Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
128
To summarise, it is relatively easy for carbon dioxide and
oxygen to diffuse into and out of the bodies of plants,
reaching and leaving every cell quickly enough down
diffusion gradients. Consequently, there is no need for
a transport system for these gases. However, transport
systems are needed for distribution of water, inorganic
and organic nutrients, as well as other substances such as
plant hormones.
Two systems: xylem and
phloem
The design of a plant’s transport system is quite different
from that of a mammal. In fact, plants have two transport
systems, xylem and phloem. Xylem carries mainly water
and inorganic ions (mineral salts) from roots to the parts
above ground. The xylem sap contained in the xylem
can move in only one direction, from roots to the rest
of the plant. The second system is phloem. This carries
substances made by photosynthesis from the leaves to
other areas of the plant. At any one time, phloem sap
can be moving in different directions in different parts
of the phloem.
In neither of these systems do fluids move as rapidly
as blood does in a mammal, nor is there an obvious pump
such as the heart. Neither plant transport system carries
oxygen or carbon dioxide, which travel to and from cells
and their environment by diffusion alone.
Structure of stems, roots and
leaves
Stems, roots and leaves are the main organs involved in
transport within plants. Organs are composed of more
than one tissue. Tissues are collections of cells specialised
for a particular function. The cells may be of the same
type, such as parenchyma, or of different types, as in
xylem and phloem.
The tissues found in stems, roots and leaves are most
easily studied using prepared slides of transverse sections
of these organs. Drawing low power plans of the organs
and representative groups of cells of the individual tissues
as seen at high power with a microscope is a useful way
of understanding the structure of the organs. Structure is
closely linked with function.
When making drawings using a microscope, you will
need to follow the advice given in Box 7.1.
Using an eyepiece graticule and a stage micrometer
will enable you to make measurements of cells, tissues and
organs and will help you to show tissues in their correct
proportions. See Worked example 1, page 7, for guidance
on making measurements.
Monocotyledons and dicotyledons
Flowering plants (angiosperms) may be monocotyledons
(monocots) or dicotyledons (dicots). Each type has its
own characteristics. For example, monocotyledonous
plants, such as grasses, typically have long, narrow
leaves. Dicotyledonous plants typically have leaves with
blades and stalks (petioles). The mechanisms of transport
through both types of plant are the same, but there are
differences in the distribution of xylem and phloem in
their roots, stems and leaves. Only dicotyledonous plants
are described in this book.
Low-power plan diagrams
Transverse sections of a typical dicotyledonous stem, root
and leaf are shown in Figures 7.2, 7.3 and 7.5–7.8. In each
case a labelled photomicrograph from a prepared slide is
shown, followed by a low-power, labelled drawing of the
same organ.
Figure 7.2 Light micrograph of part of a transverse section of a
young Ranunculus (buttercup) stem (× 60).
epidermis
cortex
fibres
phloem
cambium
xylem
vascular
bundle
Figure 7.3 Low-power plan of the Ranunculus stem shown in
Figure 7.2.