| The heart and soul of stems are
the vascular bundles. Vascular bundles play a very important
role in the survival of the plants. Without the vascular bundles,
plants would not be able to transport the important materials they need
for survival.
As we have previously discussed, the organization
of the monocot and dicot stem differs and we can recognize a stem as being
monocot or dicot by examining this organization. Likewise, the organization
within these vascular bundles differs and this can be used to identify
the bundle as monocot or dicot.
First, let's examine the types of tissues found
within all vascular bundles. All vascular bundles contain xylem.
Xylem
is a tissue that conducts water and dissolved minerals from the roots up
to the leaves. It is easy to think of xylem as a set of straws
connected end to end. Just like a straw, when water (with dissolved
minerals) is pulled through the upper part of the xylem, water is pulled
through the bottom parts of the xylem due to the surface tension of water.
What causes the water to be pulled in the first place? That is a
topic for a later lesson, but it involves a process (transpiration) where
water is removed from the leaves.
The other primary tissue found in vascular
bundles is phloem. Phloem is a collection of cells that carry
nutrients, in the form of carbohydrates, from the leaves to the roots.
Leaves are commonly thought of as the main site of photosynthesis, the
process which converts carbon dioxide into usable carbohydrates.
It should be fairly obvious that gravity plays a role in the movement of
materials through the phloem. |
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