Root Stem Leaf

5.1 Introduction to Morphology of Flowering Plants

Flowering plants exhibit enormous variation in shape, size, and structure. To understand and identify them, we study their morphology. The angiosperm plant body is differentiated into an underground root system and an above-ground shoot system (consisting of stem, branches, leaves, flowers, fruits, and seeds).

Even though flowering plants display vast diversity, they share a few basic features. The root, stem, leaf, flower, fruit, and seed are the principal organs we will study in this chapter.

5.2 The Root

The roots of most dicotyledonous plants develop from the radicle of the embryo. The primary root grows downward into the soil and gives off lateral roots, called secondary and tertiary roots — together forming the tap root system, as in mustard, mango, gram.

In monocotyledonous plants, the primary root is short-lived and is replaced by a large number of roots originating from the base of the stem. This forms the fibrous root system, as in wheat, paddy, grass.

In some plants, like grass, banyan tree, and Monstera, roots arise from parts of the plant other than the radicle — these are called adventitious roots.

Functions of the Root

• Absorption of water and minerals from soil
• Anchoring the plant body
• Storing reserve food (in modified roots)
• Synthesis of plant growth regulators

5.2.1 Regions of the Root

The root is covered at the apex by a thimble-like root cap, protecting the tender apex as it pushes through soil. Behind the cap is the meristematic region of meristematic activity. Cells of this region are very small, thin-walled with dense protoplasm. They divide repeatedly. Behind it is the region of elongation where cells rapidly enlarge and elongate, contributing to root growth in length. Further away is the region of maturation, where cells differentiate and mature; some epidermal cells form thread-like root hairs that absorb water and minerals.

5.2.2 Modifications of Root

Roots in some plants undergo various changes in their structure and shape and become modified to perform functions other than absorption and conduction.

5.3 The Stem

The stem is the ascending part of the axis bearing branches, leaves, flowers, and fruits. It develops from the plumule of the embryo. The stem bears nodes and internodes. The region of the stem where leaves are borne is called node, while the portion between two nodes is called internode. The stem bears buds, which may be terminal or axillary.

Stems are generally green when young; later they may become woody and dark brown. Functions: spreading branches bearing leaves, flowers, and fruits; conduction of water, minerals and photosynthates; some perform storage, support, protection, and vegetative propagation.

5.3.1 Modifications of Stem

5.4 The Leaf

The leaf is a lateral, generally flattened, structure borne on the stem. It develops at the node and bears a bud in its axil. The axillary bud later develops into a branch. Leaves originate from shoot apical meristems and are arranged in an acropetal order. They are the most important vegetative organs for photosynthesis.

A typical leaf consists of three main parts — leaf base, petiole, and lamina (leaf blade). The leaf is attached to the stem by the leaf base; it bears two small leaf-like structures called stipules. In monocots, the leaf base expands into a sheath covering the stem partially or wholly. The pulvinus is a swollen leaf base, found in some leguminous plants.

5.4.1 Venation

The arrangement of veins and veinlets in the lamina is called venation. When veinlets form a network, it is called reticulate venation (in dicot leaves). When veins run parallel to each other, it is called parallel venation (in monocot leaves).

5.4.2 Types of Leaves

A leaf having a single or undivided lamina is called simple. When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets, the leaf is called compound. Buds are present in the axil of the petiole in both simple and compound leaves but not in the axil of leaflets. Compound leaves are of two types: pinnately compound (e.g., neem) and palmately compound (e.g., silk cotton).

5.4.3 Phyllotaxy

The pattern of arrangement of leaves on the stem or branch is called phyllotaxy. This is usually of three types — alternate, opposite, and whorled. In alternate phyllotaxy, a single leaf arises at each node (e.g., china rose, mustard). Opposite has a pair of leaves at each node (Calotropis, guava). Whorled has more than two leaves at a node (Alstonia).

5.4.4 Modifications of Leaves

Leaves are often modified to perform functions other than photosynthesis. They are converted into tendrils for climbing (peas) or into spines for defence (cacti). The fleshy leaves of onion and garlic store food. In some plants such as Australian acacia, the leaves are small and short-lived. The petioles in these plants expand, become green and synthesize food — these are phyllodes. Leaves of insectivorous plants (pitcher plant, venus fly trap) are highly modified for trapping insects.

Worked Examples