Biology Reference
In-Depth Information
Before expansion
Without change of cell shape
With change of cell shape
Expansion
Expansion in only one axis
Isotropic expansion
Proliferation
Root cap
FIGURE 4.7
The consequences of cell expansion proximal to the zone of proliferation in a developing root of
a flowering plant: if cell expansion were to be isotropic the root would grow in girth as well as length, whereas
a change in cell shape so that cells expand only along the root axis elongates the root as a whole without broadening it.
concentrating new cell growth at one end of the cell ('focused' or 'tip' growth). Flowering
plants use tip growth as well as diffuse expansion; it is, for example, responsible for the
production of absorptive 'root hairs' from the sides of roots just proximal to the zone of
cell elongation described above (
Figure 4.8
). Tip growth is also responsible for the morpho-
genesis of the largest organisms discovered on Earth
d
multicellular fungi.
Multicellular fungi (most of which are modest in size, but some of which span square kilo-
metres of forest floor)
15
consist mainly of a network of branched hyphae that ramifies
through the soil or other substrate. Hyphae are either coenocytic or separated into cells by
transverse septa, and all of their growth takes place at the hyphal tips. Occasionally, patches
FIGURE 4.8
Root hairs of flowering plants grow from their tips.
