Plastids: Subject Matter and Classification | Cells

In this article we will discuss about the Subject Matter and Classification of Plastids:

Subject Matter of Plastids:

Plastids are unique and discrete cytoplasmic organelles found in the cells of most of the plants and certain unicellular organisms particularly protozoa, but not in the cells of higher animals. Although Comparethi (1791) noticed the presence of green granules and Sacks (1865) also observed special bodies in the plant cells yet the term plastid was first used by A.F.W. Schimper in 1883.

Plastids are coloured or colourless organelles most often occurring as spherical or disc-like bodies in free state in the cytoplasm of the eukaryotic plant cells. In fungi, bacteria and blue-green algae plastids are lacking.

However, the electron microscopic studies have revealed that in photosynthetic bacteria and blue- green algae some distinct lamellar structures bearing photosynthetic pigments undoubtedly occur in the protoplasm. These pigmented lamellae are named chromatophores by some authors. The term chromatophore was coined by Schmitz (1882).

Classification of Plastids:

The obvious variations found among plastids involve colour or pigment content, size and number. Plastids may contain one or more pigments or no pigment and on the basis of pigment content or colour, plastids have been grouped into the following types:

(i) Leucoplasts (colourless plastids)

(ii) Chloroplasts (green plastids).

(iii) Chromoplasts (plastids coloured other than green).

The plastids are generally coloured owing to the several kinds of colouring substances called pigments.

Some important pigments and their respective colours are listed below:

These pigments are found in the plastids. However, there are certain pigments, such as anthocyanins which are not found in the plastids but are dissolved in the ground substance of cytoplasm. The blue, red and pink colours of some flowers and young foliage leaves appear due to the presence of anthocyanins.

The chloroplast which contains chlorophyll pigments is the chief site of photosynthesis in plant cells. The plastids containing both chlorophylls and carotenoid pigments are active in photosynthesis but those containing only carotenoid pigments and lacking in chlorophyll are photosynthetically inactive.

The coloured plastids generally contain more than one pigment but the colour of the plastid is determined by that pigment which masks the colour of other pigments. The chloroplasts found in the cells of most green plants contain a small amount of carotenoids, such as carotenes and xanthophylls in addition to chlorophylls but carotenoids are not always visible because chlorophylls mask their colours.

Chromoplasts are pigmented plastids bearing colours other than green. They may be yellow, orange, brown or red. Yellow or orange chromoplasts occur in petals, fruits and roots of some higher plants. Generally, these plastids contain carotenoids and a reduced chlorophyll content and are photosynthetically less active.

In some cases chromoplasts contain carotenoids only and chlorophyll is lacking. The red colour of ripe tomatoes, for example, is due to the presence of chromoplasts that contain the red pigment lycopene, a member of carotenoid group.

Whenever the chromoplasts contain a small amount of chlorophylls in addition to a large quantity of carotenoids, green colour of chlorophylls is not manifested because of masking effect of the major carotenoid pigments.

Chromoplasts containing various pigments are found in different groups of algae. Phaeoplasts, brown coloured chromoplasts having fucoxanthin as major or principal pigment occur in brown algae, diatoms, dinoflagellates, etc. and rhodoplasts, red coloured chromoplasts having phycoerythrin as major pigment occur in red algae.

Certain carotenoids such as fucoxanthin and phycoerythrin absorb light for photosynthetic reaction and transfer the light energy to chlorophylls. In pigmented bacteria, however, the pigments do not show identical reactions.

i. Leucoplasts:

Leucoplasts are the plastids devoid of pigments. They are involved in the synthesis and storage of the various kinds of carbohydrates (starch), oils and proteins. If they store starch, they are called amyloplasts. When leucoplasts store oils, they are termed elaioplasts.

Leucoplasts concerned with storage of protein are called proteinOplasts or aleuroplasts. Leucoplasts are rod-like or spheroid in shape [Fig. 3.1 (A)] and occur in embryonic cells, sex cells and the cells of the storage organs. They are of common occurrence in cells that have been protected from light.

According to Schimper, “Transformation of one type of plastid into another type is possible in the following way”:

ii. Chloroplastes:

Chloroplasts are the most common and familiar plastids of most plant cells and they are of greatest biological importance, since by photosynthesis they produce chemical energy. They can be readily seen in most plant cells at low magnification under light microscope.

Morphology:

The chloroplasts remain homogeneously distributed in the groundplasm of plant cells. In some plant cells they are found concentrated around the nucleus and in some they may be found just beneath the plasma membrane. The shape, size, and distribution of chloroplasts may vary in different cells within a plant.

Shape:

Plastids in the cells of higher plants show a great deal of variation in shape. The cells of leaves contain many chloroplasts of spheroid, ovoid or discoidal shape. Some may be club shaped having narrow middle zone and bulging ends.

They may be vesicular with a colourless centre in some cells. In algae, cells often possess a single large chloroplast which may be star-shaped, band-shaped, cup-shaped, discoidal or reticulate [Fig. 3.1 (B)].

Size:

Chloroplasts show considerable variation in size. The average diameter of chloroplasts of higher plant is 4 to 6μ and thickness 1 to 3µ. The size is, more or less, constant for a given cell type in a plant but in polyploid cells they are comparatively larger than those in the corresponding diploid cells.

Plants growing in shade have chloroplasts that are larger in size and contain more chlorophylls than the chloroplasts found in plants growing in sunlight.

Number:

The number of chloroplasts appears to be relatively constant in different plants and the variation in number whenever seen is related with the physiological state of the cell. When the number of plastids is insufficient it is increased by division of existing plastids and when the number is in excess it is decreased by degeneration.

The cells in most algae possess only a single chloroplast. The cells of the higher plants may have 20 to 40 chloroplasts. Haberlandt (1882) found at an average 36 chloroplasts in each palisade cell and 20 in each spongy parenchyma cell. In moss genus Mnium, cells have been found to possess an average of about 106 chloroplasts each.

Critical observations of the plastids have shown that they are displaced and deformed by action of cytoplasmic streaming. Besides, some sort of amoeboid movement or contraction may be influenced by illumination. Isolated chloroplasts of spinach show decrease in their volume when they are exposed to light and photophosphorylation is initiated.

This effect is reversible. Considerable contraction of chloroplast takes place in dark if ATP is added to the medium. Two different proteins having contractile properties, which may account for this reversible change in the volume of plastids under different sets of conditions, have been extracted from isolated chloroplasts.

Chloroplasts show greater resistance to osmotic changes and fixatives than other types of plastids as well as mitochondria.

Ultrastructure:

There are two types of chloroplasts [Fig. 3.2 (A)]:

(a) Lamellate chloroplasts.

(b) Chloroplasts with grana.

(a) Lamellate chloroplasts:

Lamellate chloroplasts are commonly found in algae and in some cells of lower plants.

They are disc-shaped, primitive type of chloroplasts bounded by two differentially permeable unit membranes of lipoproteins. Inside the plastid membrane, plates containing pigments (chlorophylls, etc.) are orderly placed in a colourless matrix or stroma. Menke (1961) termed the lamellar units as thylakoids but Ruthsager and Palade (1957) preferred to call them discs.

In section it appears like a pair of parallel membranes joined at each end enclosing narrow space termed loculus. The thylakoids in algae are generally long and may run nearly the entire length of the chloroplasts. In many green algae, the thylakoids occur in multiple layers forming grana-like structures called stacks or bands. The number of discs in a stack varies from 2 to 20 or even more.

Usually there are four to six thylakoids in a stack. Thylakoids in the stack do not adhere and remain separated by a narrow space called interdisc or interthylakoid space. In a number of other green algae, the thylakoids do not form stacks. They come together and move apart in an irregular manner [Fig. 3.2 (B)].

In prokaryotic photosynthetic cells such as photosynthetic bacteria and blue-green algae chloroplasts do not exist as separate organelles and pigmented lamellae or vesicles are distributed throughout the cytoplasm.

As regards the relation of outer limiting membrane of the plastids with other cytoplasmic membranes, there exist considerable differences. In several cases the plastid envelope is sometimes continuous with elements of endoplasmic reticulum. Besides, connections with other cytoplasmic membranes are quite common in forms with single plastid, e.g., in chlamydomonas, rhodomonas, etc.

In Ochromonas there is a double membrane envelope outside the usual outer limiting membrane of the plastid. This outer envelope is continuous with nuclear envelope but does not contain pores.

Similar relationships exist in many groups of algae. In chlorophyceae and rhodophyceae, however, no additional envelope is known to occur outside the usual outer limiting membrane of the plastids.

Chloroplast with grana:

This type of plastid occurs generally in the mesophyll cells of the higher plants.

The mature chloroplast of the higher plant can be divided into the following three parts which are interrelated structurally and differentiated functionally (Figs. 3.3 and 3.4):

(i) The chloroplast envelope,

(ii) The stroma comprising the internal matrix material, and

(iii) The grana fretwork system comprising the internal membrane system.

The shape of chloroplast is subjected to transformation.

Sometimes formation of finger like protuberances may take place from the chloroplasts. Protuberances have been observed extending from elioplasts, mature chloroplasts, chloroplasts undergoing transition to chromoplasts, chloroplasts in tissues infected with virus, chloroplasts in manganese deficient plants and chloroplasts in starvation- stressed degenerating cells.

The protuberances extending from chloroplasts are of two types. Some appear as long thin extrusions which are bounded by the chloroplast envelope and mainly contain stroma material. They are characterized by the occurrence of many blebs from inner membrane of the chloroplast envelope.

The second type of protuberances occurs in plant possessing high photosynthetic capacity (i.e., C₄ plants) and these extensions possess an anatomizing network of tubules given out by the inner membrane of the chloroplast envelope.

The Chloroplast Envelope:

It is now well established that the chloroplast envelope consists of two unit membranes, each about 80-1oo Å in thickeness, separated by a space about I00-200 Å in width which appears electron translucent in eletron micrographs. It is across this thin envelope that photosynthetic metabolites enter and leave the chloroplast.

The studies to date on the ultra-structure of the chloroplast envelope indicate that there are numerous variations in its general organization. In most of the mature chloroplasts the inner membrane of the envelope invaginates to form small vesicular or flattened or ginger-like invaginations.

In some cases, much longer invaginations run parallel to the envelope, Schotz and Diers (1967) have reported that the invaginations are plate-like structures which sometimes enclose small enclaves and pockets of stroma.

Very little is known about the inter-membrane space of the chloroplast envelope. Recently, Sabricis Gordan and Galston (1970) using histochemical method for the fine structural localization of nucleoside phosphates have reported the presence of a Mg⁺⁺ dependent ATPase between the two unit membranes of the envelope.

The enzyme appeared to be light activated and these investigators have suggested that it might be involved in the light dependent contraction of chloroplasts.

The Stroma:

Inside the envelope there is a proteinaceous matrix called stroma, containing starch grains, osmiophilic plastoglobulin, phytoferretin granules, protein grains, ribosomes, RNAs and DNA. The stroma also contains enzymes that are involved in dark reactions of photosynthesis, carbohydrate synthesis, protein synthesis and synthesis of chlorophylls, carotenoids and other pigments (Figs. 3.3 and 3.4).

Starch grains are the commonly observed in the stroma of chloroplasts. The grains appear nearly oval in most electron micrographs although they may vary from oblong to round.

In the chloroplasts fixed with osmium tetraoxide there is characteristic occurrence of round electron opaque bodies in the stroma. These osmiophilic bodies or droplets are called plastoglobuli. The plastoglobuli appear free in stroma and may occur singly or in clusters. They may, however, occur associated with grana fretwork system.

The size of these plastoglobuii in normal photosynthesizing chloroplasts may be 100 Å or more. The size and the number of these osmiophilic droplets may increase with the advancement of age of plastids and may also increase during transition from chloroplasts to chromoplasts. It is generally suggested that plastoglobuii represent the reservoirs of excess lipid.

Plastoglobulin in chloroplasts of photosynthesizing leaves contains little or no chlorophyll or carotenoid pigments. With the advancement of plastid age or during chloroplast to chromoplast transition, the composition of the globules changes and carotenoid pigments accumulate in them.

Hyde et al. (1963) reported the occurrence of an iron protein complex in pea embryos and termed it phytoferretin because of the similarity in structure and composition to ferretin present in animal cell. Phytoferretin particles are electron opaque and measure about 100 Å in diameter.

They consist of iron containing cores and the electron translucent regions around the core, the proteinaceous shell.

They are of common occurrence in proplastids and differentiating plastids or in senescing chloroplasts and developing chromoplasts. Similar particles have been found in the stroma of chloroplasts in virus infected tissues. Hyde et al (1963) have suggested that phytoferretin represented a reservoir of stored iron in the form of a non-toxic iron containing protein.

Another characteristic feature of chloroplast is the presence of ribosomes and RNA in the stroma. The plastid ribosomes are evident particularly after staining with uranyl acetate, but not observed in permanganate fixed cells. The plastid ribosomes in thin sections measure about 170 Å in diameter and are thus smaller than the ribosomes found in ground-plasm which measure about 250 Å in diameter.

The plastid ribosomes frequently appear as free 70S particles. However, Brown and Gunnina (1966) have observed groups of ribosomes in developing chroloplasts and Bartels and Weier (1967) have also described helically arranged polysomes in the stroma of proplastids of Triticum.

Cytological, electron microscopic, autoradiographic and biochemical studies have established beyond doubt the presence of cp-DNA in developing and mature chloroplasts. At ultra-structural level DNA appears as ring-shaped irregular mesh of 25 Å fibril in the stroma.

One or several cp-DNA sites may be located in the stroma of chloroplast of several plants including Chlamydomonas, Euglena, tobacco and spinach. There is a direct correlation between the increase in the size of chloroplast and increase in the number of DNA sites. This may indicate a polyploidy or polyvalency of the cp-DNA.

The cp-DNA from spinach chloroplast has been shown to consist of linear and looped filaments. The filaments measured upto 150µ, in length and some seemed to be associated with plastid membrane. The filaments do not appear circular.

Bisalputra and Bisalputra (1967), Gunning (1965) and Kisler, Swift and Bogorad (1965) have noticed DNA regions resembling bacterial nucleoids in micrographs of thin sections of chloroplasts. cp-DNA of chloroplasts tends to have a higher adenine-thymine content than nuclear DNA.

In Euglena adenine-thymine content amounts to 80% of total base composition. Studies on DNA synthesis in isolated plastids of spinach indicate that cp-DNA replicates within the plastid independently of the nuclear DNA.

The exposure of cytoplasm of Euglena to ultraviolet radiation causes bleaching of plastid but no bleaching of plastid occurs when nucleus is exposed. This further suggests the independence of chloroplast DNA from the nuclear DNA.

Grana fretwork system:

Meyor (1883) and Schimer (1885) recognised that the internal region of the chloroplast was divided into two phases: the stroma and numerous greenish granules, the grana. The presence of grana was a serious point of controversy until the mid 1930s when Doutreligne (1935), Weier (1936) and Heitz (1936) established beyond doubt that they were not artifacts.

The lamellar nature of granum became clear only after the early electron micrographs of chloroplasts were published by Granick and Porter (1947), Steinmann (1952) and some other workers.

Electron micrographs reveal that chloroplast consists of a system or series of fine pigmented plates or lamellae enclosed within the plastid membrane. The small stacks of pigmented lamellae are called grana (singular-granum). These are flat disc-shaped or cylindrical structures, 0.3 to 1.7 µ. in diameter and about 0.2µ in thickness.

There is considerable variation in the number of grana in a single plastid, depending upon the species. The primitive forms tend to have small number of grana than the higher forms. The granum consists of a few to more than fifty disc shaped superimposed parallel membranous compartments which are called thylakoids.

These membranous lamellae are frequently fused in a stack. In some cases, the granum extends like a cylinder across the entire width the plastid. The length of thylakoids may vary in an individual granum and often one or more compartments may extend from one granum to another (Fig. 3.5). Frequently, where thylakoids extend form granum to granum the entire complex may appear branched.

The membrane bound channels interconnecting thylakoids of adjacent grana are called the frets or intergranal lamellae or stroma lamellae. The grana are bounded by marginal end membranes which isolate the internal regions of the grana from the stroma.

The marginal end membranes of grana are continuous with the membranes if the channels interconnecting the thylakoids of the adjacent grana. Recent electron microscopic studies of chloroplasts have provided evidence that the membranes within the chloroplasts form a continuum and that the loculi (cavities of the thylakoids) of the grana communicate via fretwork connections (Fig. 3.6).

A granum may consist of many compartments but the simplest granum is defined as consisting of two thylakoids and recognized by the presence of a single partition separating two loculi. The entire complex of grana together with membrane bound frets is referred to as grana fretwork system.

The nature and organization of the inter granal membranes have been a focal point of discussion. The evidences accumulated from recent studies of the ultra-structure of chloroplasts indicate that frets are more comparable to flexible channels which are often branched and extend to and connect two or more grana, with the result that the fretwork appears as highly fenestrated sheets.

Close observations of the connections of stroma channels with grana revealed that they were often connected with two or more thylakoids of the same granum. The number of frets connected to an individual thylakoid vane and tangential and serial sections indicate that the frets are arranged in a spiral fashion around the grana.

As pointed out earlier, the grana are bounded on all sides and are in contact with the stroma by a membrane which is nearly 85 Å thick. The fret membranes also measure approximately 85 Å in thickness and are continuous with the marginal end membranes of the thylakoids. Thus, it appears that these membranes are structurally and functionally identical.

The partitions of grana are apparently a complex of two membranes which in many instances are separated by a space called A-space that measures about 24 Å in width. The partitions usually nm across the granum form one margin to the other but sometimes a small gap between the end of the partition and the marginal membrane of the granum is left which results in the confluence of the loculi of adjacent thylakoids.

The two membranes of partition may be due to invagination of the marginal membrane. However, the thickness of partition including the A-space is less than the sum of the thicknesses of two marginal membranes. The A-space may appear electron translucent or electron opaque depending upon the fixation, dehydration and staining methods used.

Several views have been put forth to explain the relationship between granum lamellae and stroma lamellae. According to Skinmann and Sjostrand (1955) stroma lamellae develop from granum discs. Hodge et al. (1955) expressed that granum discs are locally swollen thylakoids.

In some cases, the stroma lamellae may or may not terminate in granum and so they may be continuous or discontinuous (Fig. 3.6). As regards their origin, the stroma lamellae and grana lamellae are similar.

Several investigators have published micrographs which apparently indicate that the granal fret membranes are tripartite structures analogous to the unit membrane consisting of central electron translucent bimolecular layer of phospholipids and two layers of proteins, one on each side of lipid layer.

The chlorophylls and other pigment molecules are located in the trilamellar limiting membranes of thylakoids and protein matrix binds chlorophylls and lipids (Weier and Banson, 1967), [Fig. 3.7 (A)]. Several investigations have made it clear that chlorophyll is present in partitions and particularly along the A- Space.

Since A-Space is generally visible when chlorophyll is retained and diminished or lost when chlorophyll is removed, the observations support the suggestion that chlorophyll is a major component of A-space in the partition. Several lines of evidence indicate that chlorophyll is also present in the fret membranes.

The loculi and fret channels are known to swell or contract in response to hypotonic or hypertonic medium which indicates that the loculi and fret channels are hydrophilic and possibly similar in nature. The semi-permeable nature of end marginal and fret membranes indicates that these membranes are, if not identical, probably closely related.

The thickness of the partition varies in different species and it is greatly affected by different chemical reagents. The efficiency of photosynthesis is a function of the wavelength of impinging light. If the chloroplasts are simultaneously activated at wavelengths 6800 Å and 6000 Å, there is a marked stimulation of the photochemical reaction.

On this ground existence of two systems of light absorption have been postulated; photosystems I (PSI) which is stimulated at wavelength shorter than 700 nm and does not yield oxygen and photosystem II (PSII) which is stimulated at wavelength shorter than 680 nm and yields O₂. Photosystem I is made up of units containing about 200 molecules of chlorophyll a, 50 molecules of carotenoids and cyt. f and PC.

One molecule of pigment P 700 is related to this unit and that pigment molecule can trap exitons and release electrons. Photosystem II is made up of units containing about 200 molecules of chlorophyll a, some 200 molecules of chlorophyll b, c or d and molecules of xanthophyll, cyt.b₆ and P.Q.

The complete set of pigments and lipids containing two photosystems is thought to be clustered in structural and functional unit. Recent methodological advancements, special embedding technique in an aqueous medium, shadow cast technique in electron microscopy have revealed more information about the cytochemical localization of both photosystems.

Cytochemical investigations have indicated that the products of photo-oxidation accumulated in the loculi of the thylakoids which suggested that photosystem I was localized at the inner face of thylakoid membrane.

The photo-reduction of ferrocyanide which indicates the localization of photosystem II causes dense deposits in the partitions between thylakoids, indicating thereby possible localization of PS II at the outer surface of the membrane.

Steinmann with the help of shadowed preparations of isolated chloroplast lamellae demonstrated the existence of a repeating structure on the inner surface of the membranes of thylakoid. Park and his associates have shown this repeating structure to consist of closely packed arrays of particles or granules.

Each granule is oblate sphere shaped 185 A long, 155 Å wide and 100 Å thick with a molecular weight of 2 x 10⁵. This sub-unit has been termed quantosome by Park and co-workers and is considered the smallest photosynthetic particle which will perform the light reactions of photosynthesis.

The quantosomes seem to be buried in the lamellae of the membranes either evenly distributed as in chloroplasts of lower plants or in packets of grana and stroma lamellae as in the chloroplasts of higher plants [Fig. 3.7 (B)].

The quantaromes are the smallest units capable of carrying out the photochemical reaction and each of them contains about 200 chlorophyll molecules as well as enzymes necessary for the light reaction of photosynthesis. Although this is not certain at present whether quantosomes represent photosynthetic units within the thylakoid membranes.

Souer and Calvin (1962) have suggested that 3 to 6 quatosomes may aggregate together to form a large particle of 500 x 100 Å.

The quantasomes may be present on stroma lamellae also but in relatively much smaller number.

Branton and Park (1967) have recognized the following three types of membranes in the plastids on the basis of the size, nature and arrangement of quantosomes in the membranes of thylakoids:

(i) Membranes with quantosomes,

(ii) Membranes with smaller particles (110 A), and

(iii) Membranes with rough surface and with a few of no particles.

In effect, elementary particles of mitochondria can be homologised with quantosomes. Both of these particles demonstrate an association of electron transport system with the unit membranes and electron transport system is accompanied by phosphorylation.

Electron transport system contain electron carriers cytochromes b₆, b₃, f (a hydrophobic protein of 1, 00,000 daltons, isolated by non- polar solvents) tightly bound to membrane structures.

Chloroplasts also contain ferredoxin, a protein of about 11,600 daltons and containing iron and sulphur but lacking in porphyrin group, plastocyanine, a copper-protein complex and two quinones-vitamin k, and plastoquinones, the latter being similar to ubiquinone.

iii. Chromoplasts:

During the transformation of chloroplast into chromoplast, yellow coloured droplets termed as globulins appear first and in the course of development chlorophylls and starch gradually decrease, large globuli are formed and arranged along the plastid membrane, lamellar structure breaks down and stroma is disorganized resulting in the empty appearance of the plastid centre.

During the transformation of leucoplasts into chromoplasts certain fibrils appear which give rise to crystals filling up the whole Plastids. The crystals are found in the form of sheet-like structures containing large quantities of carotenoids.

Origin and Development of Plastids:

Many theories have been advanced to explain the origin and development of plastids.

(а) Monotropic development:

According to this theory, the plastids are autonomous structures which can change from one form to another in either reversible or irreversible way.

(b) Development of Plastids from Proplastids:

Plastids have received much attention in ecent years and much work has been done on their development by Muihlethaler (1957), Gustafsson (1961) and many others. Several workers have suggested that the chloroplasts or any other plastids develop from tiny cytoplasmic bodies called proplastids or plastid precursors.

These are structure-less bodies which are bounded by a double-layered membrane and contain a clear matrix. They resemble mitochondria to some extent but they differ from the latter in having flattened infoldings that run parallel rather than perpendicular to the surface.

The plastid precursors are self-duplicating and contain DNA and RNA. The proplastids of higher plants are nearly 0.5 µ in diameter. With the help of electron and florescence microscopes Epistin, Hyman and Schiff (1961) have observed the proplastids in dark grown Euglenae which were 1µ in diameter and were similar to the plastid precursors of the higher plants.

Proplastids develop into the chloroplasts when they are exposed to light. Various stages in the development of chloroplast are illustrated by a series of diagrams in Fig. 3.8.

The proplastids develop first in the cytoplasm of meristematic cells. On exposure of these cells to light the proplastids appear to lengthen and there develop ‘blebs’ from the inner side of the double layered membrane. Starch grains now appear inside them.

According to Von Wettstein (1959), the steps involved in the development of chloroplasts from proplastids are as follows:

(i) Synthesis of vesicles within the plastid primordia,

(ii) Aggregation of these vesicles to form long chains of interconnected vesicles,

(iii) Rearrangement and fusion of the chains to form parallel double membranes or lamellae,

(iv) Multiplication of lamellae by linear splitting, and

(v) The growth and differentiation of lamellae into grana and stroma lamellae and in the meantime break down of starch also occurs. In higher plants the lamellae of chloroplasts appear to develop in bundles and form replicas (grana) in parallel geometric pattern.

The general pattern of chloroplast formation is similar in all the higher plants. In short, all the pigments are concentrated in grana and the grana are distributed in stroma which forms the body of chloroplast.

(c) Division and Budding:

In cryptogams new plastids originate by division and budding of the mature chloroplasts. During the division ring-like constriction appears which gradually deepens and finally divides the plastid into fragments (Fig. 3.9).

The fragments then differentiate into mature plastids. Budding of plastids takes place only under certain special circumstances, as for example, during the regeneration of a plant from excised leaves.

(d) Nuclear Origin:

Many proplastid-like bodies have been observed near the nucleus. This point has led some workers to support that proplastids originate from the nucleus due to evagination of the nuclear wall.

(e) Symbiotic Origin:

The plastids possess the essential attributes of a prokaryotic cell. They possess their own DNA, RNA, ribosomes and protein synthesizing machinery. Mutations can occur in their hereditary material causing alterations in their structures and functions.

On the basis of these striking similarities between the plastids and prokaryotes, it has been suggested that plastids have evolved from some prokaryotes like blue-green algae which have permanently settled in the plant cells.

The presumed symbiotic cells in the process of becoming plastids underwent several changes during which their autonomy was restricted and part of the genetic control of their development was assumed by the nucleus of the host cell.