In herbs and shrubs and trees secondary growth takes place as a result of 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. The increase in thickness due to addition of secondary tissues cut off by the cambium and the cork cambium in the stellar and extra stellar regions respectively is spoken of as secondary growth.
In Dicot stem secondary growth begins with the activity of cambium ring. Cambium ring begins to cut off new cells externally and internally. Those cut off on outer side are gradually modified into the elements of phloem; these constitute secondary phloem which consists of sieve tubes, companion cells and phloem parenchyma and often also some bands or patches of bast fibres. Many textile fibres of jute and bast fibres of secondary phloem. New cells cut off from cambium on inner side are modified into secondary xylem. The secondary xylem consists of scalari form and pitted vessels. Tracheids, numerous wood fibres in radial rows and some wood parenchyma. Xylem increases more rapidly in bulk than phloem and soon forms hard occupying major portion of stem.
Here and there cambium forms some narrow bands of parenchyma, running across the stem in the radial direction through secondary phloem and secondary xylem; these are secondary medullary rays. They are one, two or a few layers in thickness and one to many layers in height.
Cambium in spring becomes more active and form large pitted vessels. In winter it becomes less active. The wood formed in springs is called spring wood and in winter is autumn wood. These two kinds of appear together as concentric ring known as the annual ring or growth ring each annual ring corresponds to one year’s growth and by counting total rings the age of plant is determined. In the region of cortex cork cambium appear, it begins to divide and give off new cells on both sides of cork on outer side and secondary cortex on inner side. The cells of secondary cortex are parenchymatous in nature.
In Dicot roots secondary growth is due to addition of new tissues cut off by the cambium and cork cambium in interior as well as in peripheral regions. In the root secondary growth commences a few centimetres behind the apex. Portion of cambium near inner cambium becomes active. It begins to cut off new cells on inside. As a result the cambium and phloem are pushed outwards. Wavy band of cambium soon becomes circular or ring like. New cells cut off by cambium ring on inner side gradually become differentiated into the elements of xylem and these new elements together form secondary xylem. Secondary wood increases rapidly and soon forms main bulk of the root.
New elements cut off by cambium on outer side become modified into elements of phloem and side become modified into elements of phloem and all these form secondary phloem. It is thinner single layered pericycle divides into few rows of thin walled roughly rectangular cells which form cork cambium or phellogen. Secondary cortex of root does not contain chloroplasts.
In monocotyledons normally vascular bundles are closed. The cambium being absent the secondary growth is absent but in some plants like Dracaena and Yucca secondary growth takes place.
Dracaena Stem:
Epidermis is followed by sclerenchymatous hypodermis. Large number of closed collateral bundles are scattered in ground tissue. Cambium is on inner side and little on outside where it forms only parenchyma. On inner side it forms xylem and parenchyma in alternate patches. The inner parenchyamtous cells are called conjunctive tissue.
Later cambium on inner side changes and above xylem it starts forming phloem and then again xylem. Thus phloem becomes encircled by xylem and a ring of lap to centric vascular bundles is formed. Xylem formed earlier has bigger vessels. Around each vascular bundle is developed sclerenchymatous sheath.
Later cambium forms xylem on inner side at those places where it was previously forming parenchyma in place of xylem.
Again by change in activity it forms a ring of vascular bundles. Later in cambium more rings of vascular bundles are formed.
Cork cambium is formed below by hypodermis and forms Cork and Cork cambium in normal fashion.
In some Monocot plants like Musa rhizome or palm stems thickness takes place by formation of primary thickening meristem which initiates in the embryo itself. The procambial strands mainly develop from this meristem.
Tomilson (1961, 1971) has shown that in palmae the parenchymatous cells in the centre and the cells of bundle sheath keep on dividing and expanding which results in the thickening of the stem. He has described such growth as diffuse secondary growth.
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