The classification of plant organs                                             

 

 

 

 

 

The stems, roots and leaves of plants form an integrated structure, composed of  the dermal, ground and vascular tissue systems. You may expect to see the same tissues in each of them, but the tissues may be  rearranged and can either increase in importance or perhaps decrease in importance depending on the role of the organ concerned. An example of this would be the change in distribution of the mechanical tissues in leaves and stems, compared to roots.

We need to be able to separate and classify the components of plants into their different structures, using a relatively easily method, using a set of criteria that (usually!) work.

 

We have drawn together a set of useful characteristics which will help you to decide if the material you are looking at is of stem, root or leaf origin. Additionally, you will be able to determine organ origin as well.

 

Classification characters

1. STEM

1. Presence of a recognizable cortex.

2. Xylem and phloem occur on the same radius.

3. Fibres prominent in cortex and associated with the vascular bundles.

4. May have a starch sheath.

2. ROOT

1. (Usually) no major fibre bundle groups.

2. Xylem & phloem alternate in strands not on same radius.

3. Cortex (in young root) not differentiated – usually parenchymatous.

3. LEAF

1. High proportion of chlorenchymatous cells (mesophyll) which may be differentiated (palisade & spongy mesophyll) or undifferentiated.

2. Small bundles subtended (supported) by the mesophyll.

3. Large number of stomata (adaxial or abaxial or both sides).

4. May be dorsiventrally flattened, but could be rounded.

4. DICOTYLEDON

1. Presence of a fascicular cambium between primary xylem and primary phloem OR between secondary phloem and secondary xylem.

2. Starch sheath usually present in the young stem, separates and demarcates cortex from stele.

3. .Vascular tissue contains metaxylem and protoxylem VESSELS and TRACHEIDS.

5. MONOCOTYLEDON

1. Outer ring of vascular bundles (may be attached to  the epidermis by collenchyma or sclerenchyma fibers.

2. Vascular bundles primary only (closed, no fascicular cambium).

3. Vascular bundles contain large metaxylem (usually two vessels) as well as a prominent protoxylem lacunae.

4. Inner ‘rings’ actually spiral towards centre of the stem.

6. GYMNOSPERM

1. No protoxylem or metaxylem vessels, tracheids and fibers only in xylem.

2. Transfusion tracheids and transfusion parenchyma surround the vascular tissues.

3. Phloem consists of sieve cells, albuminous cells, and parenchyma.

4. Resin ducts always present - but take care, there are some dicots e.g. Helianthus stem which also contain resin ducts!

5. Conspicuous endodermis surrounds individual vascular bundles (usually only in leaves though).


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