Environmental Engineering Reference
In-Depth Information
g
rowth
in
V
ascular
p
lants
Vascular plants undergo two kinds of growth (growth is primarily restricted to meri-
stems).
Primary growth
occurs relatively close to the tips of roots and stems. It is
initiated by apical meristems and is primarily involved in the extension of the plant
body. The tissues that arise during primary growth are called primary tissues, and the
plant body composed of these tissues is called the
primary plant body
. Most primi-
tive vascular plants are entirely made up of primary tissues.
Secondary growth
occurs
in some plants; secondary growth thickens the stems and roots. Secondary growth
results from the activity of lateral meristems, or
cambia
, of which there are two types:
1.
Vascular cambium
gives rise to secondary vascular tissues (secondary
xylem and phloem). The vascular cambium gives rise to xylem on the inside
and phloem on the outside.
2.
Cork cambium
forms the periderm (bark). The periderm replaces the epi-
dermis in woody plants.
p
lant
h
ormones
Plant growth is controlled by plant hormones, which influence cell differentiation,
elongation, and division. Some plant hormones also affect the timing of reproduction
and germination.
•
Auxins
affect cell elongation (tropism), apical dominance, and fruit drop
or retention. Auxins are also responsible for root development, secondary
growth in the vascular cambium, inhibition of lateral branching, and fruit
development. Auxin is involved in the absorption of vital minerals and fall
color. As a leaf reaches its maximum growth, auxin production declines.
In deciduous plants this triggers a series of metabolic steps that cause the
reabsorption of valuable materials (such as chlorophyll) and their transport
into the branch or stem for storage during the winter months. When the
chlorophyll is gone, the other pigments typical of fall color become visible.
•
Kinins
promote cell division and tissue growth in the leaf, stem, and root.
Kinins are also involved in the development of chloroplasts, fruits, and
flowers. In addition, they have been shown to delay senescence (aging),
especially in leaves, which is one reason why florists use cytokinins on
freshly cut flowers—when treated with cytokinins, they remain green, pro-
tein synthesis continues, and carbohydrates do not break down.
•
Gibberellins
are produced in the root growing tips and act as a messen-
ger to stimulate growth, especially elongation of the stem, and can also
end the dormancy period of seeds and buds by encouraging germination.
Additionally, gibberellins play a role in root growth and differentiation.
•
Ethylene
controls the ripening of fruits. Ethylene may ensure that flowers are
carpelate (female), while gibberellin confers maleness on flowers. It also con-
tributes to the senescence of plants by promoting leaf loss and other changes.
•
Inhibitors
restrain growth and maintain the period of dormancy in seeds
and buds.
