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B5 Growth and Development
1. Growth
Cells, tissues and organs
When you were born you were made of thousands (millions) of cells and since birth your
body has slowly developed. Development is how an organism changes as it grows and
matures throughout its life circle (cycle). Plants and animals are made of different kinds
of cell. Each kind of cell does a different job. These cells are unspecialized (specialized).
Groups of these cells are called tissues. Examples of tissues in plants are xylem, phloem
and root. Groups of tissues arrange themselves into organs, for example the heart and
brain in humans.
From single cell to adult
A fertilized egg cell, a gamete (zygote), contains instructions for making all the different
types of cells in the body. The information is in the genetic code of the DNA inside the
cytoplasm (nucleus) of every body cell.
During the first week of growth, the zygote develops into a ball of about 100 cells (the
embryo). Up to the 16-cell stage (8-cell stage), all the cells in the embryo are
unspecialized. These cells are called adult stem cells. (embryonic stem cells) Following this
stage, the cells begin to become specialized and form tissues. By about five (two) months
into the pregnancy, the developing baby has the main organs and is now called a fetus.
Patterns of growth
Cells need to divide and make new cells, for organisms to grow bigger. Some plants
continue to grow throughout their lives:
o their stems grow taller
o their roots increase in girth (grow longer)
o their stems increase in girth
B5 Growth and Development
Plants can continue growing throughout their life because they have rings of specialized
(unspecialized) cells, called meristem cells, in the tips of the roots and shoots, and in rings
around the stem and roots, which can divide and make new cells. Plants have meristem cells
until they reach full adult size (throughout their lives) and during this time they can
regrow whole organs such as leaves and xylem vessels, even if they are damaged.
Animals also have some back-up cells, called stem cells. In newts, these stem cells become
specialized (remain unspecialized) so they can divide and develop into any kind of cell the
newt needs. This means that a newt can regrow a leg if it needs to! However, in humans,
the stem cells are unspecialized (become specialized) so they are not as useful. For
example, skin stem cells can only divide and develop into skin cells, and bone marrow stem
cells will only make new bone cells (blood cells).
Using meristems
Gardeners make use of meristem cells when growing new plants by taking cuttings, which
are shoots or leaves cut from a plant. Some grow better when the ends are dipped in preplanting (rooting) powder, which contains the plant hormones called auxins. Auxins make
the meristem cells develop in to new shoots (roots). By taking cuttings, gardeners can grow
new plants from a parent plant quickly and cheaply, which have a variety of characteristics
(are genetically identical).