Page 24 - LN-ANATOMY OF FLOWERING PLANT

as pericycle cells lying outside the protoxylem end becomes brick-shaped and
meristematic. These develop into a wavy band of vascular cambium. The vascular
cambium of the root is a secondary meristem. It continues to form secondary xylem
on the inner side and secondary phloem on the outer side.
Secondary phloem consists of sieve tubes, companion cells, phloem parenchyma
and phloem fibres. The secondary xylem contains elements like vessels, xylem
parenchyma and xylem fibres.
Activity of Vascular Cambium
The vascular cambium derived from the pericycle gives rise to only ray cells. The
formation of these ray cells is slower, than the formation of secondary vascular
tissues. Due to this, the depressed parts of vascular cambium move outwardly and
ultimately the cambium becomes circular.
Effect of Growth of Secondary Tissue
The primary phloem gets crushed due to the growth of secondary vascular tissues.
The older secondary phloem is also partially destroyed as the new phloem becomes
functional.
The primary and secondary xylems persist. Primary xylem is distinguishable by its :
exarch nature and central position. As compared to the primary xylem, the vessels of
the secondary xylem are broader and thinner. Annual rings are not very sharp.
This is because the climate of the soil does not vary much during different seasons.
Formation of Cork Cambium