Chapter 4: TRANSPORT OF MATERIALS IN FLOWERING PLANTS  
Plants need a transport system to transfer manufactured food from the photosynthesis  
sites (leaves) to other parts. The transport system is also required to move water and  
mineral salts from the roots to the rest of the plant. The transport of raw materials,  
minerals, water and manufactured food in plants occurs in the vascular system.  
VASCULAR SYSTEM  
Vascular system is a network of specialized cells or tissues that transport essential  
materials throughout the plant. The main materials that are transported in plants are  
water, mineral salts and manufactured food. The vascular system is composed of  
conducting tissues (vascular bundles) and associated supportive fibres.  
The vascular bundles are of two types, these are;  
(i)  
Xylem tissue: has the role of transporting water and mineral salts absorbed from  
the soil through the roots to all parts of the plant.  
(ii) Phloem tissue: is responsible for transporting manufactured food from the sites  
of photosynthesis to all parts of the plant.  
XYLEM  
Xylem is a tissue which transports water and mineral salts from the roots to stems and  
leaves. It also provides structural mechanical strength to the plant. The movement of  
substances in the xylem is always upward. Xylem is made up of four different types of  
cells which are: Xylem vessel elements, tracheids, xylem fibers and xylem parenchyma.  
(a) Xylem Vessel Elements. They are very efficient in transporting water and mineral  
salts of the xylem. They are very long hollow tubes made of dead cells placed end to  
end to form a pipe-like structure. Xylem vessel elements have no cytoplasm and nuclei  
at maturity to make them to be hollow tubes, which then acts as pipes. Thus, they are  
said to be dead. This enables them to transport a larger volume of water and mineral  
salts. Their walls are made up of cellulose and lignin to strengthen and make them  
rigid. This gives an additional function of supporting the plant.  
(b) Tracheids. They are series of interconnected dead, elongated, tube-like cells with  
tapering ends. They are the only transporting tissues in coniferophytes and  
pteridophytes. In angiosperms, both tracheids and xylem vessel elements are present.  
Their walls are made up of cellulose and lignin to strengthen and make them rigid.  
This gives an additional function of providing structural and mechanical support to  
the plant.  
(c) Xylem Fibres. They are dead cells with lignified walls. They do not conduct water.  
Their function is to provide protection and mechanical support to the xylem tissue.  
(d) Xylem Parenchyma. They are the only living cells of xylem. They non-specialized  
simple cells with thin cell walls. They store water and food in the form of carbohydrates.  
PHLOEM  
Phloem is a tissue which transports manufactured food materials from the leaves to the  
rest parts of the plant. The movement of substances in the phloem is by translocation.  
The movement can be in any direction. Phloem tissue is composed of various specialized  
cells which are; sieve tubes, companion cells, phloem fibres and phloem parenchyma.  
Figure 4.1: Phloem tissue  
(a) Sieve tubes. They are made up of cells that are joined end to end. However, the end  
walls of these cells have perforations or pores allowing the flow of the manufactured  
food. The perforations or pores form sieve plates.  
(b) Companion cells. These cells work in association with sieve tubes. Companion cells  
have many mitochondria that provide energy to the sieve tubes for translocation of food  
from the leaves to all parts of the plant. Companion cell communicates with sieve tube  
by means of plasmodesmata.  
(c) Phloem parenchyma cells. The function of phloem parenchyma cells is transportation  
of foods such as sugars in a dissolved form. Based on this function, they are called  
transfer cells.  
(d) Phloem fibres. They are dead cells associated with the phloem tissue. They provide  
mechanical support to the conducting cells of the phloem and strength to the stem.  
Table 4.1: Differences between xylem and phloem tissues:  
Feature Xylem  
Phloem  
It  
transports  
water  
and It transports food and substances  
dissolved minerals from the such as sugars and amino acids  
Function  
roots to aerial parts of plants.  
from leaves to the storage organs  
and growing parts.  
Xylem has tubular shaped Phloem has elongated and tubular-  
Structure  
structures called xylem vessels  
with perforated cross walls.  
It is made up of tracheids,  
shaped  
structures  
called  
sieve  
tubes with sieve plate.  
It is made up of sieve tubes,  
companion cells, phloem  
Components  
vessel  
elements,  
xylem  
parenchyma and xylem fibres.  
Xylem fibres are narrow.  
parenchyma, and phloem fibres.  
Phloem fibres are wide.  
Fibres  
It is composed of dead cells at  
maturity with narrow tubes.  
It occurs toward the inner part  
It is composed of living cells with  
little cytoplasm but no nucleus.  
It occurs on the outer side of the  
vascular bundle.  
Cells  
Location in the  
vascular bundle of the vascular bundle.  
Materials in the xylem move in  
Materials in the phloem move in  
Movement of  
materials  
only one direction which is both  
upward  
and  
downward  
upward direction. directions.  
Importance of the Transport System in Plants  
(i)  
It absorbs and transports water which is required for photosynthesis.  
It transports mineral salts needed for proper growth of plants.  
(ii)  
(iii) It transports manufactured food from the plant leaves to the storage organs.  
(iv) It removes excess water from the plant.  
THE DISTRIBUTION OF VASCULAR BUNDLES IN PLANTS  
The way the vascular bundles are arranged in the roots, stems and leaves of monocots  
and dicots differ. This arrangement also differs in the roots and stems of the two categories  
of plants.  
THE DISTRIBUTION OF VASCULAR BUNDLES IN ROOTS AND STEMS  
(a) Monocotyledonous root. The vascular structures are arranged in a circular pattern  
around the central pith consists as shown in figure blow. The pith consists of ground  
tissue called parenchyma.  
Figure 4.2: Cross-section of monocot root  
(b) Dicotyledonous root. The vascular bundles of dicot roots are located in the middle of  
the root. The phloem is found between the extensions of the xylem. The xylem is lobed,  
centrally positioned and star-shaped.  
Figure 4.3: Cross-section of a dicot root  
(c) Monocotyledonous stem. In monocot stems the arrangement of vascular bundles is  
random or scattered throughout the ground tissue. Most of the vascular bundles are  
concentrated towards the periphery of the stem and are more scattered towards the  
centre. There is no pith in the monocot stem.  
Figure 4.4: Cross-section of a monocot stem  
(d) Dicotyledonous stem. In dicot stem, the vascular bundles are arranged in a ring  
around the central pith. The phloem and xylem tissues are separated by a layer called  
the vascular cambium. The xylem tissue is on the inner side while the phloem tissue  
is on the outside.  
Figure 4.5: Cross-section of dicot stem  
THE DISTRIBUTION OF VASCULAR BUNDLES IN LEAVES  
Both monocot and dicot leaves have vascular bundles surrounded by a bundle sheath.  
The main function of bundle sheath is to protect the leaf veins.  
(a) Monocotyledonous leaves. In monocot leaves, vascular bundles form parallel veins.  
The phloem is located towards the lower epidermis of the leaf while the xylem is  
located towards the upper epidermis.  
(b) Dicotyledonous leaves. In dicit leaf, vascular bundles form net like venation. Like in  
monocot leaf, the phloem is located towards the lower epidermis of the leaf while the  
xylem is located towards the upper epidermis.  
ABSORPTION AND MOVEMENT OF WATER AND MINERAL SALTS IN PLANTS  
Plant absorb water and mineral salts from the soil by using root hairs. The root hairs are  
in contact with soil and soil water.  
Structure and functions of root hairs  
Root hairs are extensions of the epidermal cells of the root. Figure below shows the  
structure of a root hair.  
Figure 4.6: Structure of a root hair.  
Functions of the Roots  
(i) They anchor (hold) the plant firmly in the soil. This prevents it from being blown  
away by wind.  
(ii) They absorb water and mineral salts from the soil and pass them into the stem.  
(iii) They act as food storage. Example; in carrots and cassava tuber.  
(iv) They are used as reproductive organs in some plants.  
Adaptations of the Root Hair to its function  
(i) They are long and slender to provide a large surface area for the absorption of water  
and mineral salts from the soil.  
(ii) Root hairs they are large in number (numerous) to increases the surface area for  
maximum absorption of water and mineral salts.  
(iii) They are very thin in order to provide a short distance over which absorption of  
water and mineral salts takes place.  
(iv) They contain vacuoles filled with cell sap which is usually hypertonic; that is, more  
concentrated than the surrounding water. Hence, water enters the root hair cells  
from the soil by osmosis.  
(v) Root hair cells have a higher concentration of minerals than their surroundings.  
Mineral salts are therefore absorbed by active transport.  
(vi) They have permeable cell walls which are hydrophilic in nature. Hydrophilic means  
water loving. Thus, root hairs are always in search of water supply.