Page 86 - Biology Form Two

Biology for Secondary Schools

the cells. Companion cells have a high Phloem fibres
concentration of mitochondria that These are elongated and unbranched
provide the sieve tube elements with fibers with a cylindrical shape. They
energy for transporting food. are dead cells associated with the
phloem tissue. These fibres provide
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Phloem parenchyma cells mechanical support to the conducting
These are cells which are associated cells of the phloem and strength to the
with the phloem tissue. They are stem.
located near the finest branches and
terminations of sieve tubes in leaf Phloem and xylem constitute the
veins, stem, and flowers. The function major transport system in plants, but
of phloem parenchyma cells is transport they differ from each other in many
of foods such as sugars in a dissolved aspects. Table 4.1 summarises the
form. Based on this function, they are major differences between phloem and
called transfer cells. xylem tissues.
Table 4.1: Differences between phloem and xylem tissues
Feature Phloem Xylem
Function Transports food and substances Transports water and dissolved
such as sugars and amino acids minerals from the roots to
from leaves to the storage aerial parts of plants
organs and growing parts
Structure Phloem has elongated and Xylem has tubular-shaped
tubular-shaped structures called structures called xylem vessels
sieve tubes with sieve plate with perforated cross walls
Components Phloem is made up of sieve Xylem is made up of tracheids,
tubes, companion cells, phloem vessel elements, xylem
parenchyma, and phloem fibers parenchyma, and xylem fibers
Fibres Phloem fibres are wide Xylem fibres are narrow

Cells Phloem is composed of living Xylem is composed of dead
cells with little cytoplasm but cells at maturity with hollow
no nucleus tubes
Location in the Occurs on the outer side of the Occurs toward the inner part of
vascular bundle vascular bundle the vascular bundle
Movement of Materials in the phloem move Materials in the xylem of the
materials in both upward and downward main stem move in only one
directions direction which is upward
direction

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BIOLOGY FORM 2 NEW.indd 80 05/10/2024 15:36:28