Comparative Morphology in Plant, Biology tutorial

Introduction to Algae Morphology:

The body of algae is known as thallus. In unicellular algae it just comprises of single cell. All multicellular organisms start the life as single cells. When the cell divides and daughter cells from the packet surrounded in mucilaginous mass, the colony is formed. While the division of cell constantly in same plane, with daughter cells sticking together, results in the row of cells forming filament. Few cells of a filament divide only once by vertical plane followed by transverse divisions repeatedly and therefore create filamentous branched thallus. Additional, when all cells of filament experience divisions in cross and vertical planes it results in the sheet of one or more cells in thickness. This multi-cellular thallus may illustrate complicated differentiation as in seaweed. Multicellular algae illustrate stages in their development. The gradual complexity in for also points out how evolution of thallus has occurred, in algae.

Morphologically algae can be differentiated as colonial, unicellular, heterotrichous, filamentous, thalliod and polysiphoniod forms, each of the kinds is explained below:

Unicellular forms:


Single cells, cylindrical, short or long; at times very long snake forms. Cells split by construction, two daughter cells get split, hardly ever they stay together to form 2-celled filament. Individual single cells have their own mucilaginous cover around them. Many cells may be surrounded in common colorless mudlage giving impression of colony.


This single celled alga has the nucleus, cup-shaped chloroplast in which one pyreniod is generally present. Chloroplast on anterior side illustrates 2 to 3 rows of fatty red colored granules. This is called as eyespot or stigma that is useful for alga to respond to light. Cell wall is rigid and district. The small contractile vacuole is found at base of every flagellum.

Chlamydomonas cells under partly dry conditions divide and daughter cells without flagella stay surrounded by common mass of mucilage. Such colony called as palmelia phase of chlamydomonas. This is only temporary phase and on flooding with water individual cells evolve flagella and escape swimming away from colony. Therefore starting of colony construction found in Volvox can be observe in Chlamydomonas.

Colonial Algae:

When the cell divides and daughter cells formed stay together within common mucilage mass, it is called as colony. The colony may include large number of cells at times it may be so big that one can observe it without any help through eyes.


This is colonial alga, most common in polluted ponds and lakes in India. At times colonies are big and one can observe it without any help through eyes. They collect on surface of water creating fairly thick layer in few seasons (water blooms).

Single cells are spherical and colony is created due to lose aggregates of many thousand cells held by the mucilage. Colonies float on surface of water due to presence of lengthened cylindrical gas vesicle inside individual cell Reproduction is by division of cells known as binary fission.


Colonies if Volvox are spherical, similar to ball and big sufficient to be observed with unaided eye. Every colony has 1000-5000 cells arranged on outside of mucilaginous ball known as coenobium. Two kinds of cells can be observed usually vegetative or somatic and gonidia. In younger colonies cytoplasmic relations lasmodesmata between individual cells can be observed under microscope

Vegetative cells are more or less similar to Chlamydomonas with cell wall, 2 flagella, eyespot, single cup-shaped chloroplast, pyreniod, contractile cacuole and nucleus. Cells on posterior side of colony may be larger than in front.

Gonidia-cells intended for sexual reproduction are on posterior side and they lose the flagella early. They split and give rise to daughter colonies. After rupture of parent colony daughter colonies are liberated in water.

Daughter colonies made from gonidia may later grow in male colonies which produce spermatozoa or female colonies which produce eggs. Volvox colonies are usually unisexual but few species are bisexual. In volvox all cells of colony are derived from the single parental cell. They are set on surface of mucilaginous ball, related with other cells by cytoplasmic connections. Few cells act as sex cells intended for reproduction while others stays vegetative and eventually grow old and die. This discrimination into vegetative and reproductive cells is very significant feature in growth of multicellular organisms.

Filamentous Forms:

When the cell splits always cross-wise and daughter cells don't separate from each other, it results in the linear row of cell as in Nostoc, Ulothrix and Oedogonium. Though, three algae show different levels of differentiation.


This is simple filamentous form, single row of cells, uniseriate. Several filaments of Nostoc are usually surrounded within common mucilage envelop to form a colony. Few cells in between vegetative cells are altered into hertocysts. All vegetative cells are able to developing into spores known as akinetes.


This is also filamentous alga but distinguished into narrow basal holdfast by which it is joined to rock in water. Cells at apical end are comparatively broad. These suffer division and create within, the large number of motile cells intended for reproduction.


Filaments of Oedogonium are unbranched, generally distinguished at one end into the holdfast. Cylindrical cells are short or longer than broad. Development of filaments is because of division of specific cells known as cap cell that show caps (or ring like scars) on the walls. Such cells may split several times and number of caps present on the cell indicates number of divisions it has experienced.

Heterotrichous Form:

When few cells of filament split vertically it results in branch. Several filamentous forms illustrate extensive branching of main filament providing it bushy look. In some algae branches at base stay horizontal, joined to substratum called as prostrate system from which vertical system of vertical branched filamenets occur. This kind of body is called as heterotriclous habit. Heterotrichous habit is most extremely developed filamentous construction in algae.


It is heterotrichous alga that exhibits larger differentiation in plant body. Prostrate system is very much decrease. Main axis has long intermodal cells alternating with short nodal cells. Short nodal cells bear the bunch of short branches. Few of side branches may grow in long colorless hairs or setae. Major axis produces at base multicellular colorless rhizoids in large number to form the kind of cortex. Their major function is to join alga to substratum.


Coleochaete is the acquatic alga growing on surface of water plants. C.pulvinata is heterotrichous. Vertical system is in form of branched filaments. In C.scutata vertical system is absent and prostrate system is composed of short frequently branched filaments which form compact disc. In both forms few cells produce hair like bristles, called as setae from the upper surface.


Another heterotrichous algae. This prostrate system that joins algae to Substratum is composed of branched filaments. Vertical system is in form of uniseriate ro of cells branched filaments creating loose tufts of 1 mm to 10 mm or more. These thallus perhaps with unilocular or plurilocular sporangia. Branches arise just below cross walls of cells of major filament. Most of the branches terminate in lengthened hairs.

Thalloid Forms:

In cells of filament split in more than one plane, which is not only crosswise but lengthwise it results in the sheet of cells. Thallus may be one cell or numerous cells sickness. This is the very common alga discovered on rockly coasts of sea. Thallus is joined to substrate like rocks by rhizoids at base. When sheet of thallus is cut, one view two layers of cells, pressed to each other. Together they form the single sheet. Focus is brown seaweed common on rocky coasts of sea in temperate countries. Body of Fucus is large about half a metre or so in length. It contains basal discoil holdfast, short stipe and flat and dischotomously branched fronds or blades. At the tip of blade are discovered air bladders that make plant float in water. Focus is multicellular and contain complex internal structure illustrating three regions. Outer layer is epidermis, central cortex and inner medulla. Development of thallus is because of division of apical cell located in hollow depression at tip of branch. Epidermis and other layers of cortex include chromatophores that take part in photosynthesis. Cortical region stores food materials and medullary cells take part in transport of food to different regions of fronds.

Polysiphonoid Forms:

This form of algae includes more complex than earlier described forms. It is situated in red alga polysphonia that is marine in habitat.


Algae illustrate in general heterotrichous habit. Prostrate system is in form of the lengthened rhizoid that joins algae to substratum. Vertical system is extremely branched. Branches are of two types, few are long and few are short and hair-like. Main filament develops by division of single apical cell. Mature plant body is composed of central row of cells - central siphon, enclosed by vertical rows of cells, 4 to 24 - ericentral siphons. All pericentral cells are related with cells of central siphon and are also connected with each other. When cytoplasm of one cell is joined to cytoplasm of neighboring cell through pit in their wall, it is called as pit connection. In Polysiphonia though all cells are separate, their cytoplasm is related by means of pit connections. New branches may extend from cells of central siphon or from pericentral cells. Trichoblasts that are simple or branched hair-like lateral branches arise from pericentral cells.

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