Why is the Pericycle absent in the roots and stems of some hydrophytes?

Hydrophyte embraces all the different and varying plant forms that adapted to live under direct influence of free water or in waterlogged soil, e.g. a small number of ferns and angiosperm. Gymnosperms have not migrated into aquatic habitats.

Hydrophytes, also called hydrophytic plants, are classified according to forms:

Submergent, Floating and Emergent. Submergent hydrophytes grow and remain entirely submerged below the surface of water throughout life. Floating hydrophytes have leaves or shoots that float on top of water surface. Emergent hydrophytes have roots anchored to hydric soil, but a portion of shoot lies above water surface.

The different form of hydrophyte exhibits the following characteristics that help them to survive in water and waterlogged soil:

i. Increase in leaf surface that helps plants for floatation.

ii. The presence of air chambers that assist in buoyancy of the plant.

iii. Stomata, if present, nonfunctional because the plants need not regulate transpiration due to abundance of water in their environment.

iv. Reduction of xylem because the hydrophytes need not conserve water.

v. Reduction in mechanical and protective tissue as water pressure supports them.

vi. Hydrophytes have thin cuticle or lack it. Because the functions of cuticle like protection from strong sunlight, conservation of water and deterrence of insects are not required by hydrophytes. They are protected from excess sunlight by surrounding water; no question arises for water conservation as they have abundant free water in their environment; the hydrophytic angiosperms suffer little predation by aquatic animals.

Due to the thinness of cuticle the leaves absorb nutrients from the water directly. But the major bulk of minerals are absorbed by roots and transported to the leaves. As there is no transpiration pull like land plants the transportation of minerals occur through root pressure. Hydathodes are very common in the leaf tips of many submerged hydrophytes. Transportation of water occurs as a bleeding stream ending in hydathodes. The opening of hydathodes may be large.

vii. Stomata when present may be many in number and occur on upper surface of floating leaves (epistomatous).

viii. Roots are small and reduced in all respects as water can diffuse directly into leaves.

ix. Formation of special roots and these are negatively geotropic and designed to function in gas exchange. Such roots are termed as pneumatophore and present in halophytes.

x. Hydric soils have diluted oxygen and minerals and the hydrophytes adapted to absorb nutrients directly from water.

xi. The piliferous layer scarcely has (or no) root hairs. Root hairs are entirely absent from Ceratophyllum.

xii. The piliferous layer is not cutinized/suberized and so absorptive in function. Moreover the turgid piliferous zone provides mechanical strength, as mechanical cells are absent in hydrophytic roots.

xiii. The cortex of root is wide and provided with air cavities.

xiv. Xylem of root and stem is poorly developed and has thin walls. Vessels are absent in many hydrophytes. In the root of Potamogeton (Fig. 29.9A) the vascular tract in the centre is occupied by a large xylem cavity. The reduction in xylem is due to the fact that in hydrophytes no elaborate system of water absorption is required as all the epidermises of plant body are capable of water and mineral absorption.

xv. The stem of submerged hydrophytes is very weak and flexible. It moves according to the direction of flow of water currents.

xvi. The epidermis in the stem of submerged hydrophytes is uniseriate. It is not cuticularized and in some cases the cells have cellulose walls only. As a result the epidermal cells can absorb mineral salts and gases dissolved in water directly.

xvii. The cortex is wide and has large air chambers. The air chambers may be arranged symmetrically e.g. Myriophyllum (Fig. 29.9B) or scattered irregularly all over the cortex, e.g. Elodea, Hydrilla, Hippuris etc.

xviii. The cells of cortex have low osmotic pressure. The cells contain chloroplastids that help in photosynthesis.

xix. The vascular tissues in stem of submerged hydrophytes are not well developed. Endodermis and pericycle are indistinct. The vascular tissues are mostly composed of phloem.

Hydrophyte

xx. Though the endodermis is not distinct many hydrophytic angiosperms have the innermost layer of cortex that functions as endodermis. This endodermoid layer is present in submerged roots, stems and leaves. It is assumed that this layer is involved in channelizing the flow of water through reduced xylem.

Explanation: