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4 Pattern: multicellularity in the algae

4 Pattern: multicellularity in the algae

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Formation of Ontario, Canada from 1900 million years ago, 12 different species including multicellular forms, have been detected.

Multicellularity originated separately in many different groups

such as bacteria, fungi, different groups of algae, and in different

groups of animals. Among the algae several groups have both unicellular and multicellular representatives. Unicellularity is unknown in

the brown algae (Phaeophyceae) except as a short reproductive stage

of the life cycle of the multicellular organism.

The advantages of multicellularity are not hard to understand.

Multicellular individuals can be larger, thereby occupying space more

effectively and competing for light or nutrients. But one of the difficulties of being large is the control of cell function; in a large organism proportionally a large amount of genetic material is required to

produce enough gene products. By becoming multicellular multiple

nuclei provide gene products throughout the individual.

Being large enough to occupy space is not the only advantage

of multicellularity. Perhaps more important is the greater possibility of specialisation it permits. The organism can become patterned

with different zones specialised for different functions. The simplest

kinds of multicellularity are seen in some bacteria that form chains

of cells but even here some kind of specialisation is often observed.

In the bacteria, streptomycetes have branched chains in which some

branches are specialised for reproduction by fragmenting into spores.

A measure of the degree of specialisation that multicellularity permits is shown by the widely diverse forms exhibited by many of the

algae. This may take the form of sheets, ribbons or nets, or it may

be like sponge or jelly. Other algae are like fans or ears, disc-like or

spiky, or they may be branched or unbranched. In branched forms, the

branches may all be similar or different. In texture their tissues range

from delicate films to fleshy, firm or hard structures like corals. What

is remarkable is that similar patterns have evolved several times in

the different main lineages of algae, especially in the Rhodophyta and

Chlorophyta, and a single algal lineage may display several different


1.4.1 Coenocytic or siphonous forms

Multicellularity is not the only way of overcoming the problems of

largeness. Coenocytic or siphonous forms are unicellular but exhibit

some of the specialisation seen in multicellular forms. Grypania, from

2100 million years ago, may have been a coenocytic alga. It has been

likened to the unicellular green alga Acetabularia, which grows to a

length of 3 cm or more, and becomes coenocytic or siphonous in

its later stages of development; that is, the ‘cell’ contains multiple

nuclei within a single continuous cell membrane. A multiplicity of

nuclei can multiply the sources of gene products. Other examples

of coenocytic green algae are Protosiphon, which consists of a single

multinucleate sphere up to 0.3 mm in diameter, and Ventricaria, which

may grow to the size of a chicken’s egg.




Figure 1.29. Coenocytic green

algae: (a) Caulerpa taxifolia;

(b) Caulerpa prolifera; (c) Halimeda;

(d) Bryopsis plumosa frond (from

Figures 5.35, 5.36, 5.37 in Lee,




Figure 1.30. Green alga

(Chlorophyta) Codium: (a) section

through axis; (b) whole branched






Other coenocytes have a more complex form. Halimeda is a coenocytic marine green alga of coral reefs (Figure 1.29). It is subdivided

into many flat segments each between 0.5 cm and 5 cm across. Bryopsis is similar but has a differentiated frond arising from a holdfast.

The cytoplasm, with many nuclei and chloroplasts, lines the tube

like stem and branches. Caulerpa also produces fronds but they are

flattened. The large size of all these coenocytic algae places physical

stress on the individual and they show various adaptations to overcome this. Halimeda produces a calcium carbonate skeleton. Caulerpa

has stengthening ingrowths of the cell wall. The coenocytic form is

vulnerable to loss of a large part of the cytoplasm if the cell wall is

damaged. However, Caulerpa rapidly contracts the region around any

wound so that it can be sealed off.

A different coenocytic pattern is seen in the green alga Codium.

Here the body of the plant is a dense network of multinucleate filaments so that a pseudo-parenchymatous tissue is produced. Analogous forms are seen in both the fungi and the brown algae. Some

coenocytes show remarkable differentiation but overall the possession of a coenocytic body does seem to limit specialisation. In all of

them reproductive tissues differentiate after being cut off from the

rest of the coenocytic body.

1.4.2 Colonial forms

There are two basic types of colonial forms. In one, the colony is

indefinite and continues to grow by cell division and reproduces by

fragmentation. Hydrodictyon is one beautiful example in the green

algae. It produces net-like colonies up to 1 cm long. The cells of the

colony are large and coenocytic.

In the second colonial form, which is usually free floating, there

are a fixed number of cells at its origin, which do not vary much

during its life-span. Examples from the green algae include Gonium,

Eudorina (Figure 1.31) and Volvox, each with a different number of cells,

differences in ornamentation or motility, and differences in degree

of specialisation between cells.

In Gonium, which may have 4, 8, 16 or 32 cells depending on

species, the concave colony moves by the cooperative action of the

flagella of all its component cells, but each cell can divide to start a









Figure 1.31. Colonial green

algae: (a)–(c) Scenedesmus; (d)

Platydorina; (e) Pediastrum; (f)

Pandorina; (g) Eudorina; (h) Gonium.




new colony. In Scenedesmus the four cells are ornamented with spines.

In Eudorina, which can have 32, 64 or 128 cells, some posterior cells

are small and incapable of cell division. In some forms like Platydorina

there is a differentiation between the anterior and posterior part of

the colony. Volvox is the most complex free-floating colonial alga. It has

a single spherical layer of hundreds of cells, most of which are purely

vegetative, and others that divide to give rise to juvenile spheroids,

which are later released from the parent. These free-floating forms

exist in a fairly homogeneous environment, the only polarity being

the source of light, and consequently they show only weak differentiation, enabling them to orientate and swim towards light.

Figure 1.32. Hydrodictyon: (a)

portion of net-like colony; (b)

detail of net (a).

1.4.3 Filamentous forms

Filaments can consist either of a single line of cells (= uniseriate), or

multiple lines of cells (= multiseriate). Because filaments are usually

attached to a fixed substrate and have free-floating distal filaments

they effectively exist in two environments and consequently show a

greater degree of differentiation, specialisation of cells and patterning than free-floating colonial forms.

The filamentous type of multicellularity and specialisation is

present in some cyanobacteria like Anabaena. It has, along with the

normal photosynthetic cells, some large non-photosynthetic cells that

Figure 1.33. Volvox. Colonial

green alga.




Figure 1.34. Filamentous algae:

(a) Chaetomorpha (part of filament

and holdfast) (b) Ulothrix (part of

filament); (c) Fritschiella

(filamentous terrestrial alga with

differentiated aerial prostrate and

soil penetrating filaments); (d)

Sphacelaria (multiseriate filament);

(e) Draparnaldia (part of filament

showing main axis and branched

laterals produced at nodes).






are specialised for nitrogen fixation. Some cyanobacteria also develop

akinetes, enlarged cells with a thickened outer coat, that act as

resistant spores permitting survival during periods of drought or


The simplest filamentous forms in eukaryotes consist of a single

chain of cells but, as in Chaetomorpha (Chlorophyta), the lowest cell

is usually adapted as a holdfast to fix the filament to the substrate

(Figure 1.34). Bangia (Rhodophyta) has filaments at first uniseriate,

which later become multiseriate as the cells divide to produce a

cylinder of up to eight wedge-shaped cells around a central core and

surrounded by a sheath. A holdfast is made up of the elongated filaments of several basal cells. Fossils from 1200 million years ago from

the Huntington Formation of Canada are very similar to living Bangia

and represent the earliest record of the Rhodophyta.

There are many beautiful branched filamentous red and green

algae. Growth in filamentous forms occurs by the activity of an apical

cell which cuts off cells periclinally, dividing parallel to the surface

or to the main axis. Each cell can then divide anticlinally to produce

branches either singly or in whorls. There are diverse and complex

structures, with different branching patterns and different degrees

of differentiation between the main axis and branches. Cladophora

is relatively simple, with lateral branches only weakly differentiated

from the main axis. In Draparnaldia there is a clear differentiation

between the main axis and branched laterals.


Fritschiella is a terrestrial green alga with multiseriate branches

that are differentiated as either upright, prostrate, or positively

geotropic, and it also has long colourless unicellular filaments or

rhizoids that penetrate the soil.


1.4.4 Flat sheets (thalloid) and three-dimensional forms

Algae with sheets of cells display an alternative form of multicellularity. Porphyra, a red alga has sheets that are one or two cell layers thick (uni- or bistratose). The sheets are bistratose in the green

alga Ulva. Coloechaetes produces disk-like forms. A three-dimensional

tissue is also found in some red algae with filaments embedded in a

mucilaginous matrix.

However, the most complex multicellularity comes when there

are more than two-layers of cells because this permits specialisation

and differentiation between surface layers and the interior of the

organism. Chara has a thickened multiseriate axis. Although the axis

in Charales has an apical cell that divides to elongate the axis, the

thickened axis below is created, by changes in the orientation and

asymmetrical divisions further down, producing files of cells of different dimensions. This gives the axis a ‘corticated’ structure, having

a thickness of several cell layers. Chara has branched filaments (see

Figure 1.40e). Branches arise at the nodes by 90◦ changes in the orientation of the spindle in the cell division of certain sub-apical cells.

Long colourless unicellular filaments, rhizoids, are produced from

the nodes near the base of the plant.

The interior of the organism may cease to have clear layers but

forms a more mixed parenchymatous or pseudo-parenchymatous tissue that may have within it strands or islands of cells, which themselves are specialised for particular functions. In the brown algae

the main tissue is a network of filamentous cells surrounded by a

matrix. Cells may be differentiated with elongated cells in the middle for transport of mannitol and amino acids. The surface layer is

specialised as a protective epidermis. An analogous kind of patterning

has evolved in plants.


Figure 1.35. Anatomy of

Laminaria (Phaeophyta) a stratose

and parenchymatous algae (from

Lee, 1999). (a) Section through the

lamina; (b) stipe of showing

trumpet connections between

filaments and differentiated layers.


1.4.5 Inter-cellular connections and the differentiated body

A critical aspect of multicellularity is the control of inter-cellular

communication. Inter-cellular connections have evolved separately in

different lineages. For example, the cytoplasmic connections in red

algae are quite different from the plasmodesmata of green algae and

plants and plasmodesmata have multiple origins too. Other kinds

of connection between cells are also formed; ‘sieve-plates’ between

transport cells in brown algae, analogous to those of phloem sievetube cells of land plants, permit the transport of dissolved materials between cells. In the red algae there are distinct pit connections

between adjacent cells in a filament. Such pits may arise de novo where

the cells of adjacent filaments touch. The pit connections have a core

of protein and a cap of polysaccharide.


Figure 1.36. Diagrammatic

representation of inter-cellular

connections in algae and plants:

(a) in red algae (Rhodophyta);

(b) plasmodesmata in plants.




The presence of multicellular tissues with a network of connected

cells permits a complex patterning of the organism. As the individual grows it differentiates in a highly organised pattern. Different

parts of the body specialise for different functions. In photosynthetic

sedentary organisms, both algae and plants, these parts and their

functions are:













attach to the substrate as a holdfast;

penetrate the substrate as a ‘root system’;

provide an axis to which other parts can be attached;

provide a transport system connecting parts of the plant;

provide large exposed surfaces to capture light;

provide exposed regions for reproduction.

The way these parts are elaborated in plants is the subject of the

following chapters. The brown seaweeds are described below for comparison.

1.4.6 The brown algae

Figure 1.37. Pelagic brown alga

Sargassum (Phaeophyceae).

The brown algae (Phaeophyceae) are the largest and most complex

photosynthetic organisms that are not plants. Possible fossil brown

algae have been found in North American and Asian rocks as old as

650--544 million years (the Vendian or latest Proterozoic), that is, long

before the diversification of plants. Others have been described from

the Ordovician, but only from much later, in the Tertiary, is there

any degree of certainty about their presence.

The brown algae are probably the most familiar of the algal groups

since they comprise most of the large seaweeds of intertidal zones.

There are numerous marine, but also a few fresh-water, forms. The

genus Sargassum (Fucales) is unusual in having some large complex

species that are free floating and have a pelagic existence in areas of

the tropical Atlantic Ocean (the ‘Sargasso Sea’). Sargassum is like an

intertidal plant that has escaped to the open sea.

Intertidal algae are complex multicellular organisms and show

a range of adaptations for life in the intertidal zone (Figure 1.38).

A strong holdfast grips the substrate. A strong and flexible stipe and

midrib absorbs the stresses of waves. The frond is flattened to capture

light but divided to allow water to flow around it easily. Air vesicles

buoy up the alga and help to prevent tangling.

The brown algae possess chlorophylls a and b and their food

reserves are different from those of either red or green algae. The

plant body of brown algae is made up of a pseudo-parenchyma of

tightly packed filaments. The mucilage covering the whole alga aids

in preventing desiccation at low tide, as well as helping to reduce

physical damage.

They are usually divided into about 12 orders, the simplest of

which is the Ectocarpales. No members of the brown algae are adapted

to a life on land but frequently the complexity of the group is cited

as being analogous to that of the higher land plants.


The Laminariales, or kelps, are found in all the colder seas and

oceans of the world and include the two largest known algae, Macrocystis and Nereocystis. The familiar ‘tangle’ of Atlantic shores is Laminaria

digitata. They are most frequently found in the sublittoral zone just

below the low-water mark. In Laminaria, the thallus is differentiated

into a blade that, depending on the species, may be single or dissected

into many segments. There is usually a strong, pliable stipe or stem

connecting the blade to the holdfast that anchors the kelp to the

rocks. In the stipe there is a specialised conducting tissue analogous

to the phloem of land plants. It has sieve-tube elements, ‘trumpet

hyphae’, which lack a nucleus at maturity. This conducting system

probably evolved along with an increase in size. Growth of the thallus is rapid and constant abrasion by the pounding of waves against

rocks is quickly made good by very rapid meristematic growth at the


Figure 1.38. Intertidal zonation

at an exposed and sheltered rocky

site and on an unstable substrate

from the NE Atlantic coasts

showing the different distribution

of seaweeds (modified from

Hiscock, 1979).




1.4.7 Green algae (Chlorophyta)

Green algae were almost certainly the ancestors of plants. They are

very diverse and include a range of organisms, approximately 500

genera and more than 16 000 species, from unicellular flagellates

described above, to complex multicellular organisms. They have a

very wide ecological range and though most live mainly in marine,

brackish or fresh water, they are also include semi-terrestrial forms

dispersed by wind, or kinds that grow in the soil or in association,

even inside, plants and animals. Paramecium is a ciliate that engulfs

unicellular green algae and keeps them in its vacuole. The euglenoids

(Euglenophyta) have gained their chloroplast through a symbiotic

association with a green alga. Green algae are also one of the partners

in the lichen symbiosis.

The fossil record of the green algae begins in the Cambrian, predating by many millions of years the origin of the land plants. They

may have arisen even earlier but the earliest putative fossil green

algae come from the Neoproterozoic rocks 900 million years old in

Australia and 700--800 million year old rocks in Spitzbergen. Some

early fossils are referable to the Dasycladales, the order of unicellular stalked organisms that includes the living Acetabularia. About

120 fossil genera have been described. Some of the multicellular fossil algae from Spitzbergen resemble the living genera Coelastrum and

Cladophora. Many of these fossils have been preserved because they

secrete lime around the complex multicellular thallus.

The use of taxonomic names gets difficult here if we adopt a

strictly phylogenetic approach to naming groups. The green algae

that we have called the Chlorophyta, are not a monophyletic

group, because, as we have defined them, we have excluded plants

(Embryophyta). A more strict use of terms that arises from cladistic

analysis of biological variation would include all green plants and

algae in a single group (the Viridiplantae) and split this into the

two distinct sister lineages, which have been called the Chlorobionta

and the Streptobionta. Then again, if we recognise the plants as a

distinct taxonomic group, the Embryobionta, the remaining Streptobionta cease to be a proper taxonomic group. The separation of

the Chlorobionta (including only some green algae) from the Streptobionta (including some green algae and all plants) is based on what

are regarded as fundamental features of cell division and flagella

insertion, as well a molecular characters such as the DNA sequences

of mitochondrial genes.

The complications are endless because there is a ‘primitive’ group

of green algae called the ‘Prasinophytes’ by some, or the Micromonadophyta or Micromonadophyceae by others, and which includes several lineages related to either the Streptobionta or Chlorobionta or

neither. For example, Mesostigma viride has the same pigments as the

Ulvophyceae in the Chlorobionta but on the basis of DNA sequence

data this prasinophyte is the earliest divergence at the base of the

Streptophyte lineage that gave rise to plants. Mamiella, Mantoniella,

Micromonas and Pseudoscourfeldia have a distinct light-harvesting


Table 1.4 The differences between the Chlorobionta and Streptobionta plus Embryobionta


Streptobionta and Embryobionta

Nuclear membrane persists throughout mitosis


Phragmoplast transitory

Motile cells symmetrical

Flagella apical and directed forward

Cytoskeleton not in flat broad bands

No glycolate oxidase or if present outside


Nuclear membrane disintegrates in mitosis

No phycoplast

Phragmoplast (spindle) persistent

Motile cells flattened

Flagella sub-apical and at right angles to cell

Cytoskeleton with flat broad bands

Peroxisomes with glycolate oxidase (photorespiratory


pigment--protein complex indicating a separate lineage from either

the Chlorobionta or the Streptobionta.

Although the prasinophytes may not represent a monophyletic

group they are an interesting grade of organisation and share several features. They are all flagellate unicellular organisms with a variable number of flagella arising from inside an apical notch. They

can undergo encystment losing their flagella and becoming dormant

inside a round resistant wall, which looks like a spore, though it

does not contain sporopollenin. Most have an interesting scaly surface unlike that of the cell wall of plants or theca of other green

algae. The scales arise inside the Golgi apparatus and are then transferred by a vesicle to the apex of the cell from where they join the

several layers of scales that are already present.

The Chlorobionta include three groups of the green algae, the

Chlorophyceae, the Pleurastrophyceae and the Ulvophyceae, and are

represented by a range of unicellular, filamentous or stratose forms.

One way in which these groups can be separated is by how the flagella

in their motile cells are arranged. Like the hands of a clock, the

Chlorophyceae have the flagella at either 1 and 7 o’clock or 12 and

6 o’clock, while the Pleurastrophyceae have them at 5 o’clock. The

Ulvophyceae have them all at 12 o’clock. There is unlikely to be any

adaptive significance to these arrangements. This is just one of those

features that became fixed developmentally early in the evolution of

a lineage.

Molecular data indicate that the charophytes in the Streptobionta

are a monophyletic sister group to plants. Bryophyte plants share several features with them but these may have evolved in parallel. Charophyte fossils such as Palaeonitella are known from Silurian marginal

marine sediments. They underwent their first major diversification

in the Silurian and Devonian at about the same time as plants were

originating and first diversifying. Nitella-like fossils are known from

fresh-water Lower Devonian Rhynie chert. The first record of the living order Charales is from the Permian, and they underwent a major

radiation in the Mesozoic.



Figure 1.39. Derrbesia

filamentous green alga with

coenocytic cells.




Figure 1.40. Charales, the

green algal sister group to plants

shows different levels of

complexity of form: (a) Mantoniella

(Micromonadophyceae) probably

the most primitive charophyte is

scaly with two flagellae, one very

short; (b) Pyramimonas is also scaly

– the scales are produced by the

Golgi body; (c) Klebsormidium

filaments do not have holdfasts and

there are no plasmodesmata; (d)

Zygnematales show a ladder-like

(scalariform) conjugation in sexual

reproduction; (e) Chara has a

differentiated branched form with

nodes and internodes; (f)

Coleochaete – some species form a

pseudo-parenchymatous disk, and

have sheathed filaments.









There are four main groups of living charophytes (Figure 1.40),

the Klebsormidiales, Zygnematales, Coleochaetales and Charales.

The Klebsormidiales have undifferentiated uniseriate filaments and,

unlike other charophytes, they are isogamous, releasing biflagellate

gametes. The Zygnematales such as Spirogyra are multicellular and

filamentous, or bi- or unicellular organisms. They include the very

large group of planktonic organisms called the desmids. Very unusually for algae and plants the Zygnematales undergo conjugation,

the filaments aligning to each other and fusing, to bring their nonflagellate gametes together. The Coleochaetales and Charales are the

most complex filamentous and pseudo-parenchymatous multicellular charophytes. They are oogamous and have complex reproductive

structures. The Charales are called the stoneworts because they are

commonly heavily calcified.

1.5 What is a plant?

Figure 1.41. Thalloid

chlorophyta in the Ulvophyceae.

(a) Enteromorpha with a

unistratose tubular lamina; (b) Ulva

with a bistratose flat sheet.

Plants are actually very strange living creatures indeed. Their life is

alien to us. But this is their planet; they have made it and we live in

their shadow. Plants seem static so that it is easy to forget that plants

are living organisms. We associate movement with life. Though some

plants look like pebbles (Lithops), they are not stones. If plants do not

have life surging through them like animals, it trickles through them

in a constant stream. Plants combine the stability of structure with


Table 1.5 The kingdoms of life

















the fluidity of change. In the following chapters we explore this in


Before the modern period of classification any organism not obviously animal was usually regarded as a plant. This applied to fungi

and many marine animals such as anemones, bryozoans, corals, etc.

A dichotomous view of the world, basically ‘folk-taxonomy’, was allpervasive and influenced classification for two centuries. But there is

a unity between plants and other organisms. It is now known that

the similarities between animals and plants outweigh all their discrepancies. Differences in mode of life and structure are no more

than differences of expression and economy. Nowadays, three to six

kingdoms of organisms are usually recognised (or seven if viruses are


However, the most fundamental dichotomy of living organisms is

at the cellular level between prokaryotes and eukaryotes (see Section

1.3). For most people, however, the differences between prokaryotes

and eukaryotes are meaningless in comparison with the apparently

real and culturally important distinction between plants and animals.

The old dichotomous view still has relevance to our everyday lives. For

this reason the differences between plants and animals are outlined

in considerable detail below but this is not to be regarded as an

endorsement of a two-kingdom classification.

Fungi and algae are still often regarded as plants but in most modern classification systems they are placed in separate kingdoms. The

latter view is adopted in this book, so that when we write ‘plants’

this is equivalent to ‘land plants’ of other writers. Algae are placed

in the protoctista or protists, which are mainly unicellular. They are

exceedingly diverse in very fundamental ways. They have traditionally divided into heterotrophic (non-photosynthetic) and autotrophic

organisms. The latter are ‘the algae’ of diverse sorts. Algae have traditionally been considered to be ‘plants’ in a broad sense and studied

by botanists, and some at least of these algae have a direct place in

the story of plants because they are related to the ancestors of plants.

Botanists have also studied fungi, although it is now clear that they

are more closely related to animals than they are to plants. However, they too have a place in this story because of their ancient and

close ecological partnership with plants, in mycorrhizal plants and

mycotrophic plants.


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