30 BIOLOGY
not only the external features, but also internal features, like ultra-
structure, anatomy, embryology and phytochemistry. Such a
classification for flowering plants was given by George Bentham and
Joseph Dalton Hooker.
At present phylogenetic classification systems based on
evolutionary relationships between the various organisms are acceptable.
This assumes that organisms belonging to the same taxa have a common
ancestor. We now use information from many other sources too to help
resolve difficulties in classification. These become more important when
there is no supporting fossil evidence. Numerical Taxonomy which is
now easily carried out using computers is based on all observable
characteristics. Number and codes are assigned to all the characters and
the data are then processed. In this way each character is given equal
importance and at the same time hundreds of characters can be
considered. Cytotaxonomy that is based on cytological information like
chromosome number, structure, behaviour and chemotaxonomy that
uses the chemical constituents of the plant to resolve confusions, are also
used by taxonomists these days.
3.1 ALGAE
Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely
aquatic (both fresh water and marine) organisms. They occur in a
variety of other habitats: moist stones, soils and wood. Some of them
also occur in association with fungi (lichen) and animals (e.g., on sloth
bear).
The form and size of algae is highly variable, ranging from colonial
forms like Volvox and the filamentous forms like Ulothrix and Spirogyra
(Figure 3.1). A few of the marine forms such as kelps, form massive plant
bodies.
The algae reproduce by vegetative, asexual and sexual methods.
Vegetative reproduction is by fragmentation. Each fragment develops into
a thallus. Asexual reproduction is by the production of different types of
spores, the most common being the zoospores
. They are flagellated
(motile) and on germination gives 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) or non-flagellated (non-motile)
but similar in size (as in Spirogyra). Such reproduction is called
isogamous. Fusion of two gametes dissimilar in size, as in species of
Eudorina is termed as anisogamous. Fusion between one large, non-
motile (static) female gamete and a smaller, motile male gamete is termed
oogamous, e.g., Volvox, Fucus.