PRIMARY AND SECONDARY GROWTH
- The region of growth in plants is found in localized areas called meristems as shown
- A meristem is a group of undifferentiated cells in plants which are capable of continuous mitotic cell division.
- The main meristems in flowering plants are found at the tips of shoots and roots, in young leaves, at the bases of the inter-nodes.
- Vascular cambium and cork cambium.
- The meristems at the tips of the shoots and the roots are known as apical meristems and are responsible for primary growth. The cambium meristems are responsible for secondary growth.
- Primary growth occurs at the tips of roots and shoots due to the activity of apical meristems.
- These meristems originate from the embryonic tissues. In this growth there are three distinctive regions, the region of cell division, cell elongation and cell differentiation.
- The region of cell division is an area of actively dividing meristem to cells. These cells have thin cell walls, dense cytoplasm and no vacuoles.
- In the region of cell elongation, the cells become enlarged to their maximum size by the stretching of their walls.
- Vacuoles start forming and enlarging. In the region of cell differentiation the cells attain their permanent size, have large vacuoles and thickened watt cells.
- The cells also differentiate into tissues specialised for specific functions.
- Primary growth results into an increase in the length of shoots and roots.
To determine the region of growth in roots
- Germinating bean seeds with radicle of about 1cm in length, cork, pin, beaker or gas jar, water, Indian ink, blotting paper or filter paper, marker and ruler marked in mm.
- Take the germinating been seed, and using a blotting paper, dry the radical taking care not to damage the root.
- Using a marker and ruler make light ink marks 2mm apart along the length of the root.
- Make a drawing of the marked root. Pin the seedling onto the cork and place it in the beaker containing a little water. Leave it overnight. Take out the seedling and examine the ink marks.
- Measure the distances between the successive ink marks and record. Make a well labelled drawing of the seedling at the end of the experiment and compare with the drawing of the. seedling at the start of the experiment.
- Secondary growth results in an increase in width or girth due to activity of the cambium. In secondary growth new tissues are formed by vascular cambium and cork cambium.
- In monocotyledons plants there are no cambium cell in the vascular bundles.
- The growth in diameter is due to the enlargement of the primary cells.
- Secondary growth in dicotyledonous plants begins with the division of vascular cambium to produce new cambium cells between the vascular bundles.
This forms a continuous cambium ring.
These cambium cells divide to form the new cells that are added to the older ones. The cambium cells have now become meristematic.
- The new cells produced to the outer side of cambium differentiate to become secondary phloem and those to the inner side differentiate to become the secondary xylem. More secondary xylem is formed than secondary phloem.
- As a result of the increase in the volume of the secondary tissues, pressure is exerted on the outer cells of the stem.
This results in stretching and rupturing of the epidermal cells. In order to replace the protective outer layer of the stem, a new band of cambium cells are formed in the cortex. These cells, called cork cambium orphellogen originate from the cortical cells.
The cork cambium divides to produce new cells on either side. The cells on the inner side of the cork cambium differentiate into secondary cortex and those produced on the outer side become cork cells.
Cork cells are dead with thickened walls. Their walls become coated with a waterproof substance called suberin.
The cork cells increase in number and become the bark of the stem. This prevents loss of water, infection from fungi and damage from insects. The corky bark is also resistant to fire and thus acts as an insulatory layer.
- The bark is normally impermeable to water and respiratory gases. Periodically the cork cells, instead of being tightly packed, they form a loose mass. This mass is known as Jenticel.
- The rate of secondary growth in a stem varies with seasonal changes. During rainy season, xylem vessels and tracheids are formed in large numbers.
These cells are large, have thin walls and the wood has a light texture. In the dry season, the xylem and trancheids formed are few in number.
They are small, thick-walled and their wood has a dark texture. This leads to the development of two distinctive layers within the secondary xylem formed m a year, called annual rings.
It is possible to determine the age of a tree by counting the number of annual rings.
Furthermore climatic changes of the past years can be inferred from the size of the ring.
- Primary phloem Secondary phloem
- Cambium ring Medullary ray Pith
- Primary xylem Cortex