Chapter 36: Resource Acquisition
and Transport in Vascular Plants
Ancestors of the plants on the earth are the green algae that lived in an
aqueous medium and till now they do. Because they are surrounded
completely by water, and it needs nutrients (CO2, minerals and water)
and sunlight for the photosynthesis, they get their needs by diffusion.
But for land plants , it seems that they live in two worlds; a part is
above the ground and another part is under the ground.
The upper part “it is exposed to air and sunlight” gets CO2 and light
and the lower part gets water and minerals.
The trees are different from each other by many things like:
1234-
The shape of the leaf.
The length f the stem.
The way of branching.
Arrangement of leaves on the stem.
The part above the ground is called the shoot system that has the stem
and leaves, while the other part is called the root system.
Structural features of the shoot(features that makes the shoot capture
light and water-resources- with a high efficiency ).
Shoot architecture and light capture
The plants are autotrophs since we depend on them for getting our
nutrition; they make the nutrition by photosynthesis.
The minerals and water are taken by the roots from the soil and then
transported to the leaves for photosynthesis that produces sugar and
O2.
The architecture
The most important thing for the shoot is obtaining sunlight and a
good amount of water.
a- The size and the form of the leaf has a role in that since there is a
correlation between leaf size and water availability ; the largest
leaves are typically found in species from tropical rain forests(to
allow a high rate of vaporization in order not destroy the plant
from big amounts of water), whereas the smallest are usually
found in species from dry or very cold environments, where
liquid water is scarce and evaporative loss from leaves is
potentially more problematic.
b- Arrangement of the leaves around the stem. (Phyllotaxy).
If you look at stems of plants you will find nodes, and between each
two nodes there is a region called internode. The leaves are connected
to the stems by the nodes regions on the stem.
Refer to figure 36.3
You see a view of the shoot epical meristem which is going to
determine the arrangement of leaves on the stem of the plant and is
genetically determined. Each species has a specific arrangement of
leaves “so the phyllotaxy is a species specific”.
When you calculate the number of leaves in the figure notice that the
firstly born leaves have the 1st numbers. The arrangement of them has
triangular shapes.
So the phellotaxy is determined by the shoot epical meristem.
There are many types of the phellotaxy:
1- Alternate or spiral phyllotaxy” one leaf per node”. Most
angiosperms have alternate phellotaxy, with leaves arranged in
an ascending spiral around the stem, each successive leaf
emerging 137.5° from the site of the previous one.
The phellotaxy is correlated to light capture; this arrangement helps
each leaf to capture the maximum amount of light. This phellotaxy is
the most common among angiosperms.
2- Opposite phyllotaxy “two leaves per node”. They are also
arranges in a way that prevents the shading of the lower leaves
by those above.
3- Whorled arrangement “more leaves per node”.
Sometimes, plant physiologists make a useful measurement called leaf
area index, the ratio of the total upper leaf surface of a single plant or
an entire crop divided by surface area on the land on which the plant
or crop grows.
More leaves
more leaf area index.
More leaf area index is better.
Root area index values of up to 7 are common for many mature crops,
and there is little agriculture benefit to leaf are indexes higher than
this value. Adding more leaves increases shading of lower leaves to the
point that they respire more than photosynthesis, and as a result, the
non productive leaves or branches undergo programmed cell death
and are eventually shed, a process called self-pruning.
When that happens, you notice falling of the lower leaves after
becoming yellow.
c- Another factor effecting light capture is leaf orientation.
This is correlated to the environment of the plant. In low-light
conditions, the leaves are horizontally orientated. And when there is
intensive light, the leaves are vertically orientated to avoid damaging
of the leaves by that light.
In the 1st case, the light rays are vertical to the leaf surface, while in the
2nd case, the light rays are essentially parallel to the leaf surfaces.
d- The height of shoots and their branching.
Trees vary in the height of stems. some trees grow tall and has a
strong stem to be given more support and to reach the sunlight. In
an intensive forest, many trees become too long to reach the
sunlight.
More taller tree leads to more vascular tissue in the stem.
The height of the stem grows by secondary growth.
So the more growth of the tree lead to taller, thickened and more
branched stem.