Meristems at the tips of roots and shoots generate new cells through cell division, enabling plants to grow in length through primary growth. Secondary growth occurs through the vascular cambium and cork cambium, adding secondary tissues and thickening the stem. Gene expression and transcription factors control morphogenesis and the differentiation of cells into specialized plant organs and tissues.

1. BIO 304 Plant structure and physiology: Part 1 Cells and growth
Meristems generate cells for new growth
The growth of a plant differs from that of an animal in a basic way. Most animals are characterized by determinate growth;
that is, they stop growing after reaching a certain size. Most species of plants, in contrast, continue to grow as long as they
live, a condition known as indeterminate growth.
Indeterminate growth does not mean that plants are immortal. In fact, different types of plants have very different life
spans (annuals, biennials and perennials). Some perennials have life spans well beyond the longest-lived animals; for
example, some bristlecone pines and baobabs have been alive for over 4,500 years.
Primary Growth: Lengthening
Growth in all plants is made possible by tissues called apical meristems. A meristem consists of undifferentiated
(unspecialized) cells
that divide frequently,
generating new cells
and tissues. Meristems
at the tips of roots and
in the terminal and
axillary buds of shoots
are called apical
meristems. Cell division
in apical meristems
produces new cells that
enable a plant to grow
in length, a process
called primary growth
(Figure 1).
Q 1 Label Figure 1
Tissues produced by
primary growth are
called primary tissues.
Figure 2 shows a
longitudinal section
through a growing
onion root.
Q 2 Discuss Figure 2 and colour dermal, ground and vascular tissues with three different colours.
Primary growth enables roots to push through the soil. (A very similar process results in the upward growth of shoots.) At
the very tip of the root is the root cap, a thimble-like (?) cone of cells that protects the delicate, actively dividing cells of the
apical meristem. The root's apical meristem replaces cells of the root cap that are scraped away by the soil and produces
cells for primary growth. Primary growth is achieved not only by cell division but also by the lengthening of cells just above
the apical meristem (see Figure 2, centre).
These cells can undergo a tenfold increase in length, mainly by taking up water. Elongation of these cells is what actually
forces a root down through the soil. The elongating cells begin to differentiate; forming primary tissues that develop into
the epidermis, cortex, and vascular tissue (see Figure 2). Cells of this last type eventually differentiate into vascular tissues
called primary xylem and primary phloem.
Q 3 Remind yourself about the terms Phloem, Xylem, Tracheids, Parenchyma, Collenchyma and Sclerenchyma. Some
of these structures can be seen in context with the so-called primary tissue (discuss Figure 4).
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Figure 1. An overview of primary and secondary growth.

2. Secondary Growth: Thickening
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Figure 2. Primary growth of a root. Colour the dermal, ground and vascular tissues
Figure 3. Three years’ growth in a winter twig.

3. In
addition to lengthwise primary growth, the stems and roots of many plant species also thicken by a process called
secondary growth. Such thickening is most evident in the woody plants -trees, shrubby and vines- whose stems last from
year to year and consist mainly of thick layers of mature, mostly dead xylem tissue, called wood. Tissues produced by
secondary growth are called secondary tissues.
Secondary growth involves cell division in two meristems we have not yet discussed: the vascular cambium and the cork
cambium. The vascular cambium (Figures 1 + 5) is a cylinder of actively dividing cells between the primary xylem and
primary phloem. Secondary growth adds cells on either side of the vascular cambium.
Q 4 Figures 5 + 6 show important facts about secondary growth. Make notes on Figure 5 points 1-9 .
In Figures 5 + 6 the cambium has given rise to two new tissues: secondary phloem to its exterior and secondary xylem to its
interior. Yearly production of a new layer of secondary xylem accounts for most of the growth in thickness of a perennial
plant.
Annual growth rings result from the layering of secondary xylem (Figures 5 + 6) The layers are visible as rings because of
uneven activity of the vascular cambium during the year. In woody plants that live in temperate regions, such as most of
Europe, the vascular cambium becomes dormant each year during winter (because of cold), and secondary growth is
interrupted. When secondary growth resumes in the spring (when the temperature is higher), cylinder of early wood forms.
Made up of the first new xylem cells to develop, early wood cells are usually larger in diameter and have thinner walls than
those produced later in summer. The boundary between the large cells of early wood and the smaller cells of the late wood
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Fig. 4. Organization of primary tissues in young roots. Colour dermal, ground and vascular tissues (Rev. numbers) with different colours.

4. produced during the previous growing season is usually a distinct ring visible in cross sections of tree trunks and roots.
Q 5 Would you expect tropical trees to have these growth rings? Why or why not?
A tree's age can be estimated by counting its annual rings. The
rings may have varying thicknesses, reflecting the amount of
seasonal growth in different years and therefore climate
conditions. In fact, the pattern of growth rings in older trees is
one source of evidence for recent global climate change.
On Figs 4 + 5, notice that the epidermis and cortex make up the
young stem's external covering. When secondary growth
begins, the epidermis is sloughed of and replaced with a new
outer layer called cork. Mature cork cells are dead and have
thick, waxy walls, which protect the underlying tissues of the
stem. Cork is produced by a meristem tissue called the cork
cambium. Everything external to the vascular cambium (the
secondary phloem, cork cambium, and cork) is called bark.
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Figure 5. Primary and secondary growth in a stem.
Figure 6. Secondary growth produced by the vascular cambium
Q 6 Make notes on points A – D- the role of the vascular cambium
Figure 7.

5. Q 7 Figure 7 shows the anatomy of a tree trunk. Where does
transport of water and sucrose solution occur?
The bulk of a tree trunk is dead
tissue (see Fig7) the secondary
xylem. These cells no longer
transport water; they are
clogged with resins and other
metabolic by-products that
make the heartwood resistant to
rotting. The lighter-coloured
sapwood consists of younger
secondary xylem that does
conduct water.
What does the above experiment tell you about single cells from the carrot?
How did those single cells divide?
Morphogenesis in plants: control of organ development
Morphogenesis means the development of body form, and plants, like animals, have different body organs that have to
develop from undifferentiated cells. These cells are found in the meristems of plants. Differentiation is controlled by organ
identity genes.
One plant which has been studied extensively is Thale Cress Arabidopsis thaliana.
The development of flowers is controlled by 3 genes which code for proteins which act as transcription factors in
combination with each other.
Q8 What would be formed if only transcription factor A was present
What if A and B were present?
If A and C were present?
If only C was present?
These genes are very interesting to genetic engineers involved in modification of
genes in crop plants. Many of our staple foods, like maize, wheat, rice, are formed from
seeds.
Q9 Suggest why modifications to the C gene could be an advantage to a farmer.
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6. In another application, a gene called leafy controls the transcription of ABC genes. Mutation in leafy causes plants to
produce leaves but no flowers. A small amount of the protein coded for by leafy makes plants produce no flowers.
Large amount of leafy protein makes plants produce more flowers. Manipulation of the activity of the gene can make
citrus trees produce flowers earlier in their life cycle.
Check your understanding
1. Determinate growth means ……………………………………………………………………………………………
2. Indeterminate growth is when an organism continues to grow throughout its life - TRUE or
FALSE?
3. An annual plant completes its lifecycle in 2 years - TRUE or FALSE?
4. A meristem is (A) made of undifferentiated cells (B) made of unspecialized cells (C) made of
cells capable of mitosis (D) found at tips of roots and shoots (E) all of the above
5. Cell division in apical meristems and elongation of cells results in ______________________ growth.
6. Elongation of cells happens because of the entry of ______________into the cells.
7. Differentiation leads to the formation of 3 types of primary tissue which are _________________,
________________________ and _______________________________
8. The three zones of a root tip are a) _____________________________
b)________________________________________________and c)____________________________________________________
9. Which two meristems are involved with secondary growth?
10. Division of cambium produces two new tissues = ____________________________________and
_____________________________________________
11. The bark of a tree is made of (A) cork cambium, cork and periderm (B) phloem and xylem
(C) cork cambium, cork and secondary phloem (D) cork cambium, cork and secondary xylem
(E) secondary xylem and secondary phloem only
12. What is wood made of? 13. What is the function of cork?
14. Explain how annual growth rings are produced.
15. Study the experiment in the box on p5 and answer these questions.
a) What is meant by nutrient medium? b) What type of cell division happened in the single cells?
c) What is meant by a somatic cell? d) Suggest some advantages of this type of plant culture.
16. Define morphogenesis.
17. What controls differentiation of unspecialised cells into specialised plant organs?
18. Remembering work on protein synthesis from Genetics module, describe how transcription
factors can influence gene expression.
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7. In another application, a gene called leafy controls the transcription of ABC genes. Mutation in leafy causes plants to
produce leaves but no flowers. A small amount of the protein coded for by leafy makes plants produce no flowers.
Large amount of leafy protein makes plants produce more flowers. Manipulation of the activity of the gene can make
citrus trees produce flowers earlier in their life cycle.
Check your understanding
1. Determinate growth means ……………………………………………………………………………………………
2. Indeterminate growth is when an organism continues to grow throughout its life - TRUE or
FALSE?
3. An annual plant completes its lifecycle in 2 years - TRUE or FALSE?
4. A meristem is (A) made of undifferentiated cells (B) made of unspecialized cells (C) made of
cells capable of mitosis (D) found at tips of roots and shoots (E) all of the above
5. Cell division in apical meristems and elongation of cells results in ______________________ growth.
6. Elongation of cells happens because of the entry of ______________into the cells.
7. Differentiation leads to the formation of 3 types of primary tissue which are _________________,
________________________ and _______________________________
8. The three zones of a root tip are a) _____________________________
b)________________________________________________and c)____________________________________________________
9. Which two meristems are involved with secondary growth?
10. Division of cambium produces two new tissues = ____________________________________and
_____________________________________________
11. The bark of a tree is made of (A) cork cambium, cork and periderm (B) phloem and xylem
(C) cork cambium, cork and secondary phloem (D) cork cambium, cork and secondary xylem
(E) secondary xylem and secondary phloem only
12. What is wood made of? 13. What is the function of cork?
14. Explain how annual growth rings are produced.
15. Study the experiment in the box on p5 and answer these questions.
a) What is meant by nutrient medium? b) What type of cell division happened in the single cells?
c) What is meant by a somatic cell? d) Suggest some advantages of this type of plant culture.
16. Define morphogenesis.
17. What controls differentiation of unspecialised cells into specialised plant organs?
18. Remembering work on protein synthesis from Genetics module, describe how transcription
factors can influence gene expression.
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