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Anatomy of Plants

Algae and Classification of Plants

The term algae is used for aquatic unicelluar organisms that photosynthesize as do terrestrial plants. All algae contain green chlorophyll, but they also can contain other pigments that mask the color of the chlorophyll. The chlorophylls selecting different part of the spectrum are separated into type a, b, c, and d. Thus there are green, golden brown, brown, and red algae. Green algae can be single-celled, colonial, filamentous, and multicellular. It is believed that the green algae are ancestral to the first
Green Alga Green Alga plants (see "Evolution of Micro-organisms and Plants" in the appendix) because both of these groups possess chlorophylls a and b, both store reserve food as starch, and both have cell walls that contain cellulose (Figure 08). Seaweeds such as Ulva are multicellular algae carrying chlorophyll. They are anchored firmly to the rock by holdfasts. Their appearance give a false impression that they are plants having root, stem, and leaf (Figure 09).

Figure 08 Green Alga
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Figure 09 Seaweed
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Alga Lifecycle While a few of the algae such as some species of Fucus follow the diplontic lifecycle, most of them such as the green algae Chlamydomonas spend most of their life in the haploid generation. Usually, this protist practices asexual reproduction, and the adult divides to give zoospores that resemble the parent cell. During sexual reproduction, gametes of two different strains come into contact and join to form a zygote. A heavy wall forms around the zygote, and it becomes a zygospore. The zygospore is able to survive until conditions are favorable for germination and subsequent production of 4 zoospores by meiosis. The gametes shown in Figure 10 are isogametes; that is, they look exactly alike. Sexual reproduction aids the process of evolution because it offers means to produce variations in addition to mutations. Note that embryo is absent in this kind of lifecycle.

Figure 10 Green Algae Lifecycle
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Table 01 below classifies the plants according to their characteristics. Algae are included because they satisfy the most basic definition of a plant - the use of chlorophyll to produce energy. The table shows that the properties are acquired gradually by each group as they progress from algae to the flowering plants.

Phylum Chl. a & b Embryos Stele§ Seed Flower # of Living Sp.
Algae Yes No No No No 14000
Bryophytes Yes Yes No No No 22500
Pteridophytes Yes Yes Yes No No 10800
Gymnosperms Yes Yes Yes Yes No 680
Angiosperms Yes Yes Yes Yes Yes 260000

Table 01 Classification of Plants

§ Stele is the cylindrical bundle in the stems and roots = vascular tissue.

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