7.1 Classification of Plants Based on Habitats
Plants are classified into four main group types based on their adaptation to environmental factors such as water availability, temperature, light intensity, and soil conditions:
- Mesophytes: Plants that live in habitats with moderate water availability, moderate temperature, and balanced humidity (e.g., hibiscus, mango tree, sunflower).
- Hydrophytes: Aquatic plants that live floating on water or submerged in water (e.g., Eichhornia crassipes, Lotus, Hydrilla sp.).
- Halophytes: Plants that inhabit saline/salty environments high in salt concentration and frequent tidal changes, such as mangrove swamps (e.g., Rhizophora sp., Avicennia sp.).
- Xerophytes: Plants adapted to extremely hot, arid environments with minimal water supply, such as deserts or sand dunes (e.g., cactus, pineapple).
7.2 Adaptive Features of Hydrophytes, Halophytes, and Xerophytes
1. Hydrophytes (Aquatic Plants)
Hydrophytes are subdivided into floating plants and submerged plants:
- Floating Plants (e.g., Eichhornia crassipes, Nymphaea sp.):
- Stomata are localized primarily on the upper epidermis to maximize gas exchange with air.
- Leaves are broad, flat, and thin, coated with a waxy waterproof cuticle to reflect sunlight and prevent stomata clogging.
- Stems contain abundant aerenchyma tissue (spongy tissue with large air spaces) providing buoyancy to stay afloat.
- Fibrous root system traps air bubbles and anchors the plant lightly in water.
- Submerged Plants (e.g., Hydrilla sp., Elodea sp.):
- Absence of stomata and waxy cuticle on leaves; dissolved gases ($O_2$ and $CO_2$) and nutrients are absorbed directly across thin epidermal cell walls via diffusion.
- Leaves are ribbon-like or finely divided to offer minimal water current resistance and prevent tearing.
- Air spaces inside stems facilitate floating upright toward available light.
2. Halophytes (Mangrove Plants)
Halophytes survive high salt concentration, anaerobic muddy soil, high light intensity, and strong wave actions:
- Root Systems:
- Pneumatophores: Vertical, erect roots (e.g., Avicennia sp., Sonneratia sp.) growing above water surface with tiny pores called lenticels for atmospheric gas exchange in oxygen-depleted mud.
- Stilt Roots: Branched roots outgrowing from main trunk (e.g., Rhizophora sp.) providing structural support in soft mud against high tides.
- Buttress Roots: Thick expanded root bases (e.g., Bruguiera sp.) providing mechanical stability.
- Salt Excretion Mechanisms:
- Salt hydathodes/glands present on leaf surfaces actively secrete excess absorbed salt (e.g., Avicennia sp.).
- Old leaves accumulate excess salt and shed via abscission (e.g., Rhizophora sp.).
- Root membranes selectively prevent salt entry via ultrafiltration.
- Viviparity: Seeds germinate while still attached to the parent tree, producing long radicles so seedlings can anchor firmly into soft mud upon falling.
- Leaves: Thick succulent leaves with a thick waxy cuticle store fresh water and minimize transpiration.
3. Xerophytes (Desert Plants)
Xerophytes withstand extreme drought, high temperatures, and excessive water loss:
- Extensive Deep Root Systems: Roots grow deep vertically into the water table or spread widely horizontally near the soil surface to collect brief rainfall.
- Leaves Modified into Thorns/Spines: Reduces total surface area exposed to sunlight, lowering transpiration rates and defending against herbivores.
- Thick Cuticle & Sunken Stomata: Sunken stomata in leaf pits trap moist air, reducing water vapor concentration gradient between leaf interior and atmosphere.
- Succulent Stems: Green photosynthesis-capable stems equipped with parenchymal tissue specialized in storing water.