-

EXPERIMENT 26 STUDY OF
XEROPHYTES,
MESOPMYTES AND
I
I HYDROPHYTES
I

Structure

26.1 I~ilroduction
Objecrives
26.2 Categories 01PlanLs based on Habitat
26.3 Hydrophytcs
Morophological Features
Analomical Fcatures
26.4. Mesophytes
26.5 Xerophytes
Morphological Features
Anatomical Fcaturcs
26.6 SAQs

26.1 INTRODUCTION

Tllis distribulion of plants occur Jsom exlrcmcs oJ north and soul11 and east and
wesl around Ihe world. Thus certain groups or plants arc adaptcd to unusual or
extreme climalic conditions, Tllcqc plants exhibit modificalions in heir anatomy
and morpllology hat are relaled LO spccialised Junctions LO a greatcr degree.
Tlle ecological groups 01plants havc much broadcr group. But Lhcy havc to be
classificd on some basis. Watcr is onc hem. Warming (1909) classificd plant
communilies on tlle basis of Lheir dependence upon and rclalion to watcr. Water
bcing most important occupics forenlosl position in distribution of vegetation
and its structure. Warnling primarily recogniscd ll~recmajor groups of plants:
i) Hydrophy~es,ii) Mcsophytes and iii) Xcrophytcs. IIydrophytcs arc thosc
plants which l?ve partly or 1uUy submerged in water at lcast for somc wccks.
Mcsophytes grow on land wilh a moderate supply of soil moisture. Xcrophytcs
are zhase plants lhat live in dry or arid land.

Aftcr doing this cxcrcisc you will be able to:

a describe and diffcrcnciatc bctwccn hydrophytcs, nicsophytcs and xcrophy~cs,

a list varidus adaptations in hydrophyles, mcsophytes and xerophytes wilh
cxamples and illustrrllions.

26.2 CATEGORIES OF PLANTS BASED ON HABITAT

In Lhis cxpcrirncnt you will bc providcd spccimcns and slidcs. From the givcn
description you havc to classi1y Lhc plants into differcnl catagorics (dcscribcd
bclow).
Laboratory Course-I

According lo thc way In whicl~lhc hgdroph~[thd ~ \ ~ l oin[ >M A I C I . [hey arc:

subdivided into lhc following five ca[agolius:

i) ' Free-Floating IIydrophytes

Thcsc typcs of pla~ilsrcmain in conlacl will1 walcr and air, but no1 soil. Thcy
float frccly on thc watcr surfacc. Lcavcs i n somc arc vcry minulc wliilc in
othcrs quitc largc. Somc of thc frcc flpating hydrophylcs arc Trupa hi.spi:no,su,
Azolla, Eiclzhornia crassipes, Salvinia, Wolffia, Pistia, Lcmna (Fig. 26.1).

Fronds

---- -- -- ---- ----- -. --------- -- -- -- -.

..
Water :.T ==r-z--z--: x -
:
WOl/~~~l
Frond
(Plant body)

Root . -
----.- -- Pockcl - -
Eichhornia
Lrmna

Frond

Floating
Leaves
~~ ~

Leaves
A"rC?/7

Water ~
Stem ::.

Azolla Pisria

Fig. 26.1 : Free floating hydrophytes

ii) Rooted Hydrophytes wit11 Floating Leaves

The roots of these type of hydrophytes are fixed in mud, but leaves have long
petioles which keep thcm floating on the water surhce. Except leaves, the rest
of the plant body remains in water. Some examplcs are Nelumbo nucvib-a,
Nymphaea stellate (water lily), Trapa, Marsilea (Fig. 26.2).
 Study of Xeropnyres,
Mesophytes and Hydrophytes

Flower

.
Fig. 26.2 : Rooted hydrophytes with floating leaves

iii) Submerged Floating Hydrophytes

mese types of the plants are only in contact with water, being completely
submerged and not rooted in the mud. Their stems arelong and leaves
generally small. Some examples are Ceratophyllum, ULn'culmiu. (Fig. 26.3)

-------- -U- - __ _ _ _ _
Ceratophyllum

Fig. 26.3 : Submerged floating hydrophyte

 hydrophytes
i v ) Roolcd aull~~~cr.gccl

This type of hydrophytes rcrnain completely submerged in water and'rooted in

soil. In somc plants the slcm is long, bearing small leaves at the nodes. In
some planls stcm is tubcrous (corn-like) with long leaves, which are narrow,
ribbon-shaped. Common example is Vallisneria, Chara, Hydrilla, Potamogeton
(Fig. 26.4)

Flower

Water

Submerged
Leave> .-

Hydrilla

Flg. 26.4 : Rooted submerged hydrophytes

v) Rooted emergent hydrophytes

These types of plants grow in shallow water. These are such hydrophilous
forms which require excess of water, but their shoots (assimilatory organs) are
partly or completely exposed to air. The root system is completely under water,
fixed in soil. In some plants, shoots are partly in water and partly emerging i.e.
exposed to air. Whereas in some the shoots are completely exposed to air.
Some common examples are Sagittaria, Ranunculus, Cyperus (Fig. 26.5).

Now let us see the ecological adaptations in hydrophytes. Though most of the
features in hydrophytes are similar but they may differ from each other in some :
aspect. We will discuss all those features which enable them to become
hydrophytes.

26.3.1 Morphological Features

1. Roots

There is plenty of water in the surroundings of hydrophytes; thus the root

becomes of secondary importance hence less developed and insignificant in .
most of hydrophytes.
 Study of Xerophytes,
Mesophytes and Hydrophytes

1 Emergent ,

Fig. 26.5 : Rooted emergent hydrophytes

i) Roots may be completely absent or poorly developed as in Wolflia,

Salvinia. However, in emergent forms, which grow in mud, mots arc well
developed with distinct root caps.

ii) Root hairs are absent or poorly developed.

iii) Root caps are usually absent, in some cases are replaced by root pockcts as
in Eichhornia.

iv) Roots when present are generally fibrous, adventitious, reduccd in length,
and unbranched or poorly branched.

2. Stems
i) In emerged forms the stem is long, slender, spongy and flexible. In
 free-floating forms it may be slender, floating horizontally on water surfacc
or thick. short, stoloniferous and spongy. In forms which are rooted with
floating leaves it is rhizome.

ii) Vegetative propagation is by runners, stolons, stem and root tubers,

dormant apices, offsets etc. are the common merhod of reproduction. Most
or them are perennials.

3. Leaves

'i) In submcrged forms, leaves are thin, and are eithcr long and ribbon-shaped
as in Vallisneria or long and linear or finely dissected as in Potamogeton.
Floating leaves are largc, flat and entire with their upper surfaces coated
with wax as in Nymphaea; their petioles lung, flexible, and often covered
with mucilage. In some cases petioles become swollen and spongy as in
Eichhornia.

ii) Emergent forms show heterophylly with submerged, floating and aerial
- leaves as in Ranunculus.

iii) Submerged leaves are generally translucent.

4. Flowers and seeds

In submerged forms they are less common. Where flowers develop, seeds are
rarely formed.

26.3.2 Anatomical features

Now, wc will study aboul: the anatomical features of roots of the hydrophytes.

1. Roots

i) Cuticle is either completely absent or if prcsent it is thin and poorly

developed.

ii) ~piderniisis usually single layered and made up of thin wallcd

parenchymatous cells.

iii) Thc cortex is well developed, thin walled and parenchymatous, major
portion of which is occupicd by well developed prominent air cavities-tlie
'arenchyma' which offers resistance to binding slress, increases buoyancy
and allows a rapid gaseous exchange.

iv) In the given figure of Potamogeton you can see that va~cular~tissues are
poorly dcvelopcd and leasl dirrcrcntiated in subrnergcd form. The xylem
vessels are lcss common and tfachcids are present. In floating types
vascular tissue are less dcvclopcd while in emergent fonns they are much
distinct and wcll devclopcd (Fig. 26.6).

v) Thc mechanical tissues are absent except in son?e %ergcnt forms where
pith is made up of sclcrcnchymatous cells.
 O L U V J Ul A G 1 U p l l J . Y

Mesophytes and Wydrophytes

Air Chambers

, Fig. 26.6 : TS. root of Potarnogetorr pectinnlus (submerged hydrophyte). Note the absence of
root hairs and cuticle; undifferentiated broad cortex with alr chambers; vascular tissues
I
poorly developed, represented rnalnly by phloem; lack of mechanicnl tlssues.

2. Stem
I
t In Lhe given iigurc of T.S. of stcm of Hydrilkz you can sce that Fig. 26.7)

Air Chambers

Fig. 26.7 : TS. stem of H-vdrilla (submerged hydrophyte). Note, the absence of cuticle; thin-
walled epidermis; ~ndlfferentiatedcortex wlth alr chambers; abundance of thin-wnlled
elements; absence of tire( hanicsl tissues; reduced vascular elements, composed chiefly oP
phloem, xylenl being represented only by a cavlty In the centre.
Laboratory Course-I -" i) Cuticle is developed or thin or endrcly absent.

ii) Epidermis is single layered and made up of thin-walled parenchymatous

cells while in emergent form cuticle as well as epidermis is generally wcll
develop such as Typha.

iii) In'floating and emergent form you can see hypodermis may be present as
thin-walled parenchyma or collenchyma. It is completely absent in
submerged form.

iv) Cortex is well developed. One of the important feature of the cortex is that
it is thin walled and parenchymatous extensively transversed by air cavities.
The cortical cells generally possess'chloroplasts and are photosynthetic.

v) Endodermis is distinct, especially in rhizomes and similar organs.

vi) Generally, vascular bundle have no bundle sheaths. Vascular bundlcs arc 4

thin walled. But in emergent forms vascular elements are comparatively

well differentiated and developed.

vii) Mechanical tissues are generally not present.

3. Leaves
You can see form the slides provided to you of different T.S. (of various plant
leaves.)

You can see that internal structure of leaves show variation but some of the
anatomical feature is common to the most of Lhc Icavcs. From the given
transverse section you can see that;

i) Cuticle is usually absent in submerged forms such as Potamogeton but in

floating forms it is poorly developed confined to upper side and is very
thin. In emergent form also cuticle is thin (Fig. 26.8).

Mesophyll Cells
,Upper Epidermis
h

Lower Epidermis

Fig. 26.8 : T.S. leaf (only laternl wing portion shown) of Potamogeton pusillus (submerged).
Note, the absence of cutkle and stomata undifferentiated single-layered mesophyli between
two epidermnl layers.

ii) Epidermis is single-layered, made up of thin-walled cells with abundance of

chloroplasts.
!

iii) Stomata are completely absent in submerged leaves, as in Potamogeton. In

floating form stomata are confined only to the upper surface of leaf,
whereas in emergent forms they are generally found on both of the surfaces
of leaves.

iv) Mesophyll is undifferentiated in submerged leaves, and generally it is single

layered. In floating leaves, mesophyll is differentiated (a) palisade and
(b) spongy parenchyma with well-developed air cavities as in Nymphaea.
 In emergent leaves also, mesophyll is well differentiated with air cavities study of Xerophytes,
(Fig. 26.9). Mesopbytes and Hydrophytes

Stoma Cuticle Upper Epidermis

Fig. 26.9 : T.S. leaf of Nymphuea (floating-leaves). Note, the thin cuticle; stomata belng
confined only to the upper surface; thin-walled epidermal cells; abundance of air chambers
in spongy parenchyma; absence of mechanical tissues (only sclereids present): reduced
vascular elements represented mainly by phloem, xylem being much reduced

v) Vascular tissues are very much reduced and sometimes difficult to be

differentiated into xylem and phloem. Whenever differentiated into xylem,
elements are thin walled and phloem being well developed as in
Nymphaea.However, in aerial leaves', vascular elements are comparatively
well differentiated with vessels in xylem.
vi) Mechanical tissues are absent.
vii) The petioles, wherever found is well developed and also possess internally
the various tissues characteristic of a typical hidrophyle i.e. abundance of
aerenchyma, thin walled cells, lack of differentiation in vascular tissues and
absence of any lignified mechanical tissues as in Nymphaea (Fig, 26.10).

~1g.'26h0 : T.S. petiole of Nymphaea (flontlng-leaved). Note, the absence of cuticle; thin-
walled epidermal cells; reduced mechanical tissues represented only by a few layers OF
collenchymatous hypodermis; abundance of aerenchyma; vascular elements with
abundance of phloem, xylem being represented by lacunae.
Laboratory Course-I
26.4 MESOPNYTES
hlcsophytcs are plants that normally grow in habitat whcre water is ncithcr
scarce nor abundant. In such habitats Lhc porc space in soil is occupicd almost
equally by watcr and soil atmosphcrc. This condition of water and air is vcry
suitable for plant growth and hcncc in mesophytic condition h c growth of
forcsts and crop plants is bcst. In mcsophytes, no adaptation is necessary unless
Ihe habitat is spccialised in some othcr way. Mcsophytcs are very extensive on
thc surfacc of land arid most crops likc wheal, ~naizebarlcy, pcas, gram or
sugarcanc or species in grassland, meadows tropical and tempcratc forcsts are
all mcsophytcs.

Morpho-ana tomical features

i) Root systcm is wcll dcvclopcd. Roots arc gcncrally branchcd with root
caps and root hairs.

ii) Stcms are gcncrally acrial, solid and frccly branched.

iii) Leaves arc gencrrllly large, broad, thin and varicd in shapcs mostly
oricntcd horizontally, grecn, without hair of waxy coatings.

iv) Cuticlc in all acrial parts ~noderatclydevcloped.

v) Epidcrmis wcll dcvclopcd, without any hair or waxy coatings and cells
without cldorplasts.

vi) You can obscrvc that stomata are gcncra1.l~present on'both surfaces of
leavcs. Guard cell show frcquent movcments.

vii) Mcsophyll in lcavcs is wcll diffcrentiatcd into palisade and spongy

paranchyma with many intercellular spaces.

viii) Vascular and mechanical tissues both arc well devclopcd and
diffcrcntiatcd.

ix) During noon hours Lhcre may be tcmporary wilting.

26.5 XEROPHYTES

There have bccn many intcrprctations of the term xerophytcs, Sometimes they
are loosely dcfined as plants of dry habitats. A truly ecological definition
approaching as near as possiblc a quantitative basis is that xerophytcs are
plants which grow on substrata which usually becomc greatly dcpletcd of
gravitational ground watcr to a dcpth of least 20-25 cm during the course of a
normal season. Somc examplcs of xerophytes are Aloe, Euphorbia, Opuntia,
Aguve Bryophyllum, Yucca, Tradescantia (Fig 26.1 1).

Thus, in arid zoncs, all plants not confined to thc margins of streams or lakes
have been considcrcd as xcrophytcs, whcrcas in rcgions of heavy rainfall thc
class would be rcprcscntcd only by shallow-rootcd plants of sandy soils, by
'
plants of dry ridgctops, and by algnc, mosses and lichcns which grow on trees
barks or rock surfaccs ctc.

l'hc truc naturc or xcrophytcs is not clearly undcrstood. For example, it is

 difficult to decide whether a xerophyte is really xerophilous and occurs only in StudS.'fif'Xerophytes.
dry habitats and descrts or it is merely drought-resistant. Mesophytes and Hydrophytes

Aloe Euphorbia

Fig. 26.11 : Succulent xerophytes

26.5.1 Morphological Features

1 Roots
In contrast with hydrophytes which develop in conditions with plenty of watcr,
Xerophytes develop under water deficient conditions. The main purpose of roots
is to secure water, which is present in less amount and in deep layers of soil.
The root system is the most important organ for the survival of plant and thus
is very well developed. The roots have following characteristics.

i) It is very well developed and in some cases is several times longer than
shoot. Roots are long, tap roots with extensive branching ~preadover wide
areas.
I

ii) Root hair and root caps are very well developed.

2. Stems
i) Mostly the growth of stem is stunted, woody, dry, hard, ridged and covered
with thick bark.

ii)- In some as Saccharurn stem becomes underground, whereas in opuntia

(Fig. 26.11) if becomes fleshy, green, leaf-life (phylloclade) covered with
spines. In Euphorbia also (Fig. 26.11) it becomes fleshy and grecn.

iii) On stems and leaves, there are generally hairs and/or waxy coatings.

3. Leaves
i) e a v e s are very much reduced, small, scale-like appearing only for a brief
period, sometimes modified into spines. Lamina may be long, narrow or
needle-like as Pinus or divided into many leaflets as in ~ c a i a(Fig. 26.12.).
Laboratory Course-I ii) Fofiage lcaves when present, may become thick, fleshy and succulent or,
tiugh and leathery in texture.

iii) Leaf surface is mostly shiny and glazed to reflect light and heat.

iv) In some species leaves become folded and rolled in such a manner that the
sunken stomata become hidden and thus rate of transpiration is minirnised.

Acacia nelatica
- -

Fig. 26.12 : Non-succubnt perennhls

26.5.2 Anatomical featurk

1. Roots
i) Root hairs and root caps are well developed. In Opuntia root hairs develop
even at the root tips.

ii) In Asparagus roots may become fleshy to store water.

2. Stems
i) In succulent fleshy xerophytes, such as Casuarinu, following cheif
characteristics are present (Fig. 26.13).
a) Cuticle is very thick,

b) Epidermis is well developed, with heavily thickened cell walls,

c) Hypodermis is several-layered and sclerenchyrnatous.

d) Stomata an: of sunken type

e) Vascular tissues are very well develop, differentiated, heavily lignified.

vascular bundles have well developed several layered bundle sheath.

f) Mechanical tissues are very well developed

i) Bark is very well developed
ii) Oil and resin are often present
 Study of Xerophytes,
Mesopbytes and Hydrophytel

Fig. 26.13 : T.S. stem (a part) of Casuurina. Note, the thick cuticle; sunken stomata conflned
only to grooves; presence of hairs in grooves; sclerenchyrnatous hypodermls; green palisade
region of subhypodermal cortex; well-developed vascular tlssues and mechanical tissues

3. Leaves

3
i) In succulent leaves of malaco~hyllousxerophytes, such *is Peperomia,
epidermal cells of leaves serve as water-storage organ$ (Fig. 26.14).

Fig. 26.14 : T.S. leaf (lateral wing portlon only) of Peperomia showlng epidermal water
storage tissue. Note epidermlns is many layered, inner layers of which have large thln-walled
cells acting as water storage.
Laboratory Course-I Similarly, succulent leaves of Aloe have prominent water-storage regions in
their mesophyll. The cuticle is thick and outer walls of epidermal cells are
heavily deposited with cutin and cellulose.

ii) non succulent xerophytes, such as Nerium (Fig. 26.15) and Pinus have
following characteristic features:

Fig. 26.15 : T.S. leaf of Nerlum (mn-succulent perennial). Note, thlck cutlcle on both sldes;
multiserlate epldermls; stomata of sunken type, moreover sltuated lnslde the stomata1 plb,
conflned only to lower epldermls; well dlfferentivted mesophyll with pallsade on both the
sldes though abundant on the adaxial one; vascular tissues welldifferentlated.

a) The cuticle is heavy and well developed.

b) Epidermis is several layered in Nerium and in Pinus hypodermis is

several layer.

c) Mesophyll is very well differentiated into palisade and spongy

parenchyma.

d) Stomata are of sunken type confined to lower epidermis, in some

xerophyte, for example in Nerium (Fig. 26.15) stomata are situated in
pits lined with hairs.

e) Vascular tissues are very well developed, differentiated into xylem

with lignified elements and phloem.

) Mechanical tissue are very well developed, including several kinds of

sclereids.

In this exercise some example of hydrophytes, mesophyte and xerophyte have

been described. Various morphological and anatomical features are described irl
detail. Now after studying the given description you will be asked to classify
unknown specimens.
 Study of Xerophytes,
26.6 SELF-ASSESSMENT QUESTIONS 1 Mesophytes and llydrophytes

I) A hydrophytic plant is given to you. Examhc the leaves of lhe plant. How
would this condition of fincly dissected leavcs be advantageous to the
plmlt?

2) You are givcn thc prepared slide, examine the slide of cross section of thc
leaf. Draw a diagram and locate thc large air spaces. List several ways in
which these air spaces may bc of bcncfit in a hydric habitat.

3) Remove thc hydrophytic plsrils from the water, sce how flaccid i t bccornes.
Why is it so and why do submerged hydropllytes not need largc amount of
supporting tissues?
4) Examine tlic prcparcd slides oS Hydrilla stcn.~ant1 Potu/?zogetor. l ~ n \ ~ c s .
Locatc thc vascular lissucs and notc the abscncc 01 xylem. \Yhal is thc
proccss by which ulatcr cnters nqu~ticplants?

5) Examinc thc cpidcmiis and cuticlc in cross scclion and transvcssc scclion OL'
slcm and lcavcs or hydropliylic plants. Would you cxpcct lo fincl guard
cclls in submcrgcd hydrophytcs? If not, why?

6) On h a t i n g liydrophylcs whcrc will you scc stonlala and wily?

 Study of Xerophytes,
................................................................................................ Mesophytes and Hydrophytes

7) List out various morpholgical and anatomical modifications in the

hydrophytic plants you have studied.

............................................................................
....................

MESOPIIYTES

1) Some plants are given to you. Study the morphology of whole plant and
take T.S. of leaves and stem. Study various structure. Is some adaptation
are seen, if not why?

You have to study:

I a) Root system
L

I
I
b) Leaves, morphology and.transverse scctions

, i) Structure of epidermis

ii)' Ppscnce of stomata

I
iii) Differentiation of mcsophyll

' 3 C) T.S. of St-

,
I
I
i) vascular tissues
I'

ii) mechanical tissues

Xerophytic Adaptations
Xerophytes have evolved many adaptations to pre~Qntdesiccation of the plant.
You are pkvided with some slides of xerophytic plant. You should study
carefully and try to answer the following questions:

1) Examine thc slides of cross section of Pine lcaf and locate the stomata.
Draw a full labelled diagram. How does this position of stomata reduce the
rate of transpiration.
2) The prcpared slidc of xcrophytic plant show waxy material called culin. In
addition cpidcmal cells are lignified. Explain how it cut down loss of
water from plant.

3) In some xerophytic plants leaves may be fleshy, in others, the stcm is

fleshy. In the given plant, note the absence or greatly rcduccd numbcr, of
lcaves on the succulent stern. What is the primary photosynthetic organ in
these plants.
 '
4) You are provided with a succulent leaf, cut a thin cross section and mount study of Xerophytes,
it in a drop of water on a slide. Examine the C.S.under the microscope. Mesophytes and Hydrophytes
What are the xerophytic characteristics you observe microscopicalIy. Make
a labelled diagram and write down the xeropliytic adaption carefully.

5) You are provided with the six plant species. Write down the characteristic
in the provided space. Nler writing the characteristic propertics dccide the
habitat of the plant.
S pccics Air Supporting Protec~ive Lcar modification Vascular Water storage H a b i ~ r land
Chambcrs ~issucs lissucs (Stomala, motor lissucs ~issucs rcxson for
cclls, olher) choicc
xcriclhydric