Tải bản đầy đủ - 0 (trang)
6 Epiphytes, hemi-epiphytes and vines

6 Epiphytes, hemi-epiphytes and vines

Tải bản đầy đủ - 0trang

6.6 EPIPHYTES, HEMI-EPIPHYTES AND VINES



There are about 900 genera and almost 30 000 species of epiphytes

in the world, but there are no totally epiphytic families. In most plant

families epiphytes are insignificant, a spectacular exception being

the Orchidaceae. There are between 20 000 and 25 000 orchids in the

world and two out of three (70%) of them are epiphytic. Some 44% of

all vascular plant orders and 16% (or about 65 families, 11 of which

are ferns) of all vascular plant families have epiphytic species, but

only 32 seed plant families have five or more. About 20% of the pteridophytes are epiphytic. There are about 143 species of Lycopodium that

are epiphytic while only five species of Selaginella are epiphytic. Gymnosperms are rarely epiphytic and this is consistent with their slow

maturation, massive axes, anemophily and heavy seeds.

There are slightly more families with epiphytes in the Palaeotropics than in the Neotropics (43:42), and there are six times more epiphytes in Central and South America than in Africa.

Speciation of epiphytes is greatest in the Neotropics; the numbers of cacti and bromeliads account for this. Africa, with about

2400 epiphytes, and only about 50% of the families found in other

Palaeotropical areas is poorest. This was probably because of impoverishment during dry periods of the Pleistocene. Australasia is impoverished compared with the Americas (10 200 compared with 15 500).

These distributions and diversities are the result of historical accident.

Each continent has evolved its epiphytic flora independently from

terrestrial relatives (sometmes several times over); for example most

epiphytic Neotropical orchids belong to the subtribes Maxillarinae,

Oncidinae and Pleurothallidinae, whereas the epiphytic Palaeotropical orchids belong to the subtribes Dendrobiinae and Bulbophyllinae. The Palaeotropics are richer in ferns, Araceae and Asclepiadaceae

while Australasia is better represented by Rubiaceae (see Figure 6.41).

(a)



Figure 6.41. Epiphytes with a rosette form trap detritus and water: (a) epiphytic

bromeliads in neotropical forest; (b) bird’s nest fern (Asplenium).



(b)



(a)



(b)



Figure 6.40. Epiphytic orchids:

(a) pendulous orchid in flower;

(b) pseudo-bulbs.



287



288



THE LIVES OF PLANTS



Epiphytes may be classified in several different ways, for example by size as micro- or macro-epiphytes; by morphology as ‘trashbasket epiphytes’ or ‘succulent epiphytes’; by ecology, physiology or

behaviour as ‘shade-tolerant or sciophytic epiphytes’, ant-plants, ‘sunlovers or photophytic epiphytes’, stranglers, bole-climbers, etc.; by

dominant habit as proto-epiphytes (facultative and obligate); hemiepiphytes (primary and secondary); and ultra- and hyper-epiphytes,

etc.; or by a combination of many factors. The last method is probably the best because it takes into account specific adaptations and

allows us to directly inter-relate the epiphyte with its immediate

environment. However, there is a complete gradation from illadapted proto-epiphytes such as Schefflera (Araliaceae), Episcia (Gesneriaceae), through primary hemi-epiphytes such as stranglers (Ficus spp.:

Moraceae) and secondary epiphytes such as climbers to the highly

adapted hyper- and ultra-epiphytes epiphytes such as bromeliads and

orchids. A rigid classification is not only difficult, it is inadvisable.



6.6.1 The herbaceous vines and woody climbers



Figure 6.42. An unknown

climber clinging to the bark of a

tree. Many bole climbers of the

tropics attach by means of

adventitious roots, and conserve

moisture by closely hugging the

trunk of the host tree.



Lianes (lianas) and climbers have been called proto-epiphytes probably

because they begin life rooted in the soil. As they climb they establish

connections with the host or with pockets of humus and become

hemi-epiphytes. The contact with the soil may become insignificant or

be lost altogether, so that at maturity they are holo-epiphytes. Almost

all lianes are flowering plants. Gnetum is one exception. Ferns are

unusual climbers. Stenochlaena, the vine fern, has slender green rhizomes. Lygodium has an indeterminate frond that produces pinnae

continuously as it grows forward. The simplest climbers are those

lianes that lean against or scramble over their supporting trees without any intimate connection. Others produce long arching stems that

reach up to find support with hooks or thorns derived from leaves,

petioles or lateral branches to aid their scrambling. The climbing

palms (rattans), such as Calamus, are very common lianes in South

East Asia. The distal pinnae of the pinnate leaves are backward pointing spines. The rattans grow very fast and have stems which may be

well over 100 m long. The stems provide canes for the furniture, basket

making and mat industries. Another interesting group are the climbing bamboos like Dinochloa, which has a zig-zag culm and roughened

leaf sheaths to aid climbing.

A closer connection to the host is achieved by the vines with tendrils modified from leaf or stem, or which twine around the supporting tree. Unlike lianes, they may not conform to any simple

architectural model. They show varying degrees of specialisation. An

even closer connection is achieved by those climbers that produce

adventitious roots. These may penetrate the bark, as in the climbing pandanus, Freycinetia. Others, such as the familiar houseplants

Monstera and Philodendron, produce large corky aerial roots that take

advantage of pockets of humus. These root climbers tend to be highly

adapted epiphytes. They migrate up through the canopy with rounded

leaves hugging the trunk in early stages and with large out-reaching



6.6 EPIPHYTES, HEMI-EPIPHYTES AND VINES



(a)



(b)



compound leaves in later stages. Root climbers may obtain significant

amounts of water through their adventitious roots, so that they can

become holo-epiphytes relatively easily.

Some of the most important adaptations of lianes and climbers are

in their vascular anatomy. The free hanging lianes must have a pliant

stem able to withstand torsion movements. Parenchyma is abundant

in the stems of lianes and vines. In part, this may be because fibres

are not required the parenchyma may confer greater flexibility. The

xylem and phloem have to remain functional at a great age because

of the plant’s restricted ability to replace them by secondary growth.

There is a great variety of anatomical patterns, the result of differential activity of the cambium. Many have a ribbed xylem (lobed in

transverse section) as a consequence of the cambium ceasing activity

in places. The furrows between the arms of the xylem are filled with

phloem (Bignoniaceae, Apocyanaceae, Acanthaceae). Some have only

two lobes, giving a flat stem that is pressed against the supporting

tree. In others, an interfascicular cambium does not develop except

to produce extra separate bundles. In some lianes, for example in the

Sapindaceae, and in Gnetum, successive cambia are produced in the

cortex, so that they are polystelic. Some have intra-xylary phloem or

bicollateral bundles.

A very narrow stem supplies a profuse canopy with water, and

conductive ability is maximised by having large diameter vessel elements, although this is hazardous because of the liability of cavitation, i.e. the water columns breaking. Conduction is maintained

by also having narrow diameter vessel elements and/or vasicentric

tracheids. The preponderance of parenchyma and the more even distribution of phloem through the stele that results from the irregular

cambium may also confer the ability of the xylem to recover from

cavitation. Photosynthates are distributed throughout the stem. The

parenchyma provides sites for starch storage, since lianes have no

other area where it might be stored, but in addition this source of

soluble sugars may be important in the recovery of cavitated vessel

elements. Sugars transferred into the vessels will increase osmotic



Figure 6.43. (a) Rattan palm

Calamus with grapnel hooked

rhachis; (b) Monstera.



(c)



Figure 6.44. Freycinetia

(Pandanaceae) in New Guinea.



289



290



THE LIVES OF PLANTS



(a)



(b)



Figure 6.45. Woody lianes have

a specialised xylem. (a) Crinkly

lianes have flexibility; (b) dimorphic

vessel elements, broad ones for

water conduction, narrow ones

for safety.



pressure thereby encouraging the flow of water back into them. The

parenchyma also provides relatively unspecialised cells, which may

allow regeneration of the vascular tissue through the formation of

successive cambia, or after wounding.

There is distinct stratification among bole climbers. Top layers are

distinctly photophytic, for example Freycinetia (Pandanaceae). Below

this are mixed groups of aroids (Araceae), Gesneriaceae and Ericaceae,

which are themselves stratified. Below this again are the sciophytic

ferns. Most are shade-loving woody or suffrutescent perennials and

don’t display many obvious adaptations apart from aerial roots, but a

few are succulent, for example Cactaceae and some Piperaceae. Many

have increased amounts of chlorophyll or special pigmentation (see

Section 6.3), and have varying degrees of dorsiventrality. Many start

life in the soil but later lose contact with it as they root to the support

tree. In areas with lots of sunlight (gaps, etc.) the climbers are often

scrambling herbs.



6.6.2 Stranglers

Primary hemi-epiphytes such as stranglers (Ficus, Schefflera, Fagraea,

etc.) start as holo-epiphytes in the crowns of young trees, and are

carried upwards with the replacement canopy. They may have crowns

larger than the host crown, which may show considerable loss of

photosynthate through crown competition. Stranglers can maintain

their large canopy because they send their roots, which are often

free-hanging, to the soil. The roots increase in number and girth and

eventually self-graft or anastomose so that they eventually encircle

the host tree and ‘strangle’ it (Figure 6.46). Most grow in clearings,

forest fringes and gaps.



6.6.3 Hyper-epiphytes and ultra-epiphytes

Hyper-epiphytes and ultra-epiphytes are photophytic epiphytes and

include some trash-basket ferns (Drynaria spp.: Polypodiaceae),

orchids, bromeliads and ant-plants. Most of them grow in the zone

occupied by hemi-parasites such as mistletoes. Their adaptations

reach an extreme in some tiny orchids which live at the tips of

twigs in the canopy or on canopy leaves themselves, as epiphylls.

There is a whole suite of adaptations to restrict water loss and allow

them to live in the hot, dry, uppermost parts of the canopy. The

adaptations here include reduction in surface area: volume ratio;

aerial photosynthetic velaminous roots; pseudo-bulbs; succulence;

stomatal sensitivity; loss of geotropism and polarisation (epiphyllous

orchids); holdfasts; tanks and trichomes; farina; incipient carnivory;

ant symbioses; and reversed myrmecotrophy (Piperaceae, Gesneriaceae, Orchidaceae), etc.



Figure 6.46. A mature

strangling Ficus that has completely

surrounded its supporting tree.



6.6.4 Adaptations of epiphytes

The tropical rainforest is heterogeneous in four dimensions. Epiphytes

live along primary flux routes and, by virtue of their location and

scavenging capacity, their strategy is to interrupt the nutrient/water



6.6 EPIPHYTES, HEMI-EPIPHYTES AND VINES



cycle used by soil rooted perennials. In this way they retrieve nutrients

lost from the above-ground parts (for example, through leaching and

leaf drop), and intercept water and atmospheric inputs, which would

otherwise be accessible to their host.

Their low productivity and substantial powers of nutrientaccretion increase their impact on biogeochemical cycling. They are

thus major participants in the movement of mineral nutrients within

tropical forests. Attempts to analyse the structure and function of

tropical forest ecosystems cannot be wholly successful until epiphytes are given due consideration. Apart from anchorage in trees,

there is no common factor of growth form, seed type, pollen vector, water/carbon balance, source of nutrient ions or resource procurement mode. Therefore, the life-form concept of ‘epiphyte’ must

include a greater diversity of more subtle variation. Species inhabiting the same area of forest and the same tree crown may differ

in their light and humidity requirements. Many orchids, including

closely related species, form assemblages on the same host, preferring similar bark qualities, humidity and exposure.

Open-crowned, slow-growing trees with absorbent stable bark

make the best hosts. Epiphytes are generally commoner where tree

canopies are humid for most of the year, for example in swamp

forests and other humid situations such as enclosed valleys. Moisture is probably the most important criterion of all. Temporal access

to moisture, avoiding drought injury, is the most immediate challenge, and year-round, high atmospheric humidity rather than high

total rainfall is most conducive. Epiphyte diversity is greatest in wet

mid-montane forests, peaking at 1000--2000 m. Diversity diminishes

at elevations above 2000 m and where there is increasing severity of

the dry season. Cool montane cloud forest supports the most luxuriant epiphytic growth, with density peaking at 2000--2500 m where

epiphytes make up to 30% of the foliar biomass. Often diversity is

low and comprises only bryophytes, orchids and ericoids. They will

be present in drier forest where dew or mist occurs but they are less

diverse and abundant in areas with poor soils owing to their extreme

vulnerability to disturbance. In such areas specialised epiphytes such

as ant-plants or carnivorous plants such as Nepenthes spp. are more

prevalent.

Light levels and hence the leaf area index of the host are critical and many epiphytes appear to be more tolerant of low nutrients

than heavy shade. The species of host tree is also important although,

unlike parasites, most epiphytes usually have a broad host preference. Bark texture, stability and wettability are the most important

physical determinants of seedling success. Genera with exfoliating

bark such as Eucalyptus, Syzigium (Myrtaceae) are generally useless for

epiphytes.

The susceptibility of the host to leaching and the nutritional quality of the canopy fluids is important. Different amounts of nitrogen

and phosphorus can be extracted from the same kind of bark depending on nutritional status of the tree. Nutrient-charged water passes



(a)



(b)



(c)



Figure 6.47. Epiphytic orchids:

(a) with negatively gravitropic

roots that trap leaf litter;

(b) leafless epiphytic orchid;

(c) velamen of an orchid root: the

cells outside the endodermis die

and form a sponge like layer.



291



292



THE LIVES OF PLANTS



through a forest with some regularity but its movements are rapid

and it leaves little residue. Stem flow and through-fall are usually

dilute. Atmospheric inputs may be very uneven and almost every

canopy may be characterised by frequent or prolonged intervals of

extreme deprivation. Leachates may be important in breaking seed

dormancy. Some orchids may be confined to trees that can support

mycorrhizal fungi.

Perhaps the best way to begin analyses of epiphytes is to consider

the forest to be analogous to the oceans. At the top there is a photosynthetic layer (euphotic zone) where most of the production occurs.

This is the canopy. Below the canopy, in the shade zone (oligophotic

zone), photosynthesis decreases along with diversity until we reach

the bottom layers where there are only specialist scavengers that

feed on the detritus falling down. The fluctuating boundary between

the two is called the ‘morphological inversion surface’ or MIS. Above

this there are air-movement, moisture and temperature fluctuations;

below this there are stillness and uniformity. The MIS also effectively

defines the holding level for understorey and juvenile trees.

Epiphytes may also be classified by their means of obtaining

water, for example many that are ‘continuously supplied’ (CS) occur

within the shade zone and are mostly ferns or aroids. Some have

CAM (not aroids) while in others the velamen of the roots is not so

developed. Many trap organic detritus by means of ‘trash baskets’,

or have mycorrhizal associations and a prolonged life cycle. In contrast, other epiphytes which are ‘pulse-supplied’ (PS) are found in the

sun zone. The PS epiphytes usually have CAM, a reduction in surface

area:volume ratio, and a telescoping of parts (e.g. orchids, bromeliads

and cacti). Many possess velamen on their roots, absorbing trichomes,

etc., and absorb moisture from the atmosphere. The PS epiphytes are

often ant-plants (myrmecotrophs) while many also have mycorrhizal

associations.

Carbon fixation by means of crassulacean acid metabolism (CAM)

is widespread in canopy epiphytes although this phenomenon is most

frequently found in plants of arid climates. Canopy epiphytes usually

also possess succulence, low surface area:volume ratios and low transpiration rates. They usually lack a well-developed palisade layer and

most of the photosynthetic cells are spongy mesophyll. CAM is present

in 26 flowering plant families. CAM plants, like all plants, must obtain

water and CO2 but if they fully open their stomata during daylight

they transpire too much water. They therefore open their stomata at

night only and fix CO2 into malic acid by the enzyme PEP carboxylase.

Malic acid is stored in the vacuole. Starch is degraded by glycolysis

to PEP. HCO−

3 reacts with PEP to form oxaloacetate, which is then

reduced to malic acid by the enzyme malate dehydrogenase. Malic

acid disappears during the day. It diffuses out of the vacuole and is

decarboxylated with the release of CO2 . This CO2 is then utilised by

the plant in daylight via the Calvin cycle of photosynthesis. Often

CAM plants are facultative C3 and can switch to this mode in cloudy

weather or following rainstorms.



6.6 EPIPHYTES, HEMI-EPIPHYTES AND VINES



(a)



(b)



Forest canopies are unusually hostile. There are many constraints

or ‘stresses’ on epiphytes, particularly the true epiphytes of the forest

canopy. Epiphytes ameliorate stress by several strategies. Tissue concentrations of nitrogen, phosphorus and potassium may be unusually low. In the tropical forest environment, because of the level of

energy input, there is intense competition for living space. Selection

pressure has led to the evolution of specialisation towards many available niches so that the pattern of epiphyte diversity within the canopy

reflects microsite heterogeneity. Because of the intense selection pressure, there is rapid growth and rapid turnover of individuals. Disturbances such as bark exfoliation, tree movement, falling branches and

tree death provide opportunities for individuals to establish. Most epiphytes are herbaceous perennials. Woodiness only occurs in regions

of abundant moisture. They can establish in the canopy of forests

without soil or an extensive root system, and thus have an economy

which is very cost effective. In the majority the root system functions as a ‘holdfast’, analagous to that of seaweeds; for example, in

bromeliads the roots are very reduced. Because the canopy has a very

fluctuating water supply (mineral nutrients and moisture are intermittent rather than continuously available in all but the wettest climates) specialised means of nutrient and moisture procurement must

be available. The water balance is assured by considerable mechanistic diversity, for example drought-sensitive roots of ferns, velamen of

roots in orchids and aroids, ant domatia, and tanks and trichomes

of bromeliads (Figures 6.48--49). Many have structures for impounding nutrients, water and other debris, for example tanks and trashbaskets. Carnivory is rare but myrmecotrophy is common. Most epiphytes have mycorrhizal associations and for some, such as orchids,

this is vital for seedling establishment.

Epiphytes experience daily and seasonal drought. Subsequently

they have xerophytic features such as economical water use, succulence and extensive water storage capacities, pseudo-bulbs, unusual

osmotic qualities and stomatal sensitiveness. Deciduousness occurs

where there is seasonal, but not too severe, drought. Some alternate



Figure 6.48. Myrmecophytes

provide hollow organs (domatia)

as a home for ants: (a) Dischidia

(Asclepiadaceae) has hollow

leaves; (b) Myrmecodia (Rubiaceae)

has a chambered tuberous stem.



293



294



THE LIVES OF PLANTS



(a)



(b)



(c)



Figure 6.49. Tillandsia

(Bromeliaceae), the genus of

air-plants, has two main forms:

(a) upright rosettes and

(b) pendulous. Bromeliad leaves

absorb water through scale-like

trichomes, as shown in (c).



between ‘wet active’ and ‘dry inactive’ and a few are ‘resurrection

plants’ with an ability to ‘rebound’ rapidly, although poikilohydry in

epiphytes is found only in areas where moisture is abundant. Most

epiphytes are homoiohydric (avoid dessication).

Because of the patchiness of suitable microsites (as a result of

both the dispersion of host trees within the forest and the separation

of their branches), there is often a scarcity of conspecifics and so

aerial dispersal is the most frequent mode of spread (i.e. zoophilous

pollination and wind-dispersed seeds). Usually the seeds are tiny and

lack appendages (but see Aeschynanthus spp.: Gesnericeae).

Monocots (about 25% of all flowering plants) have five times the

number of epiphytic species (especially in the families Orchidaceae,

Bromeliaceae and Araceae) in comparison with the rest of the flowering plants, and twice the number with fern epiphytes. However,

there is no common monocot adaptive theme, although they possess

many features that appear to confer advantages as epiphytes. Most

species are iteroparous, with a rhizomatous, sympodial habit, and

serial perennation with determinate offshoots. Each ‘phyton’ is relatively autonomous with leaf, associated adventitious roots, buds and

subtending stem segment. The meristems receive fixed carbon, mainly

from nearby leaves (i.e. there is reduced translocation over the whole

plant). The reticulate stele gives greater capacity for functional integration and extensive vegetative renewal with a minimum of tissue

space. The meristematic regions remain as nutrient sinks whereas in

times of stress dicots will self-prune by aborting leaves, branches and

flowers.

Orchids have an affinity for acidic, humic, infertile soils (i.e. with

reduced nitrogen), and utilise NH4 rather than oxidised nitrogen,

and this may have predisposed them to epiphytism since mycorrhizae

mobilise nitrogen and phosphorus from sterile soil. Epiphytes, especially impounding ones, increase canopy humidity, which makes it

more favourable for nitrogen-fixation. Many orchids have extensive

nitrogen-fixing epiphyllae (which in turn have a symbiotic relationship with Nostoc, a blue-green alga). Epiphytes may be more important

to the forest fauna than their numbers and biomass would suggest.

Epiphytosis causes treefall and an increase in the physiognomic diversity of the forest, especially montane forest.

Bryophytes are most important in the water balance of tropical

montane forests and the dynamics of their vascular plant associations (Figure 6.50). By intercepting more than 25% of precipitation

they control and impede drainage, and can thus influence climate

on a local scale. Many species, especially epiphylls in tropical forests,

are associated with blue-green algae and fix atmospheric nitrogen.

This is the main input for nitrogen in tropical rainforests. As with

other epiphytes, bryophytes can profoundly alter the physiognomy

of forests due to ‘epiphyte load’ and consequently affect biodiversity.

When wet, mosses can be up to four times their live weight. The epiphyte biomass and interceptive capacity are proportional to annual

rainfall where the monthly average is more than 100 mm, whereas



6.7 GRASSLANDS AND SAVANNAS



(a)



(b)



tropical macro-vegetation biomass (and diversity) increases only in

areas of up to about 150 mm before tailing off.



6.7 Grasslands and savannas

The great grasslands occupy a climatic zone between forest and

desert, in the North American prairies, the Asian steppes, the African

savannas, and the South American pampas, but much of their

recent distribution has been assisted by humans. Grasses (Poaceae or

Gramineae) are one of the most familiar groups of flowering plants,

yet their identification and biology remain problematical or mysterious. For much of the year they are seen in a non-flowering state

and all appear to look alike, but they are of the utmost importance

as a food source for humans and animals, and dominate much of

the world’s vegetation. There are about 651 genera with about 10 000

species of grasses, cosmopolitan in distribution, and forming one of

the largest and most successful of flowering plant families. They occur

in every kind of habitat, from mountains to the seashore, in forests

and savanna, and in deserts, rivers and marshes, and are estimated

to be the principal component of about 20% of the Earth’s vegetation

cover.



6.7.1 Grasses

The widespread occurrence and predominance of grasses in the various types of world vegetation results from:



r adaptation to a range of soil types

r adaptation to a diversity of climates and a broad ecological amplitude

r ability to compete successfully with other plant types

r ability to survive high levels of predation



Figure 6.50. Epiphytic

bryophytes: (a) coating a branch;

(b) epiphyllous leafy liverworts and

mosses. The leaves of many plants

in the tropical forest have drip-tips

to gather up moisture and shed it

from the leaf to prevent the

colonisation of bryophytes.



295



296



THE LIVES OF PLANTS



This is brought about by the following adaptive features:



r unique morphology

r specialised physiology, especially connected with their modes of

photosynthesis

r various strategies for vegetative reproduction

r specialised flowering mechanisms

r a diversity of breeding systems



Figure 6.51. One of the more

familiar grasses, Cortaderia selloana

(pampas grass). Grasses are mostly

small tufted plants, but

occasionally may, like pampas

grass, form large robust tussocks

with a stout central axis composed

of tightly packed dead leaf bases.



All green plants utilise ribulose diphosphate for the initial capture of carbon dioxide from the atmosphere. Many grasses, however, possess an additional chemical pathway that utilises the threecarbon compound phosphoenol pyruvate (PEP). This is known as the

Hatch/Slack or C4 pathway. This pathway is possible because of its spatial separation in the leaf-blade anatomy (Kranz anatomy) from the

Calvin cycle. In grasses without this extra pathway (i.e. those with

only a C3 pathway) the Calvin cycle alone, operates in the diffusely

arranged cells of the chlorenchyma. The Hatch/Slack pathway operates in the radially arranged cells of the chlorenchyma and releases

carbon dioxide into the outer bundle sheaths, where it is incorporated into the Calvin cycle. Plants with the Hatch/Slack pathway have

a much higher rate of carbon dioxide uptake and higher growth rates

than those in which it is absent. The C4 pathway reduces photorespiration. Because it operates most efficiently in high temperatures

and high light intensities it occurs widely in tropical grasses such

as Andropogon, Panicum and Eragrostis. Temperate grasses such as Poa,

Bromus and Festuca gain no advantage from this extra pathway so they

retain the Calvin cycle alone.

Most grasses have stems that are hollow, easily bent and yield to

the wind. Stems do not increase their size by growing thicker at the

sides and longer at the top. The thickness is fixed from the beginning,

and the stem increases by the meristematic region (intercalary) at the

base just above each node. The intercalary meristem is protected by

sheathing leaf bases. Grasses do not develop taproots, and the adventitious roots are slender, relatively short, with infrequent branching.

They originate in large numbers from the base of the plant (e.g. Festuca). Alternatively, the roots may be few and very long, extending

deeply in the soil or remaining near the surface (e.g. Aristida pungens).

Grasses with such roots are able to utilise all available surface water.

Many grasses, especially perennials, have horizontal underground

rhizomes (e.g. Agropyron) or overground stolons (e.g. Cynodon), and

produce new plants at intervals along their length. Rhizomes are

often tough and serve to anchor the plant in the soil, as well as

to colonise new ground (e.g. marram grass, Ammophila arenaria; rice

grass, Spartina townsendii) (see Figure 6.52).

Grasses do not develop a permanent main stem with side branches

(except bamboos). They grow from a basal rootstock, and the leaves,

which are simple, often die back at the end of each season in perennial species. They are therefore mostly small tufted plants, but occasionally may be large, forming robust tussocks with a stout central



6.7 GRASSLANDS AND SAVANNAS



Figure 6.52. Marram grass

(Ammophila arenaria) builds dune

systems by trapping sand. It can

grow rapidly, thus preventing burial

by sand.



axis composed of tightly packed dead leaf bases (e.g. pampas grass,

Cortaderia selloana). In tropical regions of seasonal or low rainfall there

is often a danger of fire started by lightning or by humans. Grasses

are able to survive because new growth is initiated at the rootstock

below ground level. Tropical savanna grasses such as Andropogon and

Saccharum, and the giant reed Arundo donax, may have stems exceeding 3 m. The leaves are borne in two ranks at intervals along the stem.

They originate from nodes and comprise a basal sheath clasping the

stem, and a blade which is usually narrow and flat, folded or rolled.

Just above each node is the intercalary meristem. Differential growth

of this meristem allows grasses to bend upright after trampling. Leaf

blades also grow by a basal meristem situated at the junction with

the sheath, permitting the blade to grow despite the removal of distal

parts by grazing. At its upper end, the sheath passes into a parallelveined blade. The blade is typically long and narrow but may be broad

in shade-loving species. At the junction of sheath and blade is a short

membranous rim, called a ligule, that may prevent rain entering the

sheath.

Grasses only need a piece of stem bearing leaves with node and

internode in order to reproduce vegetatively. Adventitious roots readily grow from a node, while new shoots grow from buds in the axils

of the leaves. This is the way that grass cover and sugarcane is established in horticulture and agriculture, respectively (i.e. by ‘seeding’).

In many tufted or tussock species vegetative growth is by tillering.

New shoots grow out from the leaf axils at the base of the plant,

to form a rosette or tussock (Figure 6.54). Other grasses may spread

by stolons, stems that grow from the base of the mother plant and

spread horizontally over the surface of the soil, producing a new

plant at each node. This method is common in tropical grasses such

as Bermuda grass (Cynodon dactylon). In temperate grasses a similar

effect is achieved by rhizomes (under the soil surface). It has been estimated that a single plant of Festuca rubra, which spreads by rhizomes,

may be some 250 m (>800 ft) in diameter and up to 400 years old,



Figure 6.53. Bamboo is rich in

hard fibres, giving the the hollow

stems great strength.



Figure 6.54. Tussock grassland

in New Zealand.



297



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

6 Epiphytes, hemi-epiphytes and vines

Tải bản đầy đủ ngay(0 tr)

×