Video Transcript
Which of the following best
describes the structure of the phloem? (A) The phloem is comprised of
living sieve cells that form a long, continuous tube. (B) The phloem is comprised of
many dead sieve cells that have pores in their cell walls to allow movement of
substances through the plant. (C) The phloem is comprised of
dead sieve cells that form a long, continuous tube. Or (D) the phloem is comprised
of many living sieve cells that have pores in their cell walls to allow movement
of substances through the plant.
Let’s remove the
multiple-choice options for now and remind ourselves about the structure of the
phloem. You may recall that
translocation is the process whereby substances such as sucrose are transported
around a plant in the phloem. The structure of the phloem can
be represented by this simple diagram. The main transport vessel of
the phloem is called the sieve tube. The sieve tube is made of
living sieve cells, which are also known as sieve tube elements. These are stacked end on end to
form one continuous tube.
The end walls of the sieve tube
elements are called sieve plates. These contain pores which allow
substances to move from one sieve tube element to the next. Sieve tube elements also
contain plasmodesmata. Plasmodesmata are narrow pores
located between each sieve tube element and its associated companion cell. And they allow substances to
more easily diffuse into the sieve tube for transport in the phloem.
We now have enough information
to answer the question. The statement which best
describes the structure of the phloem is (D). The phloem is comprised of many
living sieve cells that have pores in their cell walls to allow movement of
substances through the plant.
