Reproduction in Plant (Part - 2) - Notes, Botany, Class 12 - Notes - Class 12

Reproduction in Plant

[ B ] MICROSPOROGE NESIS :

•  The anther appears as outgrowth like structure in the initial stage which shows spherical or oval shaped structure.
•  At this stage, it is a mass of undifferentiated and homogenous meri stematic cells whic h is surrounded by a single cell thick outer layer. This layer is known as epidermis. First of all vascular t issue are formed in middle region. Simultaneously group of  cells located just below the epidermis in vertical rows in the region of hypodermis at the four corners are become large has visible nucleus with dense cytoplasm. Due to this reason they are different from the rest of the cells. These cells are called arches porial cells .
•  These cells divide periclinally to form primary parietal cells below the epidermis and primary sporogenous cells towards the centre. Both of the cells usually undergo further divisions to form complete structure of anther except epidermis.

•  Primary parietal cells undergo further periclinal and anticlinal division to forma serie s of 3-5 la yers making the walls of the anther.
•  Out of them outer most layer of anther is formed just below the epidermis by primary parietal cells is called endothecium or fibrous layer . T he endothecium is fol lowed by 1-3 cel led th ick layer is termed middle layer . The i nn ermo st lay er of the anther which surrounds pollen sacs, is called tapetum . Later the tapetal cells play a significant role during the meiotic cell division in microsporogenous cells and in pollen development.
•  The primary sporogenous cells divide twice or more than two by mitotic division to form sporogenous cells and later sporogenous differentiated into microspore mother cells during the formation of wall of pollensac.
•  Each microspore mother cell div ide to form four haploid microspore or pollen grain by meiotic division or reduction division .
•  During this period spherical bodies are formed inside the tapetal cells before their disintegration. These spherical bodies are known as Ubisch-body . Ubischbody is made up of a complex substance called sporopollenin . It is the polym er of carotenoids .
•  After the formation of ubisch body, the tapetum layer degenerates. Ubisch bodies participate in the formation of exine of the microspores inside the pollen sacs. Now thick walled microspores are called pollen grains .
•  At the init ial stage all four microspores are at tached toget her with the help of callose layer. T h is group of microspores is called tetrad. After some time, this callose layer dissolve by callase enzyme. Which is secreted by tapetum.

Not e :- Normally each micros pore mother cell can formtetrad by meiotic division. But in some plant like Zostera, some microspore mother cells become sterile and provide nutrition to rest of microspore mother cells.

•  At this stage, it is a mass of undifferentiated and homogenous meri stematic cells whic h is surrounded by a single cell thick outer layer. This layer is known as epide rmis. First of all vascular t issue are formed in middle region. Simultaneously group of  cells located just below the epidermis in vertical rows in the region of hypodermis at the four corners are become large has visible nucleus with dense cytoplasm. Due to this reason they are different from the rest of the cells. These cells are called ar hes porial cells .
•  These cells divide periclinally to form primary parietal cells below the epidermis and primary  porogenous cells towards the centre. Both of the cells usually undergo further divisions to form complete structure of anther except epidermis.

Note Similarly, tapetum is not well developed in Gentianaceae family so some cells of sporogenous tissue become sterile and provide nutrition to remaining sporogenous cells.

Types of tetrads :- The arrangment of the microspores in tetrad condition as follows

(i)              Tetrahedral tetrad :- Four haploid microspores arranged in tetrahedral form

Example :- Dicotyledons- (Capsella).

(ii)            Isobilateral tetrad :- This condition is found in monocotyledons. Microspores are arranged at the lateral side of each other.

(iii)           Decussate Tetrad :- In this two microspores lies at the right angle of other microspores

Example :- Magnolia

(iv)           T-Shaped tetrad :- Two microspores lie longitudinally and two microspores lie at transversly in this type of tetrad. Example :- Aristolochia & Butomopsis.

(v)            Linear tetrad - In this tetrad all four pollens arranged in linear order. e.g., Halophylla, Halophia.

• All the above type of tetrads are found in Aristolochia elegans.
• Most common type of tetrads is Tetrahedral.

Facts about types of Pollen grains –

(1) In Ascl epiadace ae (Calotropis ) and Orchidaceae family, all the pollen grains joined together to fo rm " Polli nium ". Pollinium of Calotropis is also called "T rans lator apparatus" .

 ( 2 ) More than four pollen grains are found intetrad called " Polyspory " e.g. Cuscuta .

( 3 ) In some plants, four pollens of tetrad join together permanently are called "Compound pollens " e.g.

Drosera, Typha, Drimys and Elodea .

 (4) Compound pollens of Mimosaceae (Mimosa) family contains 6-8 or 64 pollen grains stick together to form a small unit that is cal led " Massullae " e.g. Mimosa and Neottia plants.

(5) Pollengrai ns of some plants present in air cause al lergy are called "aeroallergens " e.g.

Chenopodium, Parthenium, Sorghum , and Am aranthus . ["Hay fever" is caused by pollens of Ambr sia .]

( 6 ) Pollen grains of many plants are rich in nutrients. It has becomes a fashion in recent years to use pollen tabletes as food supplements. In western countries, a large number of pollen products in the form of tablets and syrups are available in the market. Pollen consumption has been claimed to increase the performance of athletes and race horses.

( 7) In Cypera ceae family only one pollen grain is formed from pollen mother cell. e.g., Cyperus .

 (8) Largest pollen - Mirabilis . 

(9) Smallest pollen - Myosotis.

 (1 0) Longest pollen – Zostera (Fili form pollen) 

(11) Eight nucleated embryo s ac type of po llen is found in Hyacinthus .[T his type of pollen grain discovered by Nemec . So it is called Nemec phenomenon ]

( 1 2) Dumble shaped pollens are found in Vincaherbacea .

[ C ] STRUCTURE OF M ICROSPORE OR POLLEN GRAIN :

Pollen grain is the first cell of a male gametophyte. 

• Pollen grain is termed as immature male g ametophyte .

Usually, they are in round shape. Pollen grain surrounded by two d istinct la yers . The outer layer (wall ) is thick, rigid and orna mented , called exine . Thi s l aye r i s formed by c utin and s porop ollenin . Sporopollenin is highly re istent material . It is nonbiodegrada ble. 

• Due to the presence of sporopollenin, fos sils o f pollen g rai n are always found in good condition . The presence of fossils of pollengrains can forecast the presence of natural reso urc e s like p et rol e um , coa ls e tc. in the eart h.
• The in ternal l aye r is thin, soft and el ast ic in nature. It is called intine. It is made up of pectin and Cellu lose or pecto- cellulose. 
•  Usually, at few place s on outer surface exine is absent or  pre sent in the form of thin layer. The se thin place s are called germ pore. The intine comes out through the any one germpore during the germination of pollen grain in the form of pollen tube .
•  The number of germpore , structure and orname ntat ion of ex ine is a sig nifi cant feature of taxonomy .
•  A detail study of pollen grains is called Palynology. 
•  Three colpus type (slit type) of germpore are present in pollen grain of most of the dicots (Capsella) . This type of p ollen grains are called tricolpate . Only one germ pore is present in monocots and pollen g rain is called monocolpate.
•  The plants in which pollination takes place by insects , their pollen grains having oily layer around the pollen grain. It is called pollen -kitt. It is composed of lipids and carotenoids.

Function of pollen kitt :-

(i) This oily layer protects the pollen grain from the harmful ultraviolet rays.

(ii) Its stic ky surface helps to attach with the insec ts.

(iii) Its yellow c olour attracts the insects. Pollen kit is pres ent on the pollens of Caps ella.

[D] DEHISCENCE OF ANTHER :

During the maturation of anther, various changes are takes place in walls of anther.

• In the beginning, middle layer degenerates due to absorpti
•  When the micropores are formed inside the pollen sacs, at the same time ubisch bodies are formed in cells of tapetum, then after it degenerates. Ubisch bodies participate in the formation of exine of pollens.
•  In this way, in a mature anther only two layers epidermis and endothecium are present in the form of outer covering.
•   The sterile tissues are present between both the pollen sacs of each anther lobe degenerate. So both pollen sacs of the each anther lobe fuse together to form single pollen sac
•  Therefore, in the T.S. of mature anther only two pollen sacs are present.
•  Dehiscence of anther takes place during the dry season. Due to the hygroscopic nature of endothecium, loss of water takes place from the cell of endothecium.
•   Walls of endothecial cells try to contract due to the loss of water but inner and radial walls do not contract due to presence of fibrous thickening whereas outer thin walled cells of endothecium contract and become concave or incurved.
• Incurving of outer walls exert pulling force or tension over the entire surface of anther. Due to tension, thin walled stomial cells breaks off and dehiscence of anther takes place and pollen grains are present in pollen sacs released into the atmosphere.
•  Dehiscence of anther in Angiosperms either longitudinal or Apical pore, or Transverse or Valvular type. Dehiscence of anther of Capsella is longitudinal.

[ E ] MICRO- GAMETO GENESIS OR DEVELOPMENT OF MALE GAMETOPHYTE :

In flowering plants , pollen grain or mi cros pore considered as first cell of male gametophyte . Germination or development of pollen grain takes place before dehiscence of anther. So it is called as precocious development . Development of pollen also takes place at mother place [inside pollensac of anther] is called In-situ development.

 ( i ) Pre Pollinat ion devel op ment – In the beginning of the the process, only nucleus of pollen grain divided by unequal mitotic division , resulting two unequal s ize of nucleus are formed. Small nucleus present near the wall is called generative nucleus a nd large nucleus pre ent inside the cytoplasm is cal led Tube or Vegetat ive nucleus .

Both the nucleus surrounded by cytoplasm and it becomes dense, then  followed by unequal cytokinesis , resulting two une qual size of the cells are formed. larger cell in which large nucleus is present known as Vegetative cell and smaller cell in which small nucleus is present , called generative cell .

Now pollen grains come in bicelled a nd binucle ated stage . In Angiosperms pollination of pollen grains take place in  icelled and binucleated stage i n 60% A ngi ospe rms and in 40% An gio sperms pollination occurs at 3-celled stage. In this generative cell divides and form two male gametes. The development gametophyte take place inside the sporangia is also known as endosporic development .

This stage of pollen grain is called immature or partially developed male gam etophyte .

Generative cell detached from the wall and changed into vermiform or spindle shaped struc ture and enter inside the vegetative cell.

 ( i i ) Post Pollination development – Further develo ment of pollen grain [Immature male gemetophyte] takes place on the sti gma of Carpel after pollination. Pollens absorb moisture and sugar content from the stigma. Due to this volume of internal contents of cytoplasm increased. It exerts pressure on the both outer layers. Because of this pressure intine comes out through any one germp ore in the form of tube like st ructure called pollen tube .

First of all vegetative nucleus ent er into the p ollen tube and as sumes terminal [tips] position. Th is spindle shaped generative now enter into the pollen tube. Inside the pollen tube, generative cell divides mi tot ically and to form a two non m oti le ma le gametes . Now male gametophyte comes in three celled structure in which one vegetative cell and two male gametes are presen t.

This three celled stage represents the mature male gametop hyte of Angiosperm [Capsellaalso]. Mal e gam etophyte i s hi ghly reduc ed and c ompletely de pen ds on sporophy te.

First ofall pollen tube disc ove red by G.B. Amici in Portulaca plant .

Longest pollen tube is found in Zeamays .