Botany Tracheo

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NonVascular and Seedless Vascular Plants

ADAPTATIONS TO LIFE ON LAND

Making the transition from a small, aquatic, Chlorophyta alga to the large, multicelled terrestrialplants of today required a number of structural and physiological changes as outlined below.

Recent* findings support the view that the most ancestral green flagellate was biflagellate and

asymmetric, and possessed an eyespot, square scales and multilayered structures. *Lemieux, C.,Otis, C.,and Turmel, M.Nature 403,649-652 (2000). This led to a number of evolutionary dead ends

possessing many of the characteristics present in the higher plants.

Conditions of Land Life

To maintain itself a living cell must obtain water and other materials by absorption through the cell

membrane. Materials must be available fairly consistently throughout all cells of an active

organism. Diffusion can be used to distribute materials if the organism is small or consists of only a

few cells. Difficulties arise in larger organisms because some cells are internal - out of direct contactwith the environment. These problems are aggravated when some parts extend into the air and are

distant from a source of water and dissolved materials. Water must be passed along from cell to cell

by osmosis and diffusion. These are short distance processes which are slow and ineffective over

distances inside the plant. Movement of materials across great distances is the first barrier to any

plant attempting to extend itself into the air. Development of vascular systems somewhat analogous

to circulatory systems in animals has overcome this problem in higher or vascular plants.

Specialization at the level of the tissues and organs was also necessary. Systems to gather in

nutrients and water (roots) and collection and conversion systems to utilize the power of sunlight(leaves)and systems to hold the plant upright and increase its competitive ability to battle for

sunlight (stem) were required. Changes in reproductive structures and behaviors led to protected

male gametophytes (pollen) and embryos that were protected and nurtured (seeds).

A COMMON SET OF FEATURES EVOLVED BY GREEN PLANTS IN RESPONSE TO THE

LAND ENVIRONMENT

1. Cuticle - waterproofing of outer layers of leaf and stem2. Pores or Stomates surrounded by Guard cells and regulated by K+ levels - aided in

water balance within leaf and entire plant body through transpiration

3. Rhizoids or roots - provided anchorage, absorption, support4. Lignin - strengthening between cells and within tissues5. Vascular tissue - xylem and phloem; move water and nutrient upwards and food

downward

6. Resistant spores and seeds - prevent dessication, contain inhibitory substances;ensure survival of the species7. Gamete forming cells enclosed within a jacket of sterile cells provides protection

from water loss8. Embryos formed and held within parent tissues - provides protection and a food

supply to developing plant

9. Root systems show a highly refined geotropic response.EVOLUTIONARY TRENDS ACCOMPANYING THE TRANSITION
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1. The sporophyte has replaced the gametophyte as the dominant stage in the lifehistory of the plant. During this shift, the size of the sporophyte progressively

increased, while that of the gametophyte decreased.

2. The increase in the size of the sporophyte was accompanied by an increase instrength and efficiency of its vascular tissue.

3. As tissues became more specialized, new organs were added progressively: simplebodies with rhizoids ---> stems ---> stems with roots and tiny leaves ---> many-

veined leaves ---> woody tissues

4. The male gametophyte evolved into a waterproof pollen grain, which traveled to thefemale gametophyte before releasing the sperm. Sperm no longer faced a long,dangerous swim to the egg.

5. The female gametophyte of all land plants retains the egg and protects the zygote asit develops into an embryo; in higher land plants, the female gametophyte itself is

retained on or in the sporophyte parent, which contributes food and protective

coatings to the seed, a new dispersal structure containing the embryo of the next

sporophyte generation.6. In the higher vascular plants, the spore, a single haploid reproductive cell that is the

dispersal stage of lower land plants, has become differentiated into megaspores,

which give rise to female gametophytes, and microspores which give rise to pollen

grains (male gametophytes). Instead of being shed, the megaspores remain in the

sporophyte, which protects the female gametophyte and plays a major role in theproduction of the seed. Seeds replaced spores as the dispersal stage in the life

history.

7. As plant structure and reproduction became more and more independent of waterand moisture in the environment, plants underwent adaptive radiation and spread

into more and more different habitats.

Sexual reproduction (fusion of GAMETES) and Asexual reproduction (by means of SPORES),

occur inALTERNATING GENERATIONS. One generation, where the plant body is known as the

GAMETOPHYTE is Haploid (n) and produces Haploid GAMETES, the Sperm and the Egg. These

gametes then fuse to form a ZYGOTE in the process of FERTILIZATION. This Diploid Zygotedevelops into a SPOROPHYTE (also Diploid). The Sporophyte undergoes MEIOSIS and forms

SPORES that develop into an adult organism called the GAMETOPHYTE This completes the

cycle.

The zygote was formed as a result of the fusion of two cells, while the Gametophyte plant developed

as the result of asexual reproduction (mitosis) of the spore.

METAPHYTA: NON-VASCULAR PLANTS - THE BRYOPHYTES

All available evidence seems to indicate thatthis divisionrepresents an evolutionary dead end.There is no evidence to indicate that this division is ancestral to any of the modern vascular plants.

It has been suggested that differences between the three major classes within the division may

necessitate reclassification into separate divisions at some time in the future.

These plantsshow many of the adaptations needed for a successful transition to a terrestrialenvironment. As a division these plants:

a. lack specialized vascular tissue or supportive tissue
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b. possess a characteristic life cycle which follows the generalized life cycle where theSPOROPHYTE is attached to and dependent upon the GAMETOPHYTE for

nutrition- the GAMETOPHYTE is dominant, conspicuous, and longer lasting

In higher, vascular plants the relative importance of the SPOROPHYTE and GAMETOPHYTE

are reversed.

Gametophyte plants may be MONOECIUS (one house) and contain both male and female organs

or, DIOECIOUS (two houses) have separate plants for each. The sex organs are multicellular andbounded by a layer of sterile cells.

Bryophytesare usually small, ranging from 2-20 cm. They lack true roots, stems, or leaves although

they have structures which accomplish many of the functions of these organs.

Leaf-like structures are usually a single cell layer thick and are called PHYLLIDS. Root-like

structures, called RHIZOIDS, serve mainly for anchorage, and stems or stalks, called SETA or

CAULIDS, serve for support.

There are three recognized classes of Bryophyta:

Liverworts, Hornworts, and Mosses

Class Musci - The True Mosses

This class is frequently confused with sea moss (red algae), reindeer moss (lichen), spanish moss

(flowering plant), and club mosses (primitive vascular plant).

Mosses are distinguished from Liverwortsand Hornworts by:

1. an algal-like PROTONEMA2. radial symmetry of the plant body3. theelaborate capsule of the mature sporophyte

There are approximately 670 genera representing about 16,000 species of mosses. They are

normally found in wooded, moist areas, usually in dense stands forming carpets or mats, and are

seldom more than 6-8 inches tall. There are some species found in desert areas. They are sensitive

to air pollution, especially sulfur dioxide.

Economically they are of importance as:

uccession

Sphagnum moss, which has a high water holding capacity and adds some acidity to the soil

The following characteristics of mosses distinguish them as transition plants between aquatic andterrestrial habitats.

1. dependence on water for sexual reproduction
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2. vegetatively well adapted for life on land3. relative size andindependence of sporophyte and gametophyte generations;

situation reversed in higher plants

It appears that the sporophyte, which eventually evolved a vascular system in higher plants, is

better adapted to life on land (terrestrial), in that spore production is well suited to that type ofhabitat. By contrast, the biflagellated gametes of the gametophyte generation are better suited to an

aquatic mode of life.

Mosses seem to retain a close algal relationship as evidenced by the structure of thePROTONEMA,

similar to that of some green algae; possible evidence for evolution from green algal forms.

Mosses established at least two critical features which later were more fully developed by land

vascular plants.

1. retention and protection of the embryo within the female parent plant2. presence of a strand of tissue occupying the center of the stemlike axis used for

support and conduction - may be prevascular tissue which evolved into truevascular tissue containing tracheids and sieve tubes; in mosses this central strand of

tissue consists of cells called HYROIDS

ALTERNATION OF GENERATIONS in Mosses

Haploid spores form within the capsule which is covered by a lid called an OPERCULUM. As

many as 50 million spores may be formed. When released the spores develop into a branched,filamentous PROTONEMA from which a leafy gametophyte develops (may be monoecious or

dioecious). Biflagellated sperm are released from the ANTHERIDIUM and attracted to the

ARCHEGONIUMby malic acid. Within the archegonium the sperm fertilizes the egg cell which

develops into a zygote which divides mitotically to form the SPOROPHYTE. Typical structures

form and spores develop within the capsule. Some species have a PERISTOME beneath theoperculum (humidity sensitive hairs regulating spore dispersal (hygroscopic), e.g. the genus

Polytrichum.

METAPHYTA: SEEDLESS VASCULAR PLANTS

Tracheophytes (vascular plants)are the largest group of plants on earth and are characterized by

the presence of tracheary elements and sieve tube elements organized into xylem and phloem in the

adult sporophyte. (see notes above on Plant Adaptations to the Land Environment)

All lines of evidence suggest that the tracheophytes developed directly from some complex,

multicellular Chlorophyta. Unfortunately, an extremely large gap exists between the algae and

vascular plants, and there are no living plants which bridge the gap. Bryophytes are somewhatintermediate but are closer to algae than even the simplest tracheophytes, and it appears that the

intermediate plants have become extinct.

In addition,tracheophytesare identified by independent and dominant sporophytes and greatlyreduced gametophytes. This is in direct contrast to the Bryophytes. The sporophyte of most

tracheophytes are differentiated into roots, stems, and leaves. Also, vascular plants are typically

land plants and, like bryophytes, have evolved terrestrial adaptations.Check out this comparative
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information on the lower tracheophytes.

Evolution in the Lower Tracheophytes (Psilophytes, Lycophytes, Sphenophytes, and Ferns)

Transition from water to land was gradual over millions of years. Those plants which evolved

adaptations permitting growth in a dry environment established themselves as the first land plants.

The basic problem confronting early land plants was availability of water. The tracheophytes did

not have the limitless supply of water which was available to the aquatic green algae. A successful

land plant must possess an efficient mechanism for absorbing materials from the soil andtransporting them throughout the plant. Once obtained from the environment the water must be

conserved.

Division Psilophyta

The evolutionary significance of the Psilophytes is that they are the most primitive known group of

vascular plants and are considered to be atransitional grouplinking the aquatic algae with more

advanced tracheophytes (club mosses, horsetails, and ferns). In some respects, the Psilophytes

substantiate the hypothesis for the evolution of the root as proposed by Lignier. In other respectsthey are of interest because they suggest how club mosses, horsetails, and ferns may have originated

by various leaf modifications and developments. Certainpsilophytes with small leavesdeveloped as

emergences and may have given rise to the Lycophyta line. Others with whorled branches may

have been the forerunners of the Sphenophyta line. The fern line may have developed from forms

in which the branch tips were flattened, possibly leading to the evolution of large leaves. Two major

genera represent this group,Psilotum and Tmesipteris.

Division Lycophyta

Represented byLycopodium and Selaginella,thelycophytesare believed to have evolved from the

psilophytes. Although members of this group are of no real economic import, the group does

illustratesome noteworthy evolutionary advancesover the psilophytes. The presence of true roots,and leaves, increased devel opment of the vascular tissue, and the organization of sporophylls into

cones (strobili) are all viewed as advances over the psilophytes. It is not clear why the lycophytes

have not given rise to more advanced groups of plants. Some have developed secondary growth and

exhibit early stages of the seed habit.

Division Sphenophyta

These plants are of little economic significance, although together with the lycophytes, theycontributed their vegetative parts to the formation of coal during the Carboniferous period. They

are considered to be a separate line derived from the psilophytes which did not give rise to other

plant groups. Major genus isEquisetum, known ashorestails or scouring rushes. Some species

accumulate silicaceous materials in their cell walls.

Division Pterophyta (Ferns)

Ferns and other pterophytes are believed to have developed from the psilophytes. Certain fossilferns resemble the psilophytes rather closely. The seed plants are considered to have derived from

certain of the extinct fern class. Modern fern groups are of little economic importance. From an

aesthetic view, they are grown exclusively for decorations. The fiddleheads and young foliage of
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many species are edible and are used as green vegetables in the Orient. A drug is derived from the

rhizomes (underground stem) of some ferns and is used to expel worms (vermifuge) especially

tapeworms, from the intestinal tract of man. Fossil ferns contributed to the deposits of the

Carboniferous period.

part is surrounded byan upgrowth of the thallus,the involucre. The development of the archesporium

beginsas a dome-like layer within the summit of thesporophyte. The sporophyte continues togrow

forseveral weeks or longer from a meristem close toits base. During this growththe archesporium dif-

ferentiates basipetally as a hollow cylinder, thecenter of which is occupied byasterile

columella.Multicellular elaters (which lack distinct spiralthickenings and are often referred toas

pseudo-elaters) differentiate amongst the sporogenouscells. When the spores at the top of the capsule

areripe, the capsule dehisces basipetally (Fig. 5.15a)along two longitudinal slits, the opening begin-ning

near the tip. As the upper part of the capsuledries, the valves separate completely above, andtheybegin

totwist longitudinally.The consequentcontortions of the valves expose the spores andelatersadhering tothe centra l co lumn, and dis-persal begins. The separation of the valves contin-ues

downward as the spores mature, and meanwhile the basal meristem generates newsporophytic tissue at

about the same rate.Consequently a single sporophyte continues to yield spores over a considerable

period. The sporeshaveconspicuously thickened and sculptured walls.

Anthoceros

and its allies further differ fromother liverworts, but resemble the mosses, in pos-sessing photosynthetic

tissue in the outer layersof the extending sporophyte. Stomata are alsopresent, as in the capsules of some

mosses. Thesporophyte is thus not entirely dependent on thegametophyte for nutrition, but, since the

sporo-phyte will still mature even if it is covered with atinfoil cap, it seems likely that it is able todraw

asubstantial proportion of its essential metabolitesfrom the parent gametophyte.

e v o l u t i o n a r y p o s i t i o n

The Anthocerotales are remarkable amongst theliverworts in recalling the features of both thealgae (thepresence of the pyrenoid in the chloro-plast) and the mosses and higher plants (the pres-

1 1 6 T H E S U B K I N G D O M E M B R Y O P H Y T A : D I V I S I O N B R Y O P H Y T A ( M O S S E S

A N D L I V E R W O R T S )

Figure 5.15.Anthoceros laevis

. (a) Female thallus withsporophyte. (b) Section of antheridial chamber. (c) Section of nearly mature

archegonium. (d) Diagrammatic longitudinalsection of a sporophyte showing the different regions.


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ence of stomata and the continued growth of thesporophyte). They are consequently thought bys o m e

t o s t a n d c l o s e t o t h e l i n e o f e v o l u t i o n leading from the algae toterrestrial vegetation. The

intermediate position of the Anthocerotalesextends even totheir ultrastructure. The electronmicroscope

confirms that the chloroplasts ofAnthoceros

resemble those of the algae, but thechloroplasts ofMegaceros

, which are several ineach cell, often lack pyrenoids and possess well-defined grana. They thus resemble

those of landplants generally. Another feature of the Anthocerotales whichhas excited much attention is

the axial formof the sporophyte, and the tendency in some speciesfor the foot toproduce rhizoid-

li ke ou tgrowths into the tissue of the gametophyte. Since thesporophyte in some forms is long-lived and

mayeven persist for a time after the death of theparent gametophyte, it may indicate how simpleaxial plants, such

as those of the Silurian andearly Devonian (p. 135), haveevolved. Alternativelythe

Anthoceros

condition may be derived, thesporophyte having become reduced and almostdeprived of its

independence. Although firm evidence relating to either pos-sibility is lacking, it seems quite plausiblethat agrowthform similar tothat shown bythe living Anthocerotales did play some part in

the evolu-tion of land plants from algal ancestors.Unfortunately the only fossils so far known attrib-utable tothe Anthocerotales are spores from the Tertiary.

Bryopsida (mosses)

The mosses are a class much greater in numberand more widely distributed than the liverworts,occurring

in almost every habitat supporting life. Apart from being the dominant vegetation in acid bogs, andalpine and arctic regions, theyare afamiliar feature of woodlands and hedgerows.

B R Y O P S I D A ( M O S S E S ) 1 1 7

Figure 5.16.

Anthoceros laevis

. Female plants bearing youngsporophytes. Scale bar 5 cm.


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Some species even survive the polluted atmos-phere of urban areas, often forming dark greencushions

between paving stones and in otherdamp crevices. Mosses in exposed situations,including

Sphagnum

in bogs, often reach tempera-tures of 3040C (86104F). Isoprene production(p. 11), which is morecommon in mosses than inany other major group of land plants, increases inthese condit ions , and

may consume up to8percent of the carbon fixed. As in other landplants, isoprene appears to beconcerned withprotecting the photosynthetic apparatus fromexcessive insolation and heating. The features

which distinguish the mossesfrom the liverworts are found in both gameto-phyte and sporophyte. The

protonemal stage of the gametophyte is often conspicuous, the matureformof the gametophyte is always

leafy, and therhizoids are multicellular. The sporophyte growsfrom an apical cell. The capsule in many

instanceshas a complex opening mechanism which affectsthe distribution of the spores, and sterile

elatersare never present. The three orders, Sphagnales, Andreaeales andBryales, differ principally in the

nature of theprotonema and the structure of the capsule. There are also differences in the

ultrastructuraldetail of the spermatozoids (which resemblethose of other bryophytes) which may

havetaxo-nomic significance.

The Sphagnales

The Sphagnales, a verydistinctive order, are repre-s e n t e d b y a s i n g l e g e n u s ,

Sphagnum, confined toacid, waterlogged habitats. The more or less con-tinuous spongy layer of peat bogs consistsverylargely of a range of

Sphagnum

species. The adult gametophyte comprises an uprightmain axis from which whorls of branches ariseatregular intervals (Fig. 5.17). The leaves, which are


A N D L I V E R W O R T S )

Figure 5.17.

Sphagnum fimbriatum. (a) Upper portion of shoot with sporophytes. (b) Stem leaf. (c) Branch leaf. (d)Arrangement of cells in

leaf, adaxial surface. (e) Section of leaf.


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clo sel y insert ed, have ape cul iar struct ure whichis diagnostic of the genus, and also, in its

finerdetails, of the many species (Fig. 5.17c and d). When firstformed, the leaves are made upof many diamond-shaped cells. These then cut off narrow daughter cells, but on two sides

only.Thedaughter cells develop chloroplasts, but themother cell remains colorless. It eventually dies, but

before doing so pores are formed in its upperand lower walls and spiral bands of thickening areoften laid

down around the cell. The architectureof the leaf ofSphagnum

thus provides a splendidexample of the phenomenon of

apoptosis

, apattern of programmed cell death being imposedupon the developing leaf. The cell-to-cell influ-enceswhich determine this pattern are as yet wholly unknown. The epidermis of the branches takes the formof a

cylindrical sheet of empty elongated cells(termed retortcells), each with an apical poreopen totheexterior. There is no evidence thatthese cells are formed byasequence of divisionssimilar

tothat seen in the devel opmen t of theempty cells of the leaf. The empty cells of the mature leaf

and branches are able totake up water and act as res-ervoirs. The weight of a saturated plant may be

asmuch as 20 time s tha t of the same plant dry .This unique water-storing capacity of the

leavesaccounts for the outstanding bog-building proper-ties of

Sphagnum. The acidity of the bogs is largelyattributable tothe presence of phenolic sub-stances in the walls

of the dead cells which act inthe same way as ion-exchange resins. The growingplants extract metallic

cations from inflowing waters and release hydrogen ions, so maintainingthe acidity at values of aroundpH 4.0 or less. Thelack of oxygen in the lower levels of the bogimpedes oxidation of the organic matter,leadingto accumulations of peat of considerable depth. Well-preserved leaves similar tothose of

Sphagnum

havebeen found in Permian deposits inRussia, indicating beyond doubt the great antiq-uity of this kind of

construction.

r e p r o d u c t i o n

The reproduction of

Sphagnum

is probably princi-pally vegetative, the decay of the older parts even-tua lly caus ing branchestosepara te and thus to become new individuals. Disks of

Sphagnum

peatfrom depths as great as 30cm (12in.) will produceinnovative growths when exposed tolight and

asaturated atmosphere. These probably arise fromthe outer cortex of the buried stems. Some

cellsevidently remain viable and capable of division,possibly for as long as 60 years.Mature plants in

favorable situations producesex organs freely.Both monoecious and dioeciousspecies occur. The

antheridia, each of which begins its development from a single apical cell,lie in the axils of leaves

towardthe tips of smallupper branches (Fig. 5.18). These antheridial branches are often stronglypigmented

and clus-tered in a conspicuous comal tuft. The femaleinflorescence consists of a bud-like

aggregateof archegonia and bracts borne laterally near thesummit of the main stem. After fertilization the

zygote yields a sporo-phyte (Fig. 5.17a) consisting principally of acapsule, containing a dome-shaped

archespor-ium, and a foot. The seta remains inconspicuous,and the function of elevating the capsule istakenon by the base of the female inflorescence which,as the capsule matures, grows up as a leaflessaxis,or

pseudopodium

(Fig. 5.19). The absence of any cellextension in the seta, and correspondingly of anymarked flux offluidinto the developing sporo-phyte, is correlated with the absence of labyrin-thine walls at the interface of

the two generationsin the foot.Release of the spores is brought about byairpressure which builds

up in the lower half of thecapsule as it dries (and may reach a level of 300kPa(3atm)). Eventually this


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pressure is sufficient todislodge the clearly differentiated lid (operculum) with explosive force, and thespores are effectivelydispersed.Developing

Sphagnum

capsules are occasion-a l l y p a r a s i t i z e d b y a f u n g u s (

Bryophytomyces

). Although the mature capsule may appear perfect,the small spores of the fungus replace those of

thehost. This gaverise toreports of heterospory inSphagnum

before the situation was understood.

Sphagnum

s p o r e s g e r m i n a t e t o f o r m a fi l a m e n t , b u t t h i s i s r a p i d l y r e p l a c e db y as ma l l thallo se protonema. This in turn gives rise toa bud which develops into the familiar

leafy gamet-ophyte, the protonema meanwhile becoming



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moribund and disappearing. Protonemata alsoappear on disks of old peat maintained in

humidcondi tions. Many appear toa ris e from spores within the compacted peat which

haveremained viable. Others are secondary in origin from viablecells in the old stems.

The Andreaeales

The Andreaeales are an order containing onlythree genera, distinguished bytheir peculiar cap-sules. The

leafy gametophyte ofAndreaea

(Fig. 5.20)rarelyexceeds 1cm (0.4in.) in height. It is usuallyfound growing on rock, chiefly in

cold, exposedand relatively dry regions. The leaves are olive- brown in color, composed of rounded cells,

and inmost species showing no distinct midrib.Sex organs are formed apically.The sporophyteresembles

that of

Sphagnum

in having a domedarchesporium (Fig. 5.20c), and in being borne on apseudopodium at maturity.

Dehiscence of thecapsule takes place byfour longitudinal slits which do not meet at the tip (Fig.

5.20b). Thehygroscopic properties of the wall cause the slitsto close in damp conditions, and to open

again indry (Fig. 5.20a). The protonema of

Andreaea

is similar tothat ofSphagnum

. Also placed in the Andreaeales is the recently


A N D L I V E R W O R T S )Figure 5.18.

Sphagnum

sp. (a) Antheridial branch. (b)Longitudinal section of antheridial branch. (c) Longitudinalsection of

archegonial branch.

Figure 5.19.

Sphagnum

sp. Longitudinal section of nearlymature sporophyte.


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discovered

Andreaeobryum

, growing in the oceanicnor thwest of Canada and Alaska . The capsul e iselevated byaseta, and

opens less regularly than in

Andreaea

. The protonema produces curious cylin-drical multicellular processes which are greenand presumablyphotosynthetic. Whereas

Andreaea

is usually found on siliceous rocks,

Andreaeobryum

prefers a calcareous habitat.Current evidence points to

Takakia

(p. 115) also being correctly placed here. This has a smallupright stem, rising from a filiform

rhizome,growing on soil partly covering rocks. It isconfined to subalpine situations in Japan andtheoceanic nor thwestof the Amer ican con tinent . The stems bear linear appendages

(phyllids),usually in groups of two or three. The archegoniaare terminal. The archesporium is domed,and thecapsule lacksaperistome and lid. Dehiscence of the capsule takes place

b y as i n g l e s l i t wh i c h extends spirally from the base of the capsule tothe tip, a feature unique in thebryophytes. Afterrupture, the calyptra is carried up at the summitof the capsule, but becomes loose and

fragmen-tary. The seta remains short and thick and doesnot attain the delicacy seen in many

bryaleanmosses. In structure and shape the foot resemblesthat of

Andreaea. Although the recognition of

Takakia

as a moss appears well founded, a distantaffinity with the Calobryales, particularly inrespect of the

archegonia terminating the leafyaxes, remains a possibility.

The Bryales

The 600 or so genera of the Bryales formawell-defined order. Although there is a commonbasicmorphology and life cycle within the order, the variation in size, detailed structure and

habitatpreferences is considerable. Many mosses areconfined topermanently damp situationsin woodlands and bysprings, but others, for example

Tortula ruraliformis

, are able to survive periods of drought in sand dunes and other arid habitats. The cells of these species

appear tohave acquiredthe capacity tocontinue metabolism at a reducedr a t e w h i l e p a r t i a l l y

d e h y d r a t e d . A t t h e o t h e r


Figure 5.20.

Andreaea nivalis

. (a) Habit of fertile plantshowing dehisced capsule in dry condition. (b) Dehiscedcapsule in wet

condition. (c) Longitudinal section of maturesporophyte.


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Botany Tracheo

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