Types and Modifications of Stem

Types of Stem

The stem may be:

  1. Aerial - erect, rigid, strong and upright as in herbs, shrubs and trees
  2. Sub aerial - weak, unable to stay upright and trail on ground as creepers or climb up as climbers
  3. Underground - buried in soil and produces aerial branches under favourable conditions only

Modifications of Stem

Stems are variously modified into underground, sub aerial and aerial stems for performing functions like manufacturing and storing food, perennation (overcoming unfavourable climatic conditions), providing mechanical support and protection and for propagating vegetatively.

Underground Modified Stems

Since underground, they may seem like roots but you can recognize them as stem due to the presence of nodes & internodes, scaly non green leaves and buds. They serve two functions:

  1. Act as perennating structures by remaining leafless and dormant in winter but giving off aerial shoots under favourable conditions.
  2. Store food and become thick and fleshy.

Rhizome

Thick, fleshy, flattened horizontally growing stem near the soil surface. Bears scale leaves on nodes, terminal and axillary buds, adventitious roots. Examples: Ginger, Turmeric

Corm

Fleshy, spherical stem with flattened base, grows vertically; bears many scale leaves, distinct nodes and internodes, buds and adventitious roots. Examples: Saffron, Yam, Gladiolus

Bulb

Reduced, flattened discoid stem with crowded nodes bearing overlapping fleshy (inner) and dry (outer) scale leaves. Terminal bud (in centre) forms foliage (green) leaves. Adventitious roots grow from discoid base. Example: Onion

Tuber

Swollen tips of underground lateral branches of stem, store food as starch, bear eyes. Each eye is a node which bears bud and scar of scale leaves. Example: Potato

Sub Aerial Modifications of Stem

Stems are weak, therefore lie prostrate on the ground or may get partially buried in the top soil. The plants bearing such stems are called creepers. Their stems serve the function of vegetative propagation.

Runner

Long, weak, slender branch with long internodes. Runs horizontally on soil surface giving off adventitious roots at nodes. Examples: Grass, Oxalis

Stolon

Weak lateral branch which grows upwards then arches down to meet the soil, strike roots and produce daughter plants. Examples:  Mint, Jasmine

Offset

Like runner but thicker and shorter, grow for a short distance then produce cluster (rosette) of leaves above and adventitious roots below; generally in aquatic plants. Examples: Water hyacinth, Water lettuce

Sucker

Underground runner which grows horizontally for a distance under soil then emerges obliquely upwards, strikes roots and forms daughter plants. Example: Chrysanthemum

Aerial Stem Modifications

Whole stem or its part (axillary or terminal bud) gets modified to perform definite functions. You can recognize them as stems by following features:

  • (i) Arise in the axil of leaf
  • (ii) Bear nodes and internodes
  • (iii) may bear leaves, buds, flowers

Stem tendrils

Thread like, spirally coiled, leafless structures (tendrils) which twine around neighbouring objects and help weak plants to climb. Example: Grape vine

Thorns

Straight, pointed, hard structures; modifications of axillary (Citrus) or terminal (Carissa) bud; act as defence organs or as climbing organs. Examples: Citrus, Duranta Carissa

Phylloclade

Green, flattened or cylindrical fleshy stem, with nodes and internodes; bears spines (modified leaves to check evaporation); carries out photosynthesis, stores water. Found in plants growing in dry regions. Example: Opuntia

Cladode

It is a phylloclade with limited growth i.e. with only one or two internodes; help in photosynthesis. Example: Asparagus