How a Pitcher Plant Turns a Leaf Into a Trap

A carrot, a potato and the spines of a cactus do not look like an ordinary root, stem or leaf — yet that is exactly what they are. Plants reshape these organs to store food, breathe in waterlogged soil, climb, defend themselves and even catch insects.

This lesson covers root types and modifications, stem modifications, and leaf structure, venation, phyllotaxy and leaf modifications.

Roots are of three types — taproots that grow from the radicle, fibrous roots that replace a short-lived primary root, and adventitious roots that grow from parts other than the radicle — and they are modified to store food, to breathe and to give extra support.

Types of roots:

- Taproot system — a main root with lateral branches, as in mustard and most dicots
- Fibrous root system — the primary root is short-lived and replaced by many roots from the stem base, as in wheat
- Adventitious roots — arise from stems or leaves, as in grasses, Monstera and the banyan tree

Storage roots:

- Modified taproots — carrot, radish and turnip
- Adventitious tuberous roots — sweet potato

Respiratory roots. Mangroves such as Rhizophora, growing in swampy, salty mud, send up pneumatophores that rise vertically above the soil to take in oxygen.

Support roots:

- Prop roots — hang down from branches and grow into the soil, as in the banyan tree
- Stilt roots — grow from the lower nodes of the stem, as in maize and sugarcane

An everyday example. The pillar-like roots holding up the spreading branches of an old banyan tree in a village square are prop roots.

The substance. A sweet potato is a root, but a potato is a stem — the potato has eyes, which are buds at nodes, and roots have no nodes.

Stems are modified underground to store food and survive bad seasons, along the ground to spread by creeping branches, and in the air to climb, protect or photosynthesise — and a modified stem can always be recognised by its nodes, internodes and buds.

Underground stems:

- Tuber — potato, with buds in its eyes
- Rhizome — ginger and turmeric, growing horizontally with scale leaves
- Corm — Colocasia (arbi), a short, upright, swollen stem
- Bulb — onion and garlic, with fleshy scale leaves around a reduced stem

Sub-aerial stems:

- Runner — grass and strawberry, creeping along the soil and rooting at nodes
- Stolon — mint and jasmine, a slender branch that arches down to the ground
- Offset — Pistia and Eichhornia (water hyacinth), a short runner ending in a rosette of leaves
- Sucker — banana, pineapple and Chrysanthemum, growing up from an underground node

Aerial modifications:

- Stem tendrils — from axillary buds, in cucumber, pumpkin, watermelon and grapevine
- Thorns — from axillary buds, in Citrus and Bougainvillea, for protection
- Phylloclade — a flattened, green, fleshy stem in Opuntia that does the work of leaves

An everyday example. Ginger bought at a sabzi market is a rhizome — its rings are nodes, and new shoots sprout from buds along it.

The substance. Thorns and spines look alike but differ in origin — a Bougainvillea thorn comes from an axillary bud, while the spines of a cactus are modified leaves.

A leaf has a leaf base, a petiole and a lamina with veins; its venation is reticulate in dicots and parallel in monocots, its phyllotaxy is alternate, opposite or whorled, and leaves can be modified into tendrils, spines, storage organs, organs of reproduction and insect traps.

Parts of a leaf. The leaf base attaches the leaf to the stem, sometimes with small stipules or, in legumes, a swollen pulvinus; the petiole holds the blade towards the light; the lamina has a midrib and veins that support it and carry water and food.

Venation:

- Reticulate — veins form a network, as in dicots such as the peepal
- Parallel — veins run side by side, as in monocots such as grasses and banana

Phyllotaxy:

- Alternate — one leaf at each node, as in china rose, mustard and sunflower
- Opposite — a pair at each node, as in Calotropis and guava
- Whorled — more than two at a node, as in Alstonia

Leaf modifications:

- Support — leaf tendrils in pea
- Protection — spines in cacti
- Storage — fleshy leaves of onion and garlic
- Reproduction — Bryophyllum leaves bear buds in the notches of the margin that grow into new plants
- Insectivorous plants — the pitcher of Nepenthes and the bladders of Utricularia are modified leaves that trap insects

An everyday example. A Bryophyllum leaf left on damp soil on a windowsill soon sprouts tiny plantlets from the notches along its edge.

The substance. Insectivorous plants still photosynthesise — they trap insects for nitrogen because they grow in poor, boggy soils, not because they cannot make their own food.