The classification of plant parts

Stems, roots and leaves are really an extension of the dermal, ground and vascular systems. We may expect to see the same tissues in each of them, but these are re-arranged and either increased in dominance, or perhaps decreased in importance, depending on the role of the organ concerned.

What we need to be able to do is to easily determine what the different plant structures are so that we can separate and classify the components of plants into their different structures. So, what we need are  a set of criteria that (usually!) work.

Characters that have been used successfully in order to classify and characterize organs from sections are listed in the Table below. 

Hopefully, these will work for you as well!

 

 

Classification

Character

STEM

1. Presence of a cortex.

2. Xylem and phloem on the same radius.

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

4. May have a starch sheath.

ROOT

1. (Usually) No major fiber bundle groups.

2. Xylem & phloem not on same radius.

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

LEAF

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

2. Small bundles subtended by the mesophyll.

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

4. May be dorsiventrally flattened.

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.

MONOCOTYLEDON

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

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

3. Vascular bundles contain large metaxylem (usually 2) vessels, as well as prominent protoxylem lacunae.

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

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 very obviously present (but take care, may be some in dicots e.g. Helianthus stem contains some).

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


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