result,
the amount of secondary xylem produced is more than secondary phloem and soon
forms a compact mass.
The primary and secondary phloems get gradually crushed due to the continued
formation and accumulation of secondary xylem. The primary xylem however,
remains more or less intact, in or around the centre. At some places, the cambium
forms a narrow band of parenchyma, which passes through the secondary xylem