Video Transcript
The diagram provided outlines the
basic structure of the xylem. Which of the following best
describes the structure of the xylem? (A) The xylem is a tissue comprised
of a long, flexible tube made up of thin-walled, living collenchyma cells. (B) The xylem is a tissue
containing a transport system of tubes made up of thick-walled, nonliving
sclerenchyma cells. Or (C) the xylem is a tissue
comprised of multiple, separate, nonliving parenchyma cells stacked on top of each
other.
The question asks us to identify
the best description of xylem, which is one of the two types of transport tissue in
plants, along with phloem, which transports sugars through the plant.
The primary function of xylem is to
transport water and minerals from the roots upwards to the rest of the plant. It also provides extra structural
support to the plant. Xylem vessels, which are the major
water-conducting cells of the xylem, are stacked end to end to form a hollow
tube. This arrangement allows water and
dissolved minerals to flow through them like a straw.
Xylem vessels are composed of dead
sclerenchyma cells. Sclerenchyma is a simple tissue
with thick-walled, lignified cells that provide structural and mechanical support in
stems, leaves, and roots. Lignin is hydrophobic and
inflexible. So, the presence of lignin in the
cell walls of xylem is what makes them waterproof and stiff.
A key concept in biology is that
structure leads to function, and this is clearly shown in xylem. Its structural properties make it
perfectly suited to perform its functions of efficient water transport and
structural support.
Now let’s have a look at our answer
choices. We can eliminate option (A) because
we’ve learned that the presence of lignin makes xylem stiff, not flexible. And it is made of nonliving
sclerenchyma cells, not living collenchyma cells. We can also rule out option (C)
because xylem is made of connected sclerenchyma cells, not separate parenchyma
cells. So, choice (B) gives the best
description of xylem structure. The xylem is a tissue containing a
transport system of tubes made up of thick-walled, nonliving sclerenchyma cells.
