External Structure and Importance of a Flowering Plant
Senior 1 · Biology
External Structure and Importance of a Flowering Plant
Lesson 4 of 5
25 min
Soak a bean and a maize grain in water overnight. Press the bean gently and it falls into two halves, like a book opening. Try the same with the maize grain and it will not open: you have to cut it. The difference is not an accident. It is the first clue to the two great groups of flowering plants.
You should find that each difference in the seed goes with a difference in the leaf and in the roots. Botanists sort flowering plants into two groups by these differences.
Definition
Cotyledon (seed leaf): a leaf that is part of the embryo inside a seed. It stores food or absorbs food for the young seedling.
Monocotyledon (monocot): a flowering plant whose seed has one cotyledon. Examples: maize, sorghum, rice, banana, onion, grasses.
Dicotyledon (dicot): a flowering plant whose seed has two cotyledons. Examples: bean, groundnut, coffee, cassava, avocado, eucalyptus.
Vascular bundle: a strand of tubes that carries water and food through the plant; the veins of a leaf are vascular bundles.
Figure 1: Five ways to tell a monocot from a dicot.
| Feature | Monocots | Dicots |
|---|---|---|
| Cotyledons in the seed | one | two |
| Leaf veins | parallel | net (reticulate) |
| Leaf shape | usually long and narrow, often with a sheath around the stem | broad, often with a petiole |
| Flower parts | in threes or multiples of three | in fours or fives, or multiples of these |
| Root system | fibrous | tap root |
| Vascular bundles in the stem | scattered | in a ring |
| Woody trunks | almost never (the banana "trunk" is made of leaf bases) | common (eucalyptus, avocado, coffee) |
| Rwandan examples | maize, sorghum, rice, wheat, banana, sugar cane, onion, pineapple, Napier grass | beans, groundnuts, soya, coffee, tea, cassava, sweet potato, Irish potato, avocado, eucalyptus |
Warning
Never classify on one feature alone. A few plants break single rules: some monocots have net-like veins, and the banana leaf has a strong midrib that makes it look like a dicot leaf. Check at least three features (veins, flower parts, roots, seed) and go with the majority.
Worked example 1: monocot or dicot?
A plant in a Musanze garden has narrow leaves with parallel veins, flowers with 6 purple tepals and 6 stamens, and a clump of fibrous roots. Classify it.
| Feature | Observation | Points to |
|---|---|---|
| Leaf veins | parallel | monocot |
| Flower parts | 6 and 6 (multiples of 3) | monocot |
| Roots | fibrous | monocot |
Answer: all three features agree: it is a monocot.
Worked example 2: is the banana a monocot?
A student argues that a banana is a dicot "because its leaf has a midrib and it grows as tall as a tree". Test the claim.
Answer: the banana is a monocot. The student relied on two features that only look like dicot features.
Rwandan farmers often grow maize and beans in the same field. The two crops suit each other partly because they belong to different groups: their root systems grow to different depths (maize roots usually go deeper than bean roots) and use nutrients differently: bean roots carry nodules whose bacteria add nitrogen to the soil. The difference also matters for weed control: some weedkillers (selective herbicides) kill broad-leaved dicot weeds but do not harm grass-family crops such as maize, rice and wheat.
Monocots vs Dicots (Neural Academy). Pause at each feature and check it against the table above.
Tip
Memory hook: monocots are the "3 P" plants: Parallel veins, Parts in threes, Plenty of thin (fibrous) roots. Dicots are the opposite on every count.
Practice: monocots and dicots
Key points
Sources and credits
Figure 1, the tables, the worked examples and the practice questions are written by Ganzaa.
