9.1.1 Draw and label plan diagrams to show the distribution of tissues in the stem and leaf of a dicotyledonous plant.(1)

These are low power diagrams that show the distribution of the different tissue types. Cell structure is not required for this syllabus statement.

Stem cross section (Helianthus spp)

stem section

Tissue types of the plant stem:



Cell diagram for comparison (the syllabus requires you know the tissue diagram)

stem section




Leaf section:






9.1.2 Outline three differences between the structures of dicotyledonous and monocotyledonous plants.(2)

differences monocot dicot



Time out for some TOK:

Take a walk around your garden or local park and using the table above classify into Monocotyledon and Dicotyledon plant. In doing so you will be following in the foot steps of John Ray(1627-1705). These differences first published in "Method of plants' 1682 by John Ray. Geoffrey Keynes described Ray's work as 'epoch-making piece of work' of 'historical significance'. Ray's work was so far ahead of its time that its importance was not recognised until very much later. Students of history should not the dates of Ray's life and consider the historical significance of this time in England (Charles II and the Act of Uniformity). Ray is also credited with one of the earliest attempt to define a species. His definition (look it up) was an early version (more than 300 years ahead) of the modern ( but very similar to) definitions provided by Ernst Mayr.

Discussion: which other scientist or scientific concepts (at the time of their publication) were not recognised as they were so a far ahead of the rest of contemporary thinking.





Illustrations of the differences between monocotyledon and dicotyledonous

Shorea siaimensismonocot leaf

dicot petal 5monocot petals


mono dicot root


(a) The fibrous branching roots of the monocotyledon

(b) The tap root structure with lateral roots of the dicotyledon .








9.1.3 Explain the relationship between the distribution of tissues in the leaf and the functions of these tissues.( 3)

leaf tissue


(a) Phloem transports the products of photosynthesis (sugars, amino acids).

(b) Xylem transports water and minerals into the leaf tissue from the stem and roots.

(c) Epidermis produces a waxy cuticle for the conservation of water.

(d) Palisade layer which is the main photosynthetic region.

(e) Spongy layer creates the spaces and surfaces for the movement of water and gases.

(f) Lower epidermis contains the stomatal pores which allow gas exchange with the leaf.

The xylem and phloem tissues combine in the vascular tissue to provide support to the leaf.








9.1.4 Identify modifications of roots, stems and leaves for different functions: bulbs, stem tubers, storage roots and

onion bulb


Stem modification:

Bulbs: Onions & Lilies







strawberry runner

stem modification:








Stem modification: e.g. Cacti








Root tip modification Stem Tuber/ Potato






carrot LS

Carrot: Tap Root modification
















9.1.5 State that dicotyledonous plants have apical and lateral meristems.(1).

meristem location


Plants grow is restricted to 'embryonic' regions called meristems. Having specific regions for growth and development (restricted to just the meristematic tissue), contrasts with animals in which growth takes place throughout the whole organism.

Apical meristems are found at the tip of the root and the shoot, adding growth to the plant in these regions.

The apical meristems are described as indeterminate , this type of growth tends to add length to root and stem in 'module' or 'units' (described below).

This tissue remains 'embryonic' for prolonged periods of time and in some cases over 100's of years. Contrast this with the more determinate growth of leaves, petals and flowers in which a very precise growth occurs.










Terminology for plant growth and development.


The diagram below is of the apical or primary meristem tissue of a plant.

The meristem tissue retains its capacity to divide and renew.

apical meristem



(a) Shoot apical meristem

(b) Leaf primordial

(c) Axillary bud primordium

(d) leaf

(e) Stem tissue







Root apical meristem:

root meristem


(a) Root cap.

(b) Root apical meristem.

(c) Ground meristem.

(d) Protoderm.

(e) Epidermal tissue of the root.

(f) Vascular tissue (central stele).











9.1.6 Compare growth due to apical and lateral meristems in dicotyledonous plants.(3)

Growth in plants is brought about initailly with cell division at the apical and lateral meristem.

Observation of shoot tip or root tips with a light microscope reveals regions with cells in the various stages of mitosis.

At the shoot tip or root tip the growth process is adding length to the stem or root.









After cell growth adds length to the shoot tip or root tip further differentation of the tissue results in additional structures added. Tissue is added in the form of 'modules' of tissue.

plant module


The tissue added includes the units described below:

1. Adds length to the stem and root

2. Added in modules.

3. Each module is added at the meristem and includes leaf (leaves), internode length of stem and axillary buds.




Stem differentiation at the apical meristem.

These diagrams illustrate that the tissue added at the apical meristem differentiates into the various primary plant body structure (AB). lateral meristem growth


primary meristematic tissue


This tissue diagram is a cross section of the stem of the primary plant body.

This means that there has been no additional secondary thickening of the cell walls.












Secondary growth added by the Lateral meristem (cambium) has two types:




9.1.7 Explain the role of auxin in phototropism as an example of the control of plant growth.(3)

notes and diagrams based on Growth and Differentiation in plants, 3rd ed, Wareing and Phillips.

A tropism is a bending-growth movement either toward or away from a directional stimulus.

Phototropism is the bending-growth towards the unilateral source of light.

Auxins are a class of plant growth hormones (growth regulating factor)

Auxins are one of atleast three major classes of plant growth regulators. Unlike animal hormones plant hormones can provide a range of responses from the tissues.


The most common auxin is IAA ( Indolacetic acid). IAA was discovered in 1932 and is believed to be the principle auxin in higher plants.

Auxin is associated with the phenomenon of phototropism.



auxin darwin



Charles Darwin studies of auxin effects are published a book called, 'The Power of movement'.

Darwin studied phototropism using the germinating stem of the canary grass (Phalaris canariensis).

The cylindrical shoot is enclosed in a sheath of cells called the coleoptile.




Darwin set out to determine which region of the coleoptile is sensitive to light.

(a) When there is a unilateral light shinning on one side of a coleoptile there is a bending growth movement towards the light.

(b) Decapitation of the tip results in no bending growth suggesting that this region is possibly sensitive to the light stimulus.

(c) The opaque cap prevents light from reaching the tip without damaging the tip as in (b). There is no bending-growth response.

(d) The buried coleoptile (except tip) show that it is not the lower stem section that is responding to light but rather the tip.

Darwin's experiments suggest that the tip is the region sensitive to light. Darwin concluded, " when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend".



Boysen Auxin

Boysen- Jensen experiments of 1913 showed that the substance traveling down the coleoptile stem was of a chemical nature.

(e) The mica plate is an un-reactive substance that is inserted on one side of the stem. With the mica in place and the unilateral distribution of light there is no bending-growth. This suggests that the growth promoting substance is prevented from moving down the shaded side of the stem.

(f) Note that when the mica placed on the exposed side it does not prevent bending-growth. In combination with the previous observation this suggests the growth promoting substance is chemical and moving down the shaded side.



Paal auxin


(g) The coleoptile is decapitated

(h) A gelatine block permeable to chemical diffusion is placed between the stem and the root tip.

(i) The reconstructed coleoptile still shows the bending-growth response with the unilateral distribution of light.

Again these Boysen-Jensen experiments add confirmation that the growth promoting substance is chemical in nature.





paal auxin


The elegant experiments of Paal (1919) confirm the work of Boysen-Jensen.

(j) Decapitation of the coleoptile tip.

(k) Replacement of the coleoptile tip but asymmetrically over one side of the coleoptile stem.

(l) With NO LIGHT, there is a bending-growth, with the overlapped side experiencing the growth.

Paal also suggested that in the dark or light from all sides the growth promoting substance was sent uniformly down the stem.



Auxin, the growth promoting substance, was first isolated by F. W. Went in 1926. The actual structure shown above for auxin was not determined until 1932.

Went Auxin

(m) Went's experiments extended the work of Paal, Boysen and Jensen by isolating the auxin onto agar gel.

(n) The gel was cut up into block as a way of quantifying the dose of auxin used.

(o) The agar block (containing auxin) are placed asymmetrically on the stem.

(p) The angle of bending-growth was measured.




Bio-assay: Went then developed a technique known as a bio-assay which the effect of a chemical (auxin in this case) is measured by its effects on living tissue.

auxin assay

This graph shows the results of changing the number of coleoptile tips placed on a single agar block. In effect this means the more tips on the block the greater the concentration of auxin.

This graph suggests:






IAA3 assay


Since Auxin (IAA3) was synthetically produced more rigorous quantitative bio-assay can be performed.

This graph measures the bending-growth against the concentration of IAA3.

Note that the graph suggests:



Transport of Auxin:

Auxin is transported through the cell membrane of the adjacent plant cells by protein carriers in the plasma membrane.

These carriers transport the anion of auxin in polar direction, from the top of the cell to the bottom of the cell.

However the stimulus of light would seem to result in the introduction of these carriers into the side of the cell membranes so that the IAA3 can now be laterally transported.

Whilst not part of the examination syllabus for IB Biology look at the many connections that can be made to the various parts of the syllabus including, cell structure; plasma membrane; cell transport.

The role of auxin:

Since IAA3 is a 'hormone' there must be some link between the signal molecule and the sub cellular responses and the cellular responses. It appears that it is the receiving cell that determines the exact cellular response rather than IAA3 having very specific responses across all cells.

As we have noted one of the major functions of auxins is the promotion of growth. Research has shown that in some tissues IAA3 promotes mitosis whilst in other tissue, it promotes cell enlargement.



Click4Biology: Topic 9.1 Plant structure and growth

Plant structure and growth

9.1.1 Dicotyledonous stem and leaf structure.

9.1.2 Differences between dicotyledonous and monocotyledonous structure.

9.1.3 Leaf tissue distribution and function.

9.1.4 Modification of root, stem and leaf.

9.1.5 Apical and lateral meristem in dicotyledonous.

9.1.6 Growth in the apical and lateral meristems of dicotyledonous plants.

9.1.7 The role of auxin in phototropism as an example of the control of plant growth.
notes and diagrams based on Growth and Differentiation in plants, 3rd ed, Wareing and Phillips.