What is secondary Meristematic Tissue? Describe its role in growth of stems

Secondary meristematic tissues are those which arise as new meristematic in non meristematic or permanent tissue, later at a certain stage of development. Sometimes the permanent tissues develop the power of division and form secondary meristems. Cork cambium forms the typical example of secondary meristem as it is developed from mature cells of epidermis, cortex of pericycle. Secondary meristems are generally lateral in position and give rise to secondary tissues which add to girth of the plant organ. The vascular cambium is stems are partly a secondary meristem. Cambium is a band of thin walled tissue lying between phloem and xylem. Its cells are arranged in radial rows and are roughly rectangular in shape, very small in size and very thin walled. Cambium is responsible for secondary growth in thickness of the plant body.

In study herbs and in all shrubs and trees secondary growth takes place as a result of the formation of new (secondary) tissues in them. Secondary tissues are formed by two meristems cambium in the stellar region and cork cambium formed later in the extra stellar or cortical region. At first a portion of each medullary ray in a line with the cambium becomes meristematic and forms a strip of cambium called interfascicular cambium. 

This joins on to the cambium proper on either side and forms a complete ring known as cambium ring. Secondary growth begins with the activity of this cambium ring. In secondary tissues the cambium ring as a whole begins to cut off new cells both externally and internally. Those cut off on the outer side are gradually modified into the elements of phloem, these constitute secondary phloem. The secondary phloem consists of sieve tubes, also some bands or patches of bast fibres. Many of the textile fibres of commerce such as jute, hemp, flax, rhea (or ramie) etc. are the bast fibres of secondary phloem.

The new cells cut off by the cambium on its inner side are gradually modified into various elements of xylem, these constitute secondary xylem the cambium is always more active on the inner side than on the outer. Xylem increases more rapidly in bulk than phloem and soon forms a hard compact mass occupying the major portion of the stem. Here and there cambium forms some narrow bands of parenchyma running across the stem in the radial direction through secondary xylem and secondary phloem; these are the secondary medullary rays. They are one, two or a few layers in thickness and one to many layers in height.

The activity of cambium increases or decreases according to favourable or unfavourable climatic conditions. In spring cambium becomes more active and forms greater number of vessels with wider cavities while in winter it becomes less active and forms elements of narrower dimensions. There are formed spring wood and autumn wood ant two kinds of wood appear together as concentric ring known as annual rings are formed year after year by the activity of cambium. Each annual rings counts one year growth and by counting total number of annual rings age of plant can be counted.

Sooner or later another meristematic tissue i.e. cork cambium (or phellogen) appears in cortical region. It originates in outer layer of collenchyma. It divides and gives of new cells on both sides. Cork on the outer side and secondary cortex on the inner. The cells of secondary cortex are parenchymatous in nature and often contain chloroplasts.

New cells cut off by cork cambium on outer side are roughly rectangular in shape and soon become suberized. They form the cork of the plant. Cork cells are dead and thick walled and are arranges in few radial rows. Cork is brownish in colour and is impervious to water.