Why a Potato Is a Stem but a Sweet Potato Is a Root

Carrots, potatoes, ginger, onions and spinach all come from plants — but they are different organs. A carrot is a root, a potato and ginger are stems, onion layers are fleshy leaves, and spinach is leaf.

This part covers root systems and the regions of a root, root modifications, stem modifications, and leaf structure and arrangement.

A tap root system grows from the radicle into one main root with branches, typical of dicots; a fibrous root system replaces the short-lived primary root with many similar roots from the stem base, typical of monocots; and adventitious roots arise from any part other than the radicle.

- Tap root — mustard, gram, mango
- Fibrous roots — wheat, rice, grasses
- Adventitious roots — grass stems, Monstera, the banyan tree

Regions of a root tip, from the tip upwards:

- Root cap — a thimble-like cover protecting the tender tip as it pushes through soil
- Region of meristematic activity — small, thin-walled cells with dense cytoplasm, dividing repeatedly
- Region of elongation — cells lengthen and enlarge, increasing the root's length
- Region of maturation — cells differentiate; root hairs grow here from epidermal cells and absorb water and minerals

An everyday example. Grass pulled from a lawn comes up with a mat of fibrous and adventitious roots that hold soil firmly.

The substance. Root hairs form only in the region of maturation, not at the very tip.

Roots can swell to store food, grow from stems or branches to support the plant, or grow upwards out of waterlogged mud to take in air — each change matching a special function.

Storage roots:

- Carrot, turnip and radish — swollen tap roots
- Sweet potato — a swollen adventitious root (tuberous root)

Support roots:

- Prop roots — hang down from the branches of the banyan tree and thicken into pillars
- Stilt roots — grow from the lower nodes of maize and sugarcane into the soil

Respiration:

- Pneumatophores — in mangroves such as Rhizophora in swampy soil, some roots grow vertically upwards above the mud to obtain oxygen

An everyday example. The mangrove forests of the Sundarbans are dotted with pencil-like pneumatophores poking out of the mud at low tide.

The substance. A sweet potato is a root because it has no nodes or buds, even though it looks like a potato.

Stems can store food underground, spread the plant, climb, protect or even take over photosynthesis, and they can always be recognised as stems because they bear nodes, internodes and buds — which roots never have.

Underground storage stems:

- Tuber — potato; its "eyes" are nodes with buds
- Rhizome — ginger and turmeric, growing horizontally with nodes and scale leaves
- Corm — Colocasia (arbi)
- Bulb — onion and garlic: a short disc-like stem bearing fleshy leaves

Stems that spread:

- Stolon — slender branches arching down to root, as in mint and jasmine
- Runner — creeping stems rooting at nodes, as in grasses and strawberry

Stems for climbing, protection and photosynthesis:

- Tendrils — from axillary buds in cucumber, pumpkin and grapevine
- Thorns — axillary buds turned woody and pointed in Citrus and Bougainvillea
- Phylloclade — a flattened green stem in Opuntia, whose leaves are reduced to spines

An everyday example. Ginger and haldi bought fresh from the market show rings and small buds — clear signs that they are underground stems.

The substance. Thorns come from buds and are stems; spines are modified leaves — a difference based on origin, not appearance.

A typical leaf has a leaf base, a petiole and a lamina with veins, and it always has a bud in its axil; leaves are classified by venation as reticulate or parallel, by type as simple or compound (pinnate or palmate), and by arrangement on the stem as alternate, opposite or whorled.

Parts of a leaf: the leaf base joins the stem (swollen as a pulvinus in legumes, a sheath in many monocots), the petiole holds the blade, and the lamina carries a midrib and veins.

Venation:

- Reticulate — veins form a network: most dicots
- Parallel — veins run side by side: most monocots

Leaf type:

- Simple — lamina entire, or cut but not down to the midrib
- Pinnately compound — leaflets along a common axis (rachis): neem
- Palmately compound — leaflets attached together at the tip of the petiole: silk cotton

Phyllotaxy:

- Alternate — one leaf at each node: china rose, mustard, sunflower
- Opposite — two leaves at each node, facing each other: Calotropis, guava
- Whorled — more than two leaves at a node: Alstonia

Worked example — leaf or leaflet? A neem "leaf" that looks like many small leaves on one stalk has a bud only where the whole stalk meets the stem — so it is one compound leaf made of leaflets.

An everyday example. The neem twig used for brushing teeth carries pinnately compound leaves.

The substance. A bud is found in the axil of a leaf but never in the axil of a leaflet — the surest test for a compound leaf.