- Call your teacher.
- Switch on the microscope making sure the lowest objective is in position (the 4x ob-
jective). - Place your slide on the stage.
- Focus the image under the 4x objective (lowest objective) and view the structure of the
celery stem. Switch to the 10x objective to look a little more closely. To see details of
the structure of plant tissue, use the 40x objective. Carefully observe all of the parts
and different tissues. - Once you are able to see specific tissue types, call your teacher.
- Make a biological drawing of your specimen as viewed under the microscope. Take
note of the magnification and draw a scale bar. Label your diagram according to the
tissues you have learnt about.
Secondary growth DUMMY
Plants, like other living organisms need to grow, and they do this by mitosis. Cell growth is
limited to the specialised cells that make up meristematic tissue. Meristematic tissue consists
of small cells that are unspecialised. These cells divide by mitosis to form new cells that can
differentiatei.e. undergo changes in their structure. When groups of cells differentiate they
form specialised tissue (e.g. xylem, phloem, epidermal cells). There are different types of
meristematic tissue:
- Primary meristematic tissueis found in the tips of roots, stems and buds. When the
cells divide, new cells are produced which causes the plant to grow in length. This is
referred to asprimary growth. - Secondary meristematic tissueoriginates from permanent tissue, usually parenchyma
tissue which divides by mitosis. Cambiumis secondary meristematic tissue that is
found in roots and stems. When these cells divide by mitosis it results in the plant
becomingwider.This is calledsecondary growth.
Secondary growth is seen clearly if you examine the stump of a tree. During every growing
season the stem of a plant increases in width. This is known as secondary thickening. To-
wards the end of the first year of growth, the parenchyma cells between the vascular bundles
become meristematic. This means that they actively start dividing by mitosis, and link up
with the cambium that occurs in the vascular bundles to form a cambium ring in the stem
(see Figure 6.13). The cells in the cambium ring start dividing to form secondary phloem
(on the outside of the cambium ring) and secondary xylem (on the inside of the cambium
ring). Each year another ring of secondary phloem and secondary xylem is formed, making
the stem grow wider.
Chapter 6. Support and transport systems in plants 167