Monocots and Eudicots

Biol 324 Introduction to Seed Plant Taxonomy

Other Groups – Monocots and Eudicots

- Most flowering plant diversity is in monocots and eudicots
- Monocots - big families: grasses (15,000 species), orchids (20,000 species)
- Eudicots - big families: legumes (18,000 species), Asteraceae (22,000 species)

- Monocots and eudicots typically pollinated by Lepidoptera (butterflies and moths) and Hymenoptera (wasps and bees - Apidae) as well as birds and mammals

Trends in monocots and eudicots
- Differentiation of the calyx and corolla from undifferentiated tepals (ie. difference between sepals and petals)
- Reduction of numbers of floral organs from indefinite to definite numbers (3, 4, 5 or multiples)
- Change in symmetry from radial to bilateral to asymmetrical
- Change from spiral to whorled arrangement of parts to whorled, with compression of receptacle

What drives this evolution? Synorganization (organization together into a unit) – can have a specific relation with pollinator, must control by producing precise and complex shapes. All pieces of the flower have a distinct job (therefore indefinite number of parts etc are rare). Can establish patterns for recognition.

[Question about generalist pollinators – it used to be thought that beetles were generalist pollinators (insects that can get inside whatever flower is available and did not have a specific relationship with the plants)….., but closer examination has revealed more
complex and specialized relationships between what used to be considered generalists (beetles) and flowers. Beetles are not so much attracted by floral structure, but other features (volatiles, heat….)

Monocots: an important and very species rich clade (clade = a branch on the phylogenetic tree) marked principally by leaf characters.
- Monophyletic group distinguished by derived character of one cotyledon
- Leaves usually parallel veined, plants herbaceous, pollen monosulcate or monoporate, perianth parts in threes (e.g. daffodils, tulips) or lost/vestigial (e.g. grasses), pistils simple or compound.
- no secondary growth. Note: palms form large woody trunks with a characteristic type of thickening, but because they are monocots have no typical vascular cambium and therefore no secondary xylem formed in rings. How do they get so big? They produce more vascular bundles with fibres between.

Leaf venation
- Monocots usually have parallel venation and often narrow, grass-like leaves
- Other angiosperms usually have reticulate (netted) venation
- Monocots with wide leaves and netted veins have evolved them secondarily
- Exceptions to the venation rule: e.g. eudicot weed plantain (Plantago) vs monocot water plantain (Alisma)
Plantago – parallel veins (leaf develops from leaf base) – analogous to monocots
Alisma - netted veins, even though a monocot

Leaf development
- In monocots the leaf generally develops mainly from the lower part of the leaf primordium and the upper part remains small or aborts. Monocot leaves have therefore developed evolutionarily and morphologically from the leaf base. The vestigial tip often present at end of a monocot leaf has therefore been considered equivalent to a reduced leaf "blade" (in eudicot terms).
- In the non-monocot groups, the leaf lamina develops from the upper part of the primordium. The lower part gives rise to the leaf base and leaf petiole (eudicot leaves are made up of three parts: base, petiole, blade)

Eudicots (buttercups, beans, brassicas, etc)
- The rest of the angiosperms comprise the Eudicots: a clade (monophyletic group) distinguished by the derived character of triaperturate pollen (i.e. tricolpate pollen, or types derived from tricolpate pollen)
- Characters: two cotyledons, leaves generally net veined, plants woody or herbaceous, perianth parts mostly in fours or fives or lost, pistils simple or compound.

Simple vs compound leaves
- All monocot leaves are basically simple, with very few exception such as the aroid Zamioculcas (Araceae). Zamioculcas develops like eudicot leaf, but this is a secondarily acquired character. Palm leaves may look compound, but are actually simple but secondarily compound by splitting (e.g. Raphis). The Swiss cheese plant (Monstera) has a false compound leaf, split secondarily by programmed cell death. Leaf cells go into cell death pathway.

- In contrast, Eudicot leaves often compound. Compound leaves are well illustrated by the Fabaceae (legume family), e.g. Leucaena retusa – bipinnate leaf (doubly pinnate)

Mechanism of compound leaf formation: reactivation of knotted homeobox (KNOX) genes in location of leaflet
- KNOX gene expression is characteristic of apical meristems to ensure continuous cell divisions (indeterminate growth) and it is downregulated during leaf primordium formation. It is not switched back on in simple leaf development (the leaf is a determinate structures, i.e. is of limited growth). However, KNOX gene expression “comes back on” later in development at leaflet organogenic sites and then again downregulates (leaflets too are determinate). Only eudicots have this feature (basal angiosperms, Magnoliids, and monocots having simple leaves).

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