Reproduction in Plant (Part - 4) - Notes, Botany, Class 12 PDF Download

Reproduction in Plant

POLLINATION

"Pollination is defined as the process of transfer of pollen grains from anther to the stigma of the same flower or of different flower of the same species."

Pollination is of two types :-

1 . SELF POLLINATION OR AUTOGAMY :

If the pollen grain are transferred from an anther to the stigma of the same flower , or different flowers of the same plant is called self pollination or autogamy .

2 . CROSS POLLINATION OR ALLOGAM Y :

When the pollen grains are transferred to the stigma of other flower of the same species is called Cross pollination or Allogamy. It takes place in between two different flowers.

Cross pollination is of two types :-

( i ) Geitonogamy : When, pollination takes place in between the two flowers of the same plant then it is called geitonogamy . From the genetical point of view geitonogamy is self pollination because all flowers of the same plant are genetically identical. But ecologically , it is considered as cross pollination .

 (i i) Xenogamy : When the pollination takes place in between the two different flowers of two different plants of the same species then it is called xenogamy . This is real or true cross pollination . Genetically , as well as ecologically , it is cross pollination .

Contrivances or Adaptation for Self Pollination : 

(i) Monocliny (Bisexuality) - It means flowers are bisexual.

(ii) Homogamy : - When both the sex organs of a flower mature at the same time. It is called homogamy . It increases chances for self pollination. E.g. Mirabilis , Catharanthus . 

(iii) Cleistogamy :- In some plants bisexual flower are formed which never opens throughout the life. Such flowers are called cleistogamous flowers , such as Commelina, Viola , Oxalis, Juncus, Drosera. Commelina plant have two types of flowers. One type of flowers are cleistogamous and anoth er are chasmogamous flowers .

• A special type of cleistogamy is found in flowers of legume plant. The sex organs are closed in a structure which is formed by joining of some petals . It is called keel . This keel never opens . So only self pollination takes place in these plants.

(iv) Bud pollination:- This pollination occurs in bud stage before the opening of flowers. E.g. Pisum, Wheat, Rice.

Contrivances for Cross Pollination 

(i) Dicliny (Unisexuality) :- Presence of unisexual flowers confirm cross-pollination. Self pollination never takes place in these flowers. It means allogamy becomes compulsory. Examples – Morus , Palms, Date Palms, Cucumber and Cucurbita, Carica .

(ii) Dichogamy :- In many bisexual flowers of the plants, stamens and carpels of a flower do not mature at the same time. Dichogamy is of two types –

(a) Protandry :- The anther of a flower mature earlier than carpels, is called protandry . Many plants of Angiosperms are cross pollinated only because of protandrus condition . E.g . Salvia, Sun flower, Cotton. 

(b) Protogyny :- The carpels of the flower mature earlier than stamens. It occurs in few  plants e.g. Ficus bengalensis , Saraca indica , Ficusreligiosa , Aristolochia, most of plants of Crucifrae and Rosaceae family .

(iii) Chasmogamy or Anthesis :- Opening [blooming] of the floral bud in the form of a flower is called anthesis .

(iv) Herkogamy :- In some plants, morphological barriers are formed in between the anther and stigma of the same flower, so self pollination can not occurs and pollen grains from the anther are unable to reach the stigma of the same flower. In this condition, only cross pollination is possible. e.g. Gloriosa, Calotropis and caryophyllaceae family.

(v) Heterostyly :- There is difference in between the length of the filaments of stamens and length of style in flowers of some plants. Some of the plants having long stamen and short style, and in some of the plants bears long style and short filam ent.  Due to this reason, self pollination can not possible in these plants e.g. Primrose, Linum, Primula .

(vi) Self sterility or self incompatibility or intraspecific incompatibility :- In this condition the pollen grains of the flower can not germinate s on the stigma of the same flower. T his condition is called self sterility . This is a parental [Genetical] characteristic feature which is controlled by genes . Such as in Pitunia, Malva, Thea, Passiflora, Vitis, Apple (Malus) .

Incompatibility involves many complex mechanisms associated with interactions of pollen and stigmatic tissues. If the incompatibility is due to the genotype of the sporophytic /stigmatic tiss ues, it is termed sporophytic incompatibility on the other hand, if it is due to the genotype of the pollen, it is termed gametophytic in compatibility . This may be due to prevention of pollen germination, retardation of growth, deorientation of pollen tube or even failure of nuclear fusion . It is controlled by genes with multiple alleles (s-allele).  Enlarged pollen tube turns upwards and degenerates in style.

(vii) Prepotency:– Growth of its pollen tube of self pollinated pollen grain is very slow and growth of the pollen tube of cross pollinated pollen grain is very fast so pollen tube of cross pollinated pollen grains reach earlier inside the ovule. This is termed prepotency . e.g, Apple, Grapes

METHODS OF CROSS POLLINATION

1 . ANEMOPHILY :
When the pollen grains are transfer from one flower to the another flower through the air is called a ne moph il y an d flowe r i s known as an emo ph ilous flow ers. Such as Ce real plants :- Maize etc.
Anemophil y is also found in all Gymnos perms . T he an emop hil ous pla nts produce en ormous a moun t of pollen grains . The pollen grains are very s ma ll , li gh t weig ht and dry and t heir stigm a is hai ry or bru shy and muci lagenous (Stic ky) .
Yellow c louds are formed i n the sky during the w i nd pollination in Pinus .

These yellow clouds are formed due to the pollen grains are called " Su lph ur Showers".
Anemophilous flowers are neither attractive nor with fra g ran c e . T hey do not have nec tar g lands .
Anemophilous flowers are generally unisexual.

E .g . - Sug ar cane, B am b oo , Coconut, Canna bis, Gras ses, Date palms , T ypha, Oak, Haze l, Cucumber, Papaya, Cotton, Tobacco,Eucalyptus.

• P ot a m og e t on a nd Myr iophyllum are aquatic plant but it is anemophilous.
• Maximum loss of pollen grains takes place only in this pollination. It is completely non dire ction al proc ess .

2 . HYDROPHILY :
When the pollination brings by water is known as hydrophily . It is of t wo types.

(i) Epihydrophily :- W hen the pollination takes place on the surface of water is called epi hydrophi ly e.g.
Vallis neria and Ru ppi a .

(ii) Hy po hy droph il y : - W he n t he pollination takes– place inside the water is called hyp ohydrophi ly . e .g.
Ceratophyllum, Najas Zo s te ra and H y dr illa .
Spe cifi c g ravit y is found in pollen grains so they remains suspended in water.

3 . ZOOPH ILY :
When the polli nati on brings by animals is called zoophi ly . Generally in zoophillous plants, flowers are very l arg e, at tr ac ti ve an d ma ximum n umbe r of ne ct ar g l an ds are pres ent (more tha n e ntomophill ous ).

(i) ENTOMOP HILY : The pollination takes place with the help of in sec ts is known as en tomophil y. Most of insec t pollination (80%) only by Hon ey be es . Mo st of entomo philou s plan ts are ornam ental plants . Orn am en tal p lan ts utiliz e th eir maximum energy in this pollination and develops different types of adaptation for attraction of insects. These flowers are at tractive i n colour. They posse s s peci a l frag ren ce . Nec tar gl a nd s i s a ls o p re s ent . e .g. Ros e, Lemo n. The pollen g rain s of i ns ec t pollin ated f lowers becom e s tic ky due to pre se nce of pollen kitt . T he surface of stigma of flowers is ro ugh .
Some of the plants develop special adaptation for insect pollination.

• In Mus saen da pl ant, a dv ert is in g f la g is present for attraction of insects.
• The lever m echa nism or turn pi pe mechan ism is foun d in Salv ia for poll ination.

The flow ers o f S al vi a have b il ab i at eco ro llat e tube. The connective of stamen is long. The anterior anther lobe of connective is fertile while posteriorly lobe is sterile. When the insect lands on the lower lip, the fertile lobe automatically brings  down to touch the back of insect and thus depositing the pollen grains on the back of insect.

• Some of the flowers have attractive br a c ts i.e. bright and coloured e.g. Boug ain villea
• Yu c ca pla nt has de velops a n ob lig a te s ym b iot ic relationship with an insect- Pronuba m o th . T he p ol linatio n i n "Yucca take s place only by Pronuba moth. This insect lays eggs in the ovary of flower. Life c ycle of bot h dep end on each other.

•  "Trap door mecha nism " is found i n species of Fic us (Peepal) for pol li nat ion [Blastophaga i nsect] because Hypanthodium type of inflorescence is present.
• In A ris t o loc h ia "Fly tra p mec hanis m " is fo und for pollinat ion. This flower is known as " Pi t fa ll f low e r" .
• Ophr ys spec ulum f lower pol li nated by Wasp [Colpa aurea] by mea ns of ps eudo-copulat ion . T he appearence and odour of the flower like female wasp [Mimicry].
• In Centau rea plant pollina tion takes place by "piston mechanism ".
• In R af fles ia , the p ol lina ti on is b ro ugh t ab out  by Ca rrion fl ie s a nd di sp ersa l o f seed s by El ep han t .
• Nymphaea , Nelum bo a nd Alis ma are also en tomop hilou s pla n ts w h i le th e y a re hydro p hytes .

(ii) Orn ithophily :-Then process of pollination is taken place by birds is known as ornithophily. e.g. Sun bird or humm ing b ird in Bi gno nia plant and by Honey b ird in S treli tzia , Calli s t em o n ( Bo ttle br us h) , B omb a x [ S ilk c ott on tr ee , ] Bu tea monosperma.

(iii) Chiropterophily :-If the pol lin a tion bring s through the b a t s (Pter o p us ) is c alled chi ropterop hily . These flowers are big in size e.g. Anthoce phalus kadamba, Bauhinia , Ki gelia plants, Adansonia, Mus a.

(iv) Myrmec ophily :-Th is pollinat ion brings about by Termi te s a n d An ts . e.g. P ros opi s (kik ar), Ac acia ( Mimo so i de ae family), Mango and some members of Rubiaceae family.

(v) Mal acop hily or Malm acop hily :-This polli na tion brings about by Sn ai ls e.g., Aspid istra lu rida and Chry sa nthem um plants.

• Erythrin a (C oral t re e) p la nt v is it ed b y C r o ws and Sq uirrels.
• In Ar is aema or Cobra Pla nt pollination by s nake c alled "O ph ioph il y".
• Ni gh t flow eri ng p la nts are pollinated by Moths. They are highly s cented.Their flower generally white coloured
• Orchids are po l l i n a ted b y W asp s/ Sn ails.
• Favourable c olour of Honey bee s is yellow , but the y are bli nd to red colour.
• Larger animals such as some primates (lamurs), arboreal (tree-dwelling) rodents, or even reptiles (gecko lizard and garden lizard) have also been reported as pollinators in some species.

FERTILIZATION
Th e fus ion of male game te wit h fe male ga mete i s ca lle d fe rti lization . Fi rst of all, fertili zation was discovered by S tras bu rg er ( 18 84) in Mon ot rapa plant. This process is completed in the fol lowing steps :-

[ A ] GERMI NATI ON OF POLLEN GRAINS :
Af te r pol li nat io n, pollen grain s g er mi na te o n th e st ig ma . Th ey a bs orb mo is tu re a nd s ug ar c on te nt s from stigma and swellup. The intine of pollen grain grows out through the any one germinal pore of exine, in the form of tube like out growth is called pollen tube. One pollen tube develops in Capsella and mo st of Ang ios perms is called mo nos iph on ous condition, but more than one pollen tubes develops in Malvaceae and Cucurbitaceae fam ily. It i s c alle d p ol ysi ph o no us .

• W hen the pollen tube come s dow n from the st igma into the st yle, first of al l vegetat ive nucleus enter, into t he pollen tube then it is followed by generative cell. The tube nucleus always occupies in terminal position in pollen tube. The vegetative nucleus controls the growth of the pollen tube. Mean while, the generative cell divide mitotic ally to form two male gametes . Both of the male gametes are non motile .
• Boron and calcium ions (mainly Boron) are essential for the growth of pollen tube and best temperature for growth of pollen tubs is 20–30 0 C . Pollen tu be shows apical g rowth and chemotropic movement.

[ B ] ENTRY OF POLLEN TUBE INTO OVUL E :
Finally, the pollen tube enters in the ovary at that time, ovule becomes mature. Inside the ovary obtur ators guides the passage of pollen tube towards the micropyle. A mature ovule in which embryo sac also matured, has three paths for the entry of pollen tube:-

( i ) POROGAMY :- In this, pollen tube enters into the ovule through the micropyle. It is known as porogamy. It is found in most of Ang ios per m s [C aps e lla ].

(i i) CHALAZOGAMY :- In this method, the pollen tube enter into the ovule through the chalaza. This method is discovered in C as ua ri na by Tre ub [1891] e .g .
Be tu la and Juglans (w alnut).

(iii) MESOGAM Y :- In this method, pollen tube enter into the ovule either through integu ments - Cucur bita or through the fun ic ulu s - P istac ia and Po pulu s .

[ C ] ENTRY OF POLLENTUBE INTO EM BRYOS AC :
Pollen tube can enter into the ovule through the any passage but inside embryosac, it enter only through the egg apparatus. After the entrance inside the ovule, it grows towards the egg apparatus because synergids cells sec rete the chemical (hormones) which attracts the growth of pollen tube. It means poll en tube shows chemotropic movement in ovule. * Any one synergid starts degenerating when the pollen tube comes near egg apparatus. The pollen tube enter into the embryosac through the degenerating synergids. * When tip of the pollen tube enters into the embryosac vegetative nucleus degenerates. The tip of the pollen tube swells and burst [Endosmosis] after reaching inside the embryosac. The pollen tube released all contents including both male gametes inside the degenerating synergids of embryosac. * Tw o d ar k gr anul es appear s in the r egion o f degener atin g synergids. These are k nown as X-bodies. They are two in no. and both X-bodies are formed by the degenerating nucleus of tube cell and synergids.

[ D ] FUSION OF GAMETES :

• Before or after the entrance of pollen tube into the embryosac, both polar nuclei of the central cell fused together to form a dipl oid nucl eus . It is known as sec o ndary nucleu s or defi nitive nucle us .
• Out of two, one male g amete fertilized with egg cell and to form a diploid zygote . This fusion is known as s yn g a my . This is true m echa nism of fer t ilization process.
• The second male gamete fused   with di ploi d s e con dary n uc leu s which is formed by the fusion of two polar nuclei. This fusion is known as triple fusion resulting, a triploid (3n) structure is formed. It is called prima ry endosperm nucleus.
• Fe rtiliz atio n takes place t w ice at a ti me in A ng ios p erm is c alled doub le f ertiliz at io n .
• Double fertilization was discovered by "N awa sc hi n " in L il iu m a nd Fr iti ll a ria plan ts.
• Do uble fer til iz atio n a nd tr iple fusion is t he specific or unive rsal characterist ic of Angios per m. T he re are f iv e nu cl ei and t hree g a me tes p arti cip ate in do ub le f er tiliz at io n.
• A zygote is formed by true fer tilization (s yngamy] develops into embryo. Triploid primary endosperm nucleus is formed by triple fusion develops into the en dos p erm which is us ed as nutrition for growing embryo.
• All the remaining cells of embryosac like antipodal cells, synergids degenerate excluding zygote and primary endosperm nucleus after the fertilization. At this time, zygote obtains food from degenerating synergids and antipodal cells.
• The fertilization in which non motile gametes are carried to female gamete through pollen tube is known as "Siphonogamy".
• Entry of more than one pollen tube into the ovule leading to occurence of super numerary male gametes is called "Polyspermy."