Question

1.Use words and illustrations to show how this nymphaea leaf and the typical dicot leaf are alike and different

2.explain one way that human can make use of plants with typical fibrous root systems, and why fibrous root plants serve that purpose particularly well. Address the same points for plants with tap root systems.

Answer 1

Answer 1) Nymphaea leaf:

Nymphaea (water lily) a floating hydrophyte, is characterized by the presence of large air chamber, branched trichosclereids, or internal hairs with deposition of calcium oxalate crystals. Vascular bundles are small, reduced and scattered in nature. Transverse section of petiole shows epidermis, ground tissue, and vascular bundles.

The epidermis is uniseriate, consisting of a rounded cell with little cutinization on outer walls. Epidermal cells contain unbranched hairs. Chloroplasts are present in epidermal cells that help in efficient photosynthesis. Below epidermis, two to three layer of collenchymas cells are present which constitute the hypodermis. These collenchymatous cells have thickened corners. Ground tissue is made up of thin-walled parenchyma cells. There are large numbers of air chambers present which contain needle-shaped trichoscleroids often with deposition of calcium oxalate. The vascular bundle is poorly developed and scattered in the ground tissue. The xylem vessels usually break down, thus are poorly developed. Xylem is represented by single lacuna but phloem elements are normal. The vascular bundles are of two types: central and peripheral vascular bundles. Central vascular bundle is large and contains two groups of phloem within central xylem but peripheral vascular bundles are small with only one outer phloem and xylem.

Diagram. Transverse section of petiole of Nymphaea leaf

Dicot Leaf:

Epidermis

The epidermis is present at the upper (adaxial) as well as lower (abaxial) surface of the leaf. Mostly epidermal layer is uniseriate, composed of compact tabular cells. The outer surface of epidermal cells is cutinized but in case of multiseriate epidermis, the upper epidermal cells are more cutinized as compared to lower epidermal cells. In Ficus, the upper epidermis is multilayered (generally three layered), where the uppermost layer is made up of isodiametric cells while lower two layers are made up elongated cells which are comparatively bigger in size. In contrast to Ficus, upper epidermis of Nerium leaf is also multiseriate but in each layer the cells are of the same type except outer layer which in having slightly smaller cells. The upper epidermal layer contains some secretary tissue such as cystoliths which contain calcium carbonate crystals. Usually, the lower epidermis is uniseriate in nature.

Stomata are distributed evenly on the lower epidermis (Magnifera and Ficus) but in case of Nerium, sunken stomata are observed, which are located in a cavity called as the stomatal pit. The trichomes develop from the cells surrounding the pit provide an effective mechanism for reducing transpiration.

Mesophyll

The ground tissue of leaf consists of specialized tissue systems known as mesophyll, differentiated into two types of cells viz. palisade and spongy parenchyma cells. Generally, the palisade cells are seen towards the adaxial surface of the leaf. Below the epidermal cell, these cells are columnar cells are present that possess no or very little intercellular space. Palisade cells contain abundant chloroplast and thus act as primary photosynthetic tissue. Single layer of palisade cells are present towards the lower epidermis in Nerium and even calcium oxalate crystals are also present in this lower layer of palisade cells. The palisade layers are two layered in Magnifera and Ficus, three to four layers in Oleander Nerium.

The spongy cells occur towards the lower epidermis in Mangifera and Ficus or between the palisade cells in Nerium. Spongy cells are thin-walled, irregular in shape, isodiametric, loosely arranged with conspicuous intracellular space between them. The number of the chloroplast is fewer in mesophyll cells as compared to palisade cells. The spongy cells help in gaseous availability for photosynthetic machinery.

Conducting system

The conducting system consists of tissues present near or at the center of the midrib region. Shapes of conducting strands are of various types such as ring form, a crescent-shaped ring or in the form of scattered patches. Within the ring shaped conducting strands parenchymatous cells are present in the center of the ring. The vascular bundles in the leaves are generally collateral and closed type with xylem lying on the upper side (adaxial surface) and phloem on the lower side (abaxial surface). The xylem is composed of various kinds of vessels, tracheid’s, phloem fibers and parenchyma cells. Xylem not only helps in conduction of water and mineral nutrients but also provide mechanical support to the leaves. The phloem is composed of sieve tubes, companion cells and phloem parenchyma cells. Phloem helps in translocation of prepared food materials from the mesophyll cells to different parts of the plant.

Answer 2)

A fibrous root system is the opposite of a taproot system. It is usually formed by thin, moderately branching roots growing from the stem. A fibrous root system is universal in monocotyledonous plants and ferns. The fibrous root systems look like a mat made out of roots when the tree has reached full maturity.

Most trees begin life with a taproot, but after one to a few years change to a wide-spreading fibrous root system with mainly horizontal surface roots and only a few vertical, deep anchoring roots. A typical mature tree 30–50 m tall has a root system that extends horizontally in all directions as far as the tree is tall or more, but well over 95% of the roots are in the top 50 cm depth of soil.

A few plants with fibrous root systems:

• Coconut palm
• Grass

Fibrous roots grow fairly close to the surface of the ground. Leaves with parallel venation have fibrous roots.

Forages have a fibrous root system, which helps combat erosion by anchoring the plants to the top layer of the soil, and covering the entirety of the field, as it is a non-row crop. In a fibrous root system, the roots grow downwards into the soil, and also branch off sideways throughout the soil. This forms a mass of fine roots, with no distinct tap root, because the embryonic root dies back while the plant is still young and growing.

A taproot is a large, central, and dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. In some plants, such as the carrot, the taproot is a storage organ so well developed that it has been cultivated as an edible plant, for which its storage capacity has been exaggerated by selection for size and palatability.

The taproot system contrasts with the adventitious or fibrous root system of plants with many branched roots, but many plants that grow a taproot during germination go on to develop branching root structures, although some that rely on the main root for storage may retain the dominant taproot for centuries, for example Welwitschia.