3.1 Algae 🔵
Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic organisms. They occur in a variety of habitats, including:
- Moist stones
- Soils
- Wood
- Fungi (lichen)
- Animals (e.g., on sloth bear)
Algae have a highly variable form and size, ranging from:
- Colonial forms like Volvox
- Filamentous forms like Ulothrix and Spirogyra (Figure 3.1)
Some marine forms, like kelps, form massive plant bodies.
Reproduction
Algae reproduce by:
- Vegetative reproduction: by fragmentation. Each fragment develops into a thallus.
- Asexual reproduction: by production of different types of spores, the most common being zoospores. They are flagellated (motile) and on germination give rise to new plants.
- Sexual reproduction: takes place through fusion of two gametes. These gametes can be:
- Flagellated and similar in size (as in Ulothrix)
- Non-flagellated (non-motile) but similar in size (as in Spirogyra)
- Fusion of two gametes dissimilar in size (as in species of Eudorina)
- Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete (as in Volvox, Fucus)
Importance
Algae are useful to man in a variety of ways:
- At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
- They increase the level of dissolved oxygen in their immediate environment.
- They are of paramount importance as primary producers of energy-rich compounds, which form the basis of the food cycles of all aquatic animals.
- Many species of Porphyra, Laminaria, and Sargassum are used as food.
- Certain marine brown and red algae produce large amounts of hydrocolloids (water-holding substances), e.g., algin (brown algae) and carrageen (red algae) which are used commercially.
- Agar, one of the commercial products obtained from Gelidium and Gracilaria, is used to grow microbes and in preparations of ice-creams and jellies.
- Chlorella, a unicellular alga rich in proteins, is used as food supplement even by space travelers.
Classification
Algae are divided into three main classes:
3.1.1 Chlorophyceae 💚
- Members of Chlorophyceae are commonly called green algae.
- The plant body may be unicellular, colonial, or filamentous.
- They are usually grass green due to the dominance of pigments chlorophyll a and b.
- The pigments are localized in definite chloroplasts.
3.1.2 Phaeophyceae 🟡
- Members of Phaeophyceae or brown algae are found primarily in marine habitats.
- They show great variation in size and form.
- They range from simple branched, filamentous forms (Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 meters.
- They possess chlorophyll a, c, carotenoids, and xanthophylls.
3.1.3 Rhodophyceae 🔴
- Members of Rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body.
- Majority of the red algae are marine, with greater concentrations found in warmer areas.
- They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates.
Table 3.1 Divisions of Algae and their Main Characteristics
3.2 Bryophytes 🌿
Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills (Figure 3.2).
Characteristics
Bryophytes are also called amphibians of the plant kingdom because these plants can live in## Bryophytes 🌿
Characteristics:
- Non-vascular plants that do not have true roots, stems, or leaves
- Plant body is thallus-like and prostrate or erect
- Attached to substratum by unicellular or multicellular rhizoids
- Lack true roots, stems, or leaves
- May possess root-like, leaf-like, or stem-like structures
Life Cycle:
- Main plant body is haploid and produces gametes (gametophyte)
- Sex organs are multicellular
- Male sex organ is called antheridium and produces biflagellate antherozoids
- Female sex organ is called archegonium and produces a single egg
- Antherozoids are released into water and come into contact with archegonium
- An antherozoid fuses with the egg to produce a zygote
- Zygote develops into a multicellular body called a sporophyte
- Sporophyte is not free-living and is attached to the photosynthetic gametophyte
- Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores
- Spores germinate to produce gametophyte
Economic Importance:
- Some mosses provide food for herbaceous mammals, birds, and other animals
- Species of Sphagnum provide peat that has been used as fuel and as packing material for trans-shipment of living material
Ecological Importance:
- First organisms to colonize rocks
- Decompose rocks, making the substrate suitable for the growth of higher plants
- Form dense mats on soil, reducing the impact of falling rain and preventing soil erosion
Liverworts 🌿
Characteristics:
- Thalloid plant body
- Dorsiventral and closely appressed to the substrate
- Leafy members have tiny leaf-like appendages in two rows on stem-like structures
Asexual Reproduction:
- Fragmentation of thalli
- Formation of specialized structures called gemmae (sing. gemma)
- Gemmae are green, multicellular, asexual buds that develop in small receptacles called gemma cups
- Gemmae become detached from the parent body and germinate to form new individuals
Sexual Reproduction:
- Male and female sex organs are produced either on the same or on different thalli
- Sporophyte is differentiated into a foot, seta, and capsule
- After meiosis, spores are produced within the capsule
- Spores germinate to form free-living gametophytes
Mosses 🌿
Characteristics:
- Predominant stage of the life cycle is the gametophyte
- Gametophyte consists of two stages: protonema stage and leafy stage
- Protonema stage develops directly from a spore and is a creeping, green, branched, and frequently filamentous stage
- Leafy stage develops from the secondary protonema as a lateral bud and consists of upright, slender axes bearing spirally arranged leaves
- Attached to the soil through multicellular and branched rhizoids
Vegetative Reproduction:
- Fragmentation
- Budding in the secondary protonema
Sexual Reproduction:
- Sex organs are produced at the apex of the leafy shoots
- After fertilization, the zygote develops into a sporophyte, consisting of a foot, seta, and capsule
- Sporophyte is more elaborate than that in liverworts
- Capsule contains spores
- Spores are formed after meiosis
- Elaborate mechanism of spore dispersal
Examples of Mosses:
- Funaria
- Polytrichum
- Sphagnum## Angiosperms 🌼
Angiosperms, also known as flowering plants, are a group of plants that have evolved to produce flowers, fruits, and seeds. In contrast to gymnosperms, angiosperms have ovules and pollen grains that are developed in specialized structures called flowers.
Characteristics of Angiosperms
• Seeds are enclosed in fruits: Angiosperms produce seeds that are protected by a fruit or a covering. • Wide range of habitats: Angiosperms can be found in diverse environments, from the smallest Wolffia to tall trees of Eucalyptus (over 100 meters). • Provide various products: Angiosperms are a source of food, fodder, fuel, medicines, and other commercially important products.
Classification of Angiosperms
Angiosperms are divided into two classes:
Importance of Angiosperms
Angiosperms play a vital role in our ecosystem and provide numerous benefits to humans and the environment.
"Angiosperms are an exceptionally large group of plants occurring in wide range of habitats."
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