BOTANY Higher Secondary Second Year - Textbooks Online

2.5. Secondary growth in dicot stem
The primary structure of the plant body is caused by the activity of
apical meristems. The primary permanent tissues produced by the apical
meristems cause the growth in length and to some extent in thickness.
This is called primary growth. Further thickness is observed mostly in
dicot plants. This thickness is caused by the addition of new tissues by the
activity of the lateral meristems like vascular cambium and cork cambium.
The new tissues, which are formed by the lateral meristems, are called
secondary tissues.
Thus the increase in thickness, due to the addition of secondary tissues
by the activity of vascular cambium and phellogen in the stelar and cortical
regions is called secondary growth. The process and the structures
associated with the secondary growth in dicot stems are given below:
Formation of vascular cambial ring
The cambium occurring between the xylem and the phloem in the
vascular bundle, is called fascicular cambium. In between the vascular
bundles, a few parenchymatous cells of the medullary rays that are in line
with the fascicular cambium become meristematic and form a strip of
cambium. It is called interfascicular cambium. This interfascicular cambium
joins up with the fascicular cambium on both sides and forms a continuous
ring. It is called a vascular cambial ring.
Formation of secondary tissues
The vascular cambial ring becomes active and begins to cut off new
cells both towards the inner and outer sides. The cells, which are cut off
towards the outerside, get differentiated into the secondary phloem, whereas
those cut off on the inner side are differentiated into secondary xylem.
Secondary phloem consists of sieve tubes, companion cells, phloem
parenchyma and phloem fibres. Secondary xylem occupies the major part
of the woody stem. It consists of vessels, tracheids, xylem fibres and
xylem parenchyma.
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