Page 132 - Biology_F5

Comparative studies of natural groups of organisms

(e) They possess large vacuole for Distinctive features of phylum
osmoregulation. Chlorophyta
(f) They occur in a diverse range of sizes and Chlorophytes are distinguished from other
forms, including unicellular, filamentous, species of Protoctists by the following
colonial and thalloid forms. features
(g) They reproduce both sexually and (a) They have spiral chloroplasts with
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asexually. Sexual reproduction involves pyrenoids.
specialised cells called sporocytes (b) They have central suspended nucleus
which undergo meiosis to produce with cytoplasmic strands.
haploid motile or flagellated cells called (c) They show an isomorphic and
zoospores. These grow mitotically to haplontic alternation of generations.
form a gamete producing body called
gametophyte, capable of producing Sturcture of Spirogyra
either male or female reproductive cells Task 3.5
(gametes). Morphologically, female Use internet sources to view simulation,
gametes can be either larger than male video or image of Spirogyra. Note its
ones (anisogamous) or similar in size structure.
(isogamous). Female and male gametes
are flagellated, they can swim and
unite to form a diploid zygote, which Spirogyra is a genus of filamentous green
settles and grows mitotically to form a algae, which has helical or spirally arranged
multicellular body called sporophyte chloroplast as a characteristic feature
that can again produce sporocytes. The of this genus. They have thin strands of
gametes, if not fertilised, lose their cytoplasm in which the prominent nucleus
flagella and grow into a new gamete- is suspended, and their spiral chloroplasts
producing body. Asexual reproduction embedded in the cytoplasm bear the
occurs by vegetative fragmentation structure called pyrenoid for starch
where individual cells or short chains storage. The cell wall contains cellulose
of cells separated from the main body at the inner layer, and the outer layer
are capable of growing into a new body. contains pectin, which is responsible for
(h) Life cycle of chlorophytes is the slippery surface of algae. Surrounding
diverse. For instance, Ulva shows an the cell wall, there is mucilage, which
isomorphic alternation of generations thickens the cell membrane, store water
in which sporocyte-producing body and food. Moreover, Spirogyra has long,
called sporophyte generation is unbranched filaments with cylindrical
morphologically identical to gamete- cells that are joined end to end. Each cell
producing body called gametophyte has a central vacuole. The cells are long
generation. Ulothrix shows haplontic and thin filaments, and sometimes these
life cycle, in which sporophyte filaments develop root-like structures for
and gametophyte generation are attachment to the substrate (Figure 3.18).
morphologically different.

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