7

Dispersal of Seeds

:

Project: Study of dispersal of seeds by aarious agencies

tntroduction

The flowers of angiosperms form fruits from ovary and seeds from ovules after pollination and

fertilization. As seeds'contain the embryo, or the future plant, their dispersal is important for

maintaining the continuity of species through generations and for the establishment of plants a!

different lolations. h many pl-,jts, the fruits anJseeds have certain special features with the help of

which they are distributeioiver considerable distance from the parent plant. After gefting favourable

conditions, they germinate and give rise to new plants'

Wind, water, animals and explosive mechanisn are the main agencies tha! h91p in the dispersal of

seeds. Some seeds are light and &spersed by wind. The seeds which are slightly heavy develop wings.

Some develop hooks, hiirs, spines, which enable them to stick to the fur of animals and are thus get

carried fromtne place to *oih"r. The various agencies for seed dispersal and the characteristics of the

seeds they disperse are as follows.

Agencies for dispersal

Wiilil Water AnimalsmechanismI

I

r. Ho<iked fruits and seeds

2. Sticky fruits and seeds1. Light and small seeds 1.

2. Presence of wings 2.

3. Presence of hairs4. Presenceofballoonlike 3.

appendages5. Different mechanisms

like parachute mechanism,

censer medranism and rollingmechanism

Light seeds

Presence offloating devices 3. Edible fruitsProtective 4. Athactive fruits and seeds

covering 5. SpinY seeds

imperviousto water

Bursting offruit with force,

to release seeds

The dispersal of seeds increases the chances of survival of the plant species in various

environmenis, as it prevents overcrowding and competition among the members of the same species

for ligh! ak,water, mineral salts, etc.

ExPeriment 7.1

ObiectiveTo study dispersal of seeds by various agencies

Apparatus and materials requiredOifierent types of seeds, fl"shy fruits, a pair of forceps, Petri dishes, a hand lens, glass slides, a knife,

and water r>27

Biology Lab Manual for Class 12

Procedure

1. Collect various types of seeds from your neighbourhood and study them for fheir modes of

dispersal.

2. Cttthe fleshy fruits and observe their seeds. Study and find out their mode of dispersal.

ObservationsRecord the following:

(i) the external aPPearance of the seed,

(ii) the size of the seed,

(iii) the weight of the seed,

(iv) the presence of outgrowths like spines, hooks, wings, hairs, etc',

(v) the type of fruit, if available,

(vi) the occurrence of the seed, in a pod (legume) or in a capsule,

(vii) identification of the seed, i-f possible,

(viii) other special features.

On the basis of above characters, identify the agency through which the dispersal of the seed takes

place.

In the next few pages some description about fhe agencies of seed dispersal and the specific

features of the seeds they disperse are given for your convenience'

Dispersal of seeds by wind The wind is probably the most important agency of seed dispersal in

nature. Seeds have dispersal by wind following features.

L. Lightweight and minute seeds Some seeds are so light that they are blown by air like dust, e.g., seeds

of orchids, grasses, etc.

Z.Wings The seeds of some plants like Moringa, Cinchonat, Oroxylum, etc., have wings. Th9 ry:ngs Teth'r, diy, light and membran6us, due to which these seeds are easily caflied away to long distances by

the wind.

j. Hairs The seeds ol Calotropis, Gossypium (cotton), etc., possess hairs which help them to float in air

and eventually carry them to long distances.

4. Batloonlike appendages In some plants, certain floral parts become inflated, which keep the fruits or

the seeds floating in th; air, e.g., Plrysalis, Colutea, etc. In Physalis, the persistent calyx is inflated like a

balloon.

S.pappusandparachutemechanisrr [r the members of Asteraceae (Compositae) like Taraxacum,

,"pui"' are modified into tufts of hair called pappu! *_hiol, persistent and Jound attadred to

siigle-seeded fruits. The pappus forms a miniature umbrellalike white outgrowth which acts like a

parachute that allows the wind to carry the seeds to great distances.

6. Censer mechanism hr some plants lke Argernone mexicana (prickly PoPpy), Aristolochia, Papaoer

(poppy), etc., fruits,rupture after maturity fui the seeds do not come out' When the fruit is shaken

"iol^eniiy by the wind, the seeds get scattered in all directions.

7. persistent styles In plants like Clematis, Naraaelia, etc., .the,style of the flower persists with the fruitand becomes hairy which hetp in their dispersal.

. _:, ;:{ j

B. Rolling mechanism or twmble weeds Some wild herbs like Amaranthus albus, Chenopodium albumiett.,

get detaihed from soil when dry, and roll over the ground due to the force of wind. \Atrhile doing so

they scatter seeds all along the way.

9. propeller outgrowths The fruit of Ailanthus possess propeller outgrowths due to which they are

propelled to a long distance.

WINGED SEEOS

BALLOONLIKE APPENDAGES

FERSISTEI,IT STYLE$

Dispersal of Seeds

PAPPUS AND PARACHUTK.MECHANISM

ROLLING MECHAN'SM

Colton

CENSER MECHANISM

Pores

PaWwt

tD:7,Soedsg'

D-29

HAIRY SEEDS

iT-rf,.}aftl1f/

:**":'..'",€#

@.{'+. ,

u&s

Ari$/Iolmhia

Fig.7.1 Dfferent types of fruits and seeds showing adaptations for their dispersal by wind

Biology I"ab Manual for Class 12

Dispersal of seeds by water Water is another agency for the dispersal of fruits and seeds, mainly for

uqriti. plants growing near the river banks or seashores'

7. Eloating iteoices Thefruits and seeds of some plants develop some kind of floating devices such as

the fibrous growth on the outer coverin g of cocos"nucifta (coconut) or the sPongy structure inNelumbo

iucyero(btfis). 11 ur" ,t*"tores protectihe fruit and help it to float on water'

2.Water-tesistantprotectioecoaering The outermost layer of some fluits is smooth, waxy and

impervious to water, as in coconut. : -.-B.Lightseeits The seeds of sonneda"t" ltkeNymphaea (water lily) are small and light, and contain

air-filled aril which keep them floating in water'

Fibrous oulercovedng

C*cas nuc$era{Coeonut}

ireiufftbo rlrcif*ra {Lstus}

Eig.72 Fruits showing adaptations for their dispersaf by water

Dispersal of seeds by animals The various adaptations for dispersal of fruits and seeds by animals are

as follows.

l. Stick7 frltits anil seeds Many plants produce sticky fruits_or seeds which adhere to the feet or beak

of birds and are kansported from one place to another, e.g., Boerhaaoia, Cleame, etc'

2. Haokeit fruits anil seeds Many fruits and seeds have spines, hooked bristles, stiff hairs, thoms, etc',

to attaclr themselves to the skin, fur, or wool of the roaming animals or human beings, e'g',Xmtthium'

Aclryranthus,Tribulus, Aristida, etc. i

S. ia*U l*fts The edible fruits are consumed but the seeds being resistant to digestive enzymes- come

out with the faeces of the animal , e.g., Ficus bmghalmsis (banyao), F . religinsa (peepal), gu1ya, ap|!e, etc,

fuig" ,".a, are thrown away after ionsuming"the juiry and'flet"hy p*il e'g', mango, litchi' etc' Nuts of

oak (Quercus), chestrut, ut"., ,t" often collecied by rodeitts T{ ftuy hide these dry seeds at different

pla"""-wh"r. some of the seeds germinatd. under suitable conditions.

Disperialof seed$bllexplosivemechanism ftr6 its$f some plants sudi 1s lm?atims (balsam)'

vlr{r: G*rri;*, .1",.-b;;;th sudden jerk due to whic-h the'seede are scatte.red in all directions. The

p;{; "t;r;*il,l"A*+"iolentty with a

"i""tir,g sound: rn.Eibsllium elatariutn (sqtrirtins cucumber), the

ir,,go" pressure within ttre fruit, after maturifu, canse-s it to burst at the poiit of attadrment of the

p"iicet. Rs a result, the,seeds come out like a fountain along with a mucilaginous mass'

CarPel

D-31

STICKY FRUIT

stickv

YM

ryffBoefiaavia

Dispersal of Seeds

HOOKED FRU}T$ AND SEEDS

Hooks SPines

Fig.T.SDfferenttypesoffruitsandseedswithadaptationsfortheirdispersalbyanimals

Seeds

ffiPuPalia

)Gnthiua Adtyfinthus

Hoot(s

Martynh

Spines

Urena

Ads,tda

Geranium

Ecballium elaartum

Fig,7,4 Different types of fruits and seeds with adaptations for their dispersal by explosive mechanism