Primary growth is the first form of growth to occur. A whole plant can be built up by primary growth, and in most monocotyledonous plants and herbaceous dicotyledons it is the only type of growth. It is a result of the activity of the apical, and sometimes intercalary meristems.
The seedling undergoes primary growth to form the herbaceous (non-woody) green plant. Primary growth is initiated by divisions of meristematic cells at the apex of stem and root, the apical meristems.
Growth and development of shoot
The shoot is composed of two organs : the stem and leaves. The primary growth of the shoot is concentrated at the shoot tip.
All tissues of the stem and leaves arise from terminal meristem. Leave primordia originate on the lateral flanks of the apical meristem. Shoot apical meristem and the young primordia is commonly termed the bud. The leaf primordia subsequently grow and differentiate into leaves and lateral buds. The point of attachment of the leaf and lateral bud on the stem is the node. The stem between the nodes is referred to as the internode.
Growth of the stem axis is by cell enlargement in the internodal region caused by intercalary meristem. There is a progressive increase in the distance between the young nodes and the apex of the shoot is pushed upward. Branch primordia develop in the axil of old leaves. The primary meristem forms dermatogen, periblem and plerome (Hanstein’s histogen theory). Dermatogen forms epidermis, periblem forms cortex and plerome forms stele the part inner to endodermis. Central part of plerome which forms vascular tissues is called procambium. Stem consists of vascular tissue in the centre. During the formation of vascular tissue, the innermost cells of the procambial strands of the stem form the protoxylem. Cells at the outer side of the procambial strands form the protophloem. Metaxylem and metaphloem tissues are formed subsequently.
Cells of the procambial strand may also differentiate into fibres, a form of sclerenchyma. The formation and enlargement of more robust metaxylem and metaphloem tends to crush the first-formed vascular tissue, particularly the protophloem. The differentiation of phloem involves the formation of phloem sieve tubes and accompanying companion cells.
Growth and development of root
The apical meristem of the root is a mass of irregularly arranged cells, which divide and cut off new cells in all directions. The cells which cut off ahead of this meristem form the root cap. In monocots and many other plants, calyptrogen forms root cap. Dermatogen, periblem and plerome form the root tissues in a fashion similar to stem development.
The bulk of the cells cut off by the root apical meristem gives rise to the central procambial strand, and to the tissues of the cortex. Protoxylem and protophloem develop from the procambial strand cells followed by metaxylem and metaphloem.
The central vascular tissue is referred to as the stele. The stele is surrounded by a single layer of cells, the endodermis, and immediately within this layer is a layer of cells, the pericycle.
Development of cambium in dicot plants After primary growth is completed and the mature primary tissues of stem and root are formed, some procambial strand cells remain in the vascular bundles of the stem and in the stele of the root. These meristematic cells lie between the metaxylem and metaphloem, and are known as cambium. Cambium cells are capable of further growth, leading to secondary growth of roots and stems.
The developmental phase in plant life when rapid primary growth occurs is called juvenile phase. This phase begins with young seedling and continues until the plant begins reproductive development. In some plants, juvenile phase extends for a long period, sometimes indefinite, e.g., English ivy. Juvenile phase is followed by maturity. At maturity plant shows reduced vegetative growth and active reproductive growth.
At maturity : Flowers and fruits develop in monocarpic plants (flower once in their life) and they die after flowering and fruiting, while eucarpic plants resume vegetative growth again after flowering and fruiting. In dicots of angiosperms and gymnosperms, the primary body forms the fundamental structure and increase in girth and diameter takes place by the formation of secondary tissue. This formation of secondary tissue is called secondary growth.
Secondary growth and development
Secondary growth is the growth in thickness due to the formation of secondary tissues by lateral meristems. With the exception of some annuals, most of the dicots and gymnosperms show secondary growth in their roots. It takes place by the production of two types of secondary tissues. They are secondary vascular tissues and periderm. These tissues are formed by meristems, vascular cambium and cork cambium respectively. Secondary growth results in the formation of a large amount of secondary xylem called wood and of external cork layer known as bark.
Secondary growth in dicot roots
Secondary growth in dicotyledonous roots occurs by the initiation and activity of two secondary meristems– (i) vascular cambium and (ii) cork cambium (or phellogen).
Stelar growth by vascular cambium
The process of stelar secondary growth in dicotyledonous roots begins with the initiation of vascular cambium strips which develop from the parenchymatous cells present along the inner edges of primary phloem. The number of cambial strips depends on the number of phloem or xylem strands.
The cells of cambium strips (vascular cambium) divide repeatedly to produce new cells both towards inner as well as outer side. The cells produced towards innerside (centripetally) differentiate into secondary xylem elements and those produced towards outerside (centrifugally) differentiate into secondary phloem.
Subsequently, the cells of pericycle lying towards outer side of protoxylem divide by tangential division. The inner derivatives of these cells become meristematic and function as cambium. These join with cambia derived from phloem strands to form a complete ring of cambium. Thus, a continuous wavy ring is produced which is present below the phloem but above the xylem and is secondary in origin. Soon, the cambium becomes circular by its divisions and secondary tissue formation. Some cambial cells may function as ray initials and produce secondary medullary rays. Secondary vascular tissue assumes the shape of continuous cylinder interrupted at places where wide vascular ray connect primary xylem to cortex through pericycle and endodermis.
Extrastelar growth by cork cambium Addition of more and more tissues of secondary xylem and secondary phloem in the central part of root exerts pressure towards periphery. The tissues outside this zone of active growth get crushed and are sloughed off. To avoid injury of secondary phloem, the roots develop a new cambium ring in the pericycle, called cork cambium (phellogen). The cells of cork cambium (phellogen) divide to form cells of cork (phellem) towards outer side and secondary cortex (phelloderm) towards inner side. It results in the formation of outer protective covering consisting of multilayered cork, cork cambium and multilayered secondary cortex. These three layers make the periderm.
Secondary growth in dicot stems Stelar growth by vascular cambium The dicotylcdonous stems are characterized by presence of fascicular cambium (primary meristem) derived from procambium of shoot apical meristem between xylem and phloem. The parenchymatous cells beside the fascicular cambium become meristematic and develop a new cambium, called interfascicular cambium (secondary meristem). The cambium ring formed by joining of fascicular and interfascicular cambium, is called vascular cambium. It is composed of two types of cells– the fusiform initials and the ray initials.
The cells of cambium mostly divide by periclinal divisions. Out of the two cells produced from single cell of cambium, one differentiates into secondary tissue and the other remains cambial cell. In this way the cambial cells, by repeated divisions, add new cells either towards centre or towards periphery and the cambium still remains single layered.
Formation of secondary xylem and secondary phloem
The fusiform initials of cambium ring divide by tangential divisions and add new cells. The new cells produced toward inner side (i.e., towards primary xylem) may remain meristematic for sometime and differentiate into elements of secondary xylem. On the other hand, the new cells produced towards outer side (i.e., towards primary phloem) remain meristematic for a limited period and differentiate into secondary phloem. The primary phloem gets crushed.
Formation of secondary medullary rays
Ray initials of intrafascicular vascular cambium divide by tangential division and add new cells which form secondary medullary rays. They are usually one to few layers in thickness and one to several layers in height. The medullary rays form the radial system responsible for radial conduction of solutes. They maintain connection between pith and cortex.