Pollen Morphology of Gymnosperms

Gymnosperms along with angiosperms are classified as seed plants. Seed plants can be simply defined as plants producing seeds. The seed is an organ originated from an ovule. It has a resistant outer seed coat (testa) enclosing mainly a nourishing tissue called endosperm for the developing embryo.

Thus, ovule can be defined as a megasporangium (nucellus) containing a single functional megaspore (embryo sac) enveloped by coat (integumant). In other words, an ovule develops into a seed after fertilization in seed plants. In fact achievement of seed habit from heterospory is a very important step in the evolution of plant life. This is accompanied by evolution of pollen ultimately resulting in the diversity of plants. Conventionally microspores produced in the microsporangia of male cones in gymnosperms are termed as pollen grains. Paleobotanical evidence shows that fossil pollen of almost all groups of gymnosperms such as cycads and conifers in addition to extinct pteridosperms have been well preserved in the sediments dating back to the Devonian period.

As mentioned earlier, basically the terminology used for describing pollen morphological characters in gymnosperms and angiosperms is the same. In the present chapter only a brief account of pollen morphological variation observed in various groups of modern and fossil groups of gymnosperms will be given. Fossil gymnospermous pollen have been extensively used in palynostratigraphy, which are of great significance in the exploration of fossil fuels such as coal and oil.

It seems appropriate to discuss pre pollen and pollen of pteridosperms or the seed ferns before describing the pollen morphology of modern gymnosperms. The geologically oldest pollen has been reported from the Devonian rocks. This pollen is often referred as pre pollen. It appears to have retained the morphology of spores and germinated proximally as do spores of vascular plants. Chaloner (1970) has summarized the distinction between spores, pre pollen and true pollen and their significance in plant evolution. There appears to be evolutionary trend from proximal emergence of pollen tubes (proximal germination) in pre pollen to distal emergence of pollen tubes (distal germination) in true pollen occurring in advanced gymnosperms.

POLLEN MORPHOLOGY OF FOSSIL GYMNOSPERMS (THE PTERIDOSPERMS OR CYCADOFILICALES)

The most outstanding characters of the pollen grains of the cycadofilicales are their large size and pluricellular structure. There is a lot of variation in their size, ranging from 70-500 mm in diam. in the different species which have been described. There is also variation in their cellular structure; in some the cavity of the grain contains only two cells, while in others it contains as many as 30. Nevertheless, it is almost universally true that the entire cavity is filled with cellular tissue with well-developed cell walls.

Many of the pollen grains of this group are found in the pollen chamber, which probably increased in size after reaching there. Saporta and Marion (1935) have described pluricellular pollen grains in the pollen chamber of Pachytesta, which are 500 mm long. In the pollen chamber of Aetheotesta, Renault (1896) described similar grains about 400 mm long and without exines. The pollen grains are thus fairly large compared to that of angiosperms.

Cycadofilicinean pollen grains from the staminate inflorescenes of Crossotheca were one-celled and in all their characters which have been preserved, they are remarkably like many of the fern spores of today. In an exceptionally well-preserved fructification in coal ball, Rothwell (1972) has shown the occurrence of pollen tubes in pollen Idanothekion of Callistophytaceae. It seems likely that in the cycadofilicales the pollen grains left their anthers as single celled spores, and germination, or any extensive cellular proliferation, did not take place until after they arrived at their destination.

The next stage of the evolution of the pollen grains was accompanied by a reduction of the prothallial tissue until it became represented by two or three nuclear divisions without the formation of any cell walls, which took place prior to shedding, for example in the grains of cycads and Ginkgo, leaving the broad furrow, which had been provided to accommodate it a useless organ and very probably an encumbrance to the grain.

DESCRIPTION OF POLLEN GRAINS FOUND IN CROSSOTHECA HONINGHAUSII

Grains globular or slightly oval, 50-70 mm in diam. Their outer surface roughened by numerous closely placed, very minute, blunt points. Each grain is provided with a distinct triradiate crest, though this is often difficult to see on account of the crumpling of the pollen grain wall (Fig. 6.1). It is believed that the presence of the triradiate crest indicates that these grains were developed as members of tetrahedral tetrads. Nucellular tissue is not observed in any of them, but this may be due to the fact that they were immature when fossilized, since they were dissected out of unopened anthers. Crossotheca hoeninghausii is known to be the staminate inflorescence of Lyginodendron oldhamium Williamson.

Stephanospermum caryoides: Grains found in the pollen chamber of the seed. Grains ellipsoidal, flattened 91 x 72 mm provided with a wind like bladder, which completely encircles the grain (looks like grains of Podocarpinae) (Fig. 6.2).

Pachytesta gigantea Renault. Grains ellipsoidal, 320-400 mm long and 270-310 mm broad. One sees in the interior a number of walls dividing the cavity into a certain number of cells. The grains do not have any exine (Fig. 6.3).

POLLEN OF CORDAITALES

Fig. 6.1 Pollen of Crossotheca Honinghausii showing triradiate mark.

Fig. 6.1 Pollen of Crossotheca Honinghausii showing triradiate mark.

Fig. 6.2 Monosaccate pollen of Stephanospermum caryoides.

Fig. 6.3 Pollen of Pachytesta gigantea showing pluricellular internal structure.

Fig. 6.3 Pollen of Pachytesta gigantea showing pluricellular internal structure.

The pollen grains of the Cordaitales are generally ellipsoidal in shape, rather large, measuring about 100 mm in length, and with a characteristically roughened exine. They are always provided with a single, deep, longitudinal furrow and exhibit a pluricellular internal structure.

As compared with the grains of Cycadofilicales they possess certain rather striking differences. There is generally a little less prothallial tissue, and its development takes place earlier. There is no doubt that the germination of cordaitalean pollen grains took place prior to their release from the anther.

These pollen grains possess a single longitudinal furrow, which appears to function as a proliferation chamber, enabling the prothallus to develop without rupturing the spore wall, as it did in the grains of the Cycadofilicales, which were without a furrow. In the grains of Cordaitales the floor of the furrow became pushed up by the development of the prothallus within, finally separating from the rest of the surface of the pollen grain along its rim and opening as a lid, permitting the escape of the antherozoids.

In these characteristics the pollen grains of Cordaitales show a distinct advance over those of the Cycadofilicales, and a step toward the form of the conifers and angiosperms. But they still differ from the grains of the conifers in their pluricellular internal structure with well-developed walls partitioning off the whole device for their wide-open furrow.

Dolerotheca Renault.: Grains ellipsoidal, about 280 mm long. Exine finely roughened.

On one side are two deep furrows joining together at one end and marking out an elliptical shaped operculum by which dehiscence may take place. The interior of the grain is divided into 8-10 cells with thin walls (Fig. 6.4).

Fig. 6.4 Pollen of Dolerotheca fertile with longitudinal furrow.

POLLEN MORPHOLOGY OF BENNETTITALES

The pollen grains of the Bennettitales are, with one or two exceptions, scarcely different from those of the cycads. They are boat shaped and provided with a single longitudinal furrow as in Cycadeoidea etrusca (Fig. 6.5) and Cycadeoidea dacotensis (Fig. 6.6a), which appears to have been as ineffective in closing as that of the cycads and Ginkgo. Their prothallial tissue was much less extensive than in the grains of Cycadofilicales, or even of the Cordaitales but considerably more extensive than that of the Cycadales as in Cycadeiodea dacotensis (Fig. 6.6b).

In size, they range from 20-67 mm in length. They are thus generally a great deal smaller than those of the Cycadofilicales or even the Cordaitales

Fig. 6.5 Pollen of Cycadeoidea etrusca showing longitudinal furrow.

(b)

Fig. 6.6 Cycadeoidea dacotensis pollen: a. Showing two longitudinal furrows, b. Optical section of pollen showing multicellular internal structure.

but are larger than those of the Cycadales and Ginkgoales. In the development of their prothallial tissue and their size they thus occupy a position intermediate between the Cycadofilicales and the Cycadales. Though they are found in the Carboniferous, they become more abundant in the Mesozoic.

MORPHOLOGY OF POLLEN OF EXTANT GYMNOSPERMS

Living gymnosperms are broadly divided into four orders: Cycadales, Ginkgoales, Coniferales and Gnetales. Prominent pollen morphological features of certain members of these orders will be described here.

POLLEN GRAINS OF CYCADALES

In Cycadales the pollen grains are small, broadly ellipsoidal, boat shaped, with a single deep longitudinal furrow reaching from end to end, essentially as in the grains of Ginkgo. The pollen grain is bilaterally symmetrical in the sense that its two sides and two ends are exactly alike; but its remaining two sides are dissimilar, since one of them bears the furrow and other does not.

When the pollen grain is dry, the edges of the furrow arch inward toward each other, tending to close the opening, and may even touch in the middle. When the pollen grain is moistened the furrow gapes widely open as in Cycas (Figs. 6.7 - a, b, c). In Zamia (Fig. 6.8) and Ceratozamia the spore coat is quite smooth. In Cycas it is minutely warty on the outside suggesting the flecked surface of the grains of Juniperus.

In the grains of the cycads the exine is thinner than in other grains. This one furrowed or monocolpate type of grain, besides occurring throughout the Cycadales, is characteristic of many Monocotyledons and primitive Dicotyledons for example the Palmaceae, Magnoliaceae and Nymphaeaceae.

Cordaitales ExampleSmooth Pollen Grain

Fig. 6.7 Pollen of Cycas: a. Ventral view showing wide, longitudinal furrow, b. Longitudinal view, c. Optical section showing furrow.

Ginkgoales

The pollen grains of the Ginkgoales, like those of the cycads, are notable for their single deep and broad unprotected furrow and their lack of prothallial tissue (Figs. 6.9 - a, b, c). At maturity they exhibit nothing of the pluricellular tissue, which characterized the grains of the extinct Cordaitales and Bennettitales. The male gametophyte is reduced to nominally three cells, that is one vegetative, one generative and one tube cell (Fig. 6.10).

Fig. 6.8 Polllen of Zamia showing longitudinal furrow.

Longitudinal Furrows

Fig. 6.9 Pollen of Ginkgo biloba: a. Lateral longitudinal view, b. Ellipsoidal monosulcate pollen, c. Median longitudinal section showing a single furrow.

The furrow, on the other hand, is of practically the same form as that of the grains of the Cordaitales, except that there being no internal tissue developed within, it remains deeply invaginated until the pollen tube begins to emerge.The pollen grains of Ginkgo biloba are similar in all their major features to that of Cycas but may be distinguished from the latter by their more elongated shape, smoother surface, and the slightly wavy margins of its furrow.

Ginkgo Biloba
Fig. 6.10 Section of pollen of Ginkgo biloba at shedding stage. (Three-celled male gametophyte).

CONIFERALES

The pollen grains of the Coniferales are known for their extraordinary diversity of form. As compared with the grains of the more primitive gymnosperms, we find little trace of the pluricellular gametophytic tissue, which was very evident among the Cycadofilicales and Cordaitales.

POLLEN DEVELOPMENT

In majority of conifers the tetrad of microspores is produced by simultaneous division resulting in a tetrahedral arrangement of the developing pollen grain. In this tetrad the proximal pole (ventral) is toward the centre and the distal pole (dorsal) is away from the centre of the tetrad. However in nonsaccate pollen, polarity of mature pollen grains is not very clear. The tetrahedral type includes both saccate and nonsaccate pollen whereas the bilateral type produces only nonsaccate pollen. The pollen wall in conifers and other gymnosperms has an outer exine composed of sporopollenin and an inner intine consisting of hemicellulose.

SACCATE POLLEN

In living conifers, pollen that develops two or more, usually hollow, extensions of the infratectum (sacci, wings or bladders) occur only in Phyllocladus, Pinus, Picea, Abies, Cedrus, Pseudolarix and Podocorpus. The pollen lack sacci in Larix, Tsuga, and Pseudotsuga and Saxegothaea. The sacci are developed laterally from the body of the grain, that is their proximal limit determined by the limits of contact of the cells in the original tetrad as shown in Fig. 6.11.

In saccate pollen of gymnosperms normally there are two sacci. However three sacci occur in pollen of Dacrycarpus, Microstrobus, and Microcachrys. In the bisaccate pollen of Pinus the proximal part is described as the 'cap',

Polar, axis

Pollen Organ Callistophytaceae

Ventral root of sac Furrow or sulcus

Fig. 6.11 A typical bisaccate pollen showing different features.

Polar, axis

Dorsal root of sac

Ventral root of sac Furrow or sulcus

Fig. 6.11 A typical bisaccate pollen showing different features.

the distal part, delimited by sacci is referred as 'germinal furrow' (Fig. 6.12 and 6.12a). The inflated portion of the saccus is supported internally by the alveolar or honeycomb-like structure of the ektexine. The outer surface of the saccus is smooth but minutely porous.

Gymnosperm Longitudinal Section

PROXIMAL

Fig. 6.12 Mature saccate pollen grain of Pinus in median section showing different features.

PROXIMAL

Fig. 6.12 Mature saccate pollen grain of Pinus in median section showing different features.

The non-saccate pollen is essentially spherical or biconvex and of simple organization. The intine in this pollen is much thicker than the granular exine. The pollen of Araucariaceae has a moderately thick intine and little differentiated sexine. The pollen grains are multicellular.

In the Abietineae, the wings when present are nearly always two, one on each side of rur , c . r . .. Fig. 6.12a Pinus Pollen (SEM).

the furrow and forming for it a protective cover when the grain dries. While in the grains of the Podocarpineae, there may be 2, 3, 4, 5 or 6 bladders, and in the pollen of one species of Podocarpus some grains have a single bladder encircling them completely like a frill. But in both tribes there are genera with grains entirely lacking bladders. Similar wings are known to have occurred on the microspores of some Palaeozoic Cycadofilicales and Lycopodiales and still occur on the spores of some of the modern ferns and lycopods, and their presence among the Coniferales suggests the great antiquity of the group.

The capacity to develop wings has apparently been inherited from the remote past, but only in these few genera has it been called forth here, apparently, in response to a need of protection for the broad, open furrow with which such wings are here associated. Modern gymnosperm families characterized by inaperturate pollen are the Araucariaceae, Cephalotaxaceae, Cupressaceae, Taxodiaceae and some genera of the Pinaceae.

Araucariaceae

The Araucariaceae are undoubtedly among the most ancient living conifers. The pollen grains of both Araucaria and Agathis are without a true furrow or pore. In the grains of Agathis no vestige of a furrow can be found, but in those of Araucaria there is an annular thickening which corresponds in position to the furrow rim in the expanded Cycad grains and which appears to be the vestige of the cycadean furrow rim. The pollen grains are approximately spheroidal, non-saccate without any vestige of a germ pore.

Abietineae

Within the group are found three distinct types of pollen grains. The grains of five genera such as Pinus, Cedrus, Picea, Abies and Pseudolarix resemble each other in their common possession of bladders and in all the major features of their construction, such as the differentiation between the dorsal and ventral side and their possession of a single long furrow. In contrast to these, the pollen grains of Tsuga have no true furrow or wings but resemble those of the winged grained Abietineae in the character of their exine. They are also referred as monosaccate. The pollen grains of Larix and Pseudotsuga are also entirely without furrow or bladders having perfectly smooth exine and rather thick intine.

Taxodiaceae

The pollen grains of both the Taxodiaceae and Cupressaceae are entirely without prothallial cells, which decidedly has given them a modern aspect among the Coniferales. A unique feature of some of the pollen of Taxodiaceae is the presence of a conical Fig. 6.13 Non saccate pollen projection (termed papilla or ligula) rising of sequoia showing papilla-like from the spherical body of the grain as in projection. Sequoia (Fig. 6.13). The wall layers at the tip of the papilla are thinned where a germinal pore may be present.

The pollen grains of Taxodiaceae are nearly spherical or sometimes angular with thin exine and thick intine. In the grains of Taxodium the furrow appears to be reduced to vanishing point, represented by only a small protuberance.

Podocarpaceae

The pollen grains of the Podocarpaceae are known for the possession of bladdery wings in most of their species, causing them to bear a superficial resemblance to the grains of the Abietineae. But in neither the Abietineae nor the Podocarpineae are the grains of all species winged.

Podocarpus sp. usually have two wings. P. dacrydioides normally have three wings, Microcachrys the bladders may be 3-4-5-6. Phyllocladus - two bladders. Saxegothaea has no wings. The bladders are associated with an ancestral type of furrow. The pollen grains of Podocarpus have two lateral sacs, which float upwards in a fluid as in Pinus. The pollen grains of Saxegothaea are more or less spherical and non-saccate (Fig. 6.14).

Cupressaceae

The pollen grains of cupressaceae are similar to those of Taxodiaceae. The intine is very thick.

Cephalotaxaceae

In Cephalotaxus the plant is monoecious. The pollen grains are round with thin exine and thick intine. When the grains are left in water or in a weak sucrose solution, the exine cracks and the intine and its contents are released.

Pinaceae

Pinus: It has characteristic needle-like leaves (Fig. 6.19). Pollen grains are fairly large and possess two ventrally borne air-filled sacs with a single long furrow situated between them (Fig. 6.20). The dorsal surface has a thick and rugged exine. The sacs diverge sharply in moist conditions, but in dry pollen they are pressed together. The sacs on their inner surface are thrown into a reticulum of ridges, which impart rigidity and prevent them from collapsing. The pollen grains of Pinus are very buoyant due to the presence of sacs. They rise upwards and float in the fluid. Thus winged pollen grains are characteristic of Pinus and other genera of Pinaceae. Whether or not wings are organs of flight is doubtful though they do impart a buoyancy to the grains.

Takeshi N. et al. (1996) thoroughly explored the pollen morphology of modern gymnosperms from the Himalayan region. Pinus roxburghii and P. wallichiana pollen grains were examined by them with Scanning Electron Microscope (SEM) in an attempt to identify fossil pollen grains of the Himalayan region. The exine sculpture of P. roxburghii is rugulate whereas that of P. wallichiana is smooth or slightly rugulate.

The Morphology Gymnosperms
Fig. 6.14 Saxegotheca pollen which is spherical, non saccate
Morphology Gymnosperms

side

Fig. 6.15 Pinus pollen showing details of measurement of various dimensions.

side

Fig. 6.15 Pinus pollen showing details of measurement of various dimensions.

Various GymnospermsMicroscope Pollen Identify
Fig. 6.19 A branch of Pinus with needles. Fig. 6.20 Bisaccate pollen of Pinus.

Various dimensions of Pinus pollen grains, which are taken into account, were clearly described by them as shown in the Fig. 6.15. They stated that saccus height, saccus width at base, corpus breadth, and surface sculpture of 'Cappa' (proximal side of corpus) were the important characteristics for the description of the Himalayan Pinus pollen grains. It is also known that the size and orientation of body and sacci differ in different species of Pinus (Figs. 6.16 and 6.17 and Table 6.2).

In Larix and Pseudotsuga the pollen grains are non-saccate and larger but they too, are borne by the wind as effectively as the saccate ones. In gymnosperms the non-saccate condition is common but this in no way hinders the pollen flight. These sacs fold under dry conditions during pollination. Wodehouse (1935) remarks, "If the bladders are organs of flight, pollen grains are possibly the only flying organisms of which it can be said that they fold up their wings and fly away". According to Doyle and O'beary (1935) and Doyle (1945) the sacs have a definite floation function, which is adapted to the inverted position of the ovules. The distended sacs of the pollen grain make it float up in the fluid - filled inverted micropyle. In the process the sacs point upwards so that the germ pore comes directly in contact with the nucellus.

MORPHOLOGY OF SACCATE POLLEN AND POLLINATION IN CONIFERS

Saccate pollen are restricted to, but not ubiquitous in, Phyllocladaceae, and are associated with the development of an inverted pollination drop except in Phyllocladus.

Tomlinson (1994) studied these pollen grains with greater emphasis on the pollination mechanism. He found that the saccate pollen is also non wettable, in the sense that it floats on water and it functions as a distinctive antigravity device. Pollen entry into the micropyle is dependent on resorption of the pollination drop. In contrast, non-saccate pollen is wettable and sinks in water. In coniferous families such as Cephalotaxaceae, Cupressaceae, Sciadopityaceae, Taxaceae and Taxodiaceae, there is a pollination drop without preferred orientation. Pollen swells and ruptures in water, shedding the exine by a mechanism determined by the apparently simple wall structure. In nonsaccate pollen of other conifers such as Agathis, Araucaria, Larix, Pseudotsuga, Tsuga and Saxegothaea there is no pollination drop, though pollen is wettable. They do not rupture in water and there is either extended siphonogamy or micropylar invagination (Table 6.1).

Pollen reception in most of the living gymnosperms is mediated by pollination drop, exuded by the micropyle of the ovule. Therefore, the site of pollen reception is wet, and in a way pollination is hydrophilous. Doyle (1945) had emphasized the significance of the pollination drop, showing the diversity of mechanisms that exist for the engulfment of the pollen by the micropyle. An overview of gymnospermous pollen with special emphasis on conifers was summarized by Ueno (1960).

Picea

The pollen grains are larger but similar to those of Pinus. The sacs are small in proportion to the size of the grain. The furrow forms a shallow groove between the sacs (Wodehouse 1935).

Table 6.1 A summary of information on coniferous pollen is incorporated below.

Pollen Structure

Family

Grains nonsaccate; exine granular:

Intine considerably thicker

than exine; numerous orbicules

adhering to the exine

Cephalotaxaceae, Cupressaceae,

Sciadopityaceae,

Taxaceae, Taxodiaceae

Intine equal to or somewhat

thicker than exine; orbicules

usually absent

Araucariaceae

Grains usually saccate; exine

tectate, alveolar

Phyllocladaceae, Pinaceae,

Podocarpaceae (sacci

absent: Larix, Pseudotsuga,

Saxegothaea, Tsuga)

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Responses

  • allan milne
    Are pollen grains present in seed plants?
    5 years ago
  • Amerigo
    Does conifers have pollen grain?
    5 years ago
  • Esmeralda
    How pre pollen is differ from pollen and spore?
    3 years ago
  • abdullah nuguse
    Why do pollengain of pinus contain two lateral tilted bladder like?
    3 years ago
  • Ulrich
    Can plants be grown from fossilised pollen?
    3 years ago
  • wolfgang junker
    What is the role of the air sacs on pollen grains of pinus?
    3 years ago
  • callimaco genovese
    What are angiosperm pollen grains?
    3 years ago
  • zahra
    Which gymnospermous family has winged pollens?
    3 years ago
  • xander dickson
    What is winged pollen grain?
    3 years ago
  • daniela
    What are winged pollen grains?
    3 years ago
  • savanna
    Which gymnosperm polengrain has wings?
    3 years ago
  • dennis reinhardt
    Why the pollen.grain of pinus us winged?
    2 years ago
  • juho
    What makes intine of pollen so rigid?
    2 years ago
  • Federico
    What is winged pollen grains in pinus?
    2 years ago
  • amy
    Which gymnosperm pollen grain has wings?
    2 years ago
  • valentin
    Which Gymnosperm produces winged pollen?
    2 years ago
  • dewayne marion
    Which type of pollen grains found in pinus wallichiana?
    2 years ago
  • esa
    What is prothalial cell in pollen?
    2 years ago
  • tiblets
    What is the fuction of prothallial cell in pollen of gymno?
    2 years ago
  • emilia
    What is pollen chamber in gymnosperms?
    2 years ago
  • jonathan
    Is a pollen grain multicellular?
    2 years ago
  • Medhane Abaalom
    Why variation occure in pollen grain of gymnosperm?
    2 years ago
  • stewart
    How many saccate pollen grains are present in pinus?
    2 years ago
  • paul gilbert
    What is intine in the pollen grain of pine?
    2 years ago
  • myrtle
    Why do only some pollen grains have wings?
    2 years ago
  • armas
    What are polen drops in gymnosperm?
    2 years ago
  • panu
    How can gymnosperms pollen be distinguished from angiosperm pollen?
    2 years ago
  • tobias
    What are the types of variation in gymnos structure?
    2 years ago
  • BETTIE
    Why do angiosperm pollen grains not have wings?
    2 years ago
  • lawrence
    What is the function of pollen chamber in gymnosperm ovules?
    1 year ago
  • kelsey king
    What is the function of pollen chamber of cycus sl.?
    1 year ago
  • JERE
    What is winged pollen grains in gymnosperm?
    1 year ago
  • sheshy
    Which plants contain winged pollen?
    1 year ago
  • Lyle
    What is the example of winged pollan grain?
    1 year ago
  • semhar massawa
    Which plantshave winged pollen grains?
    1 year ago
  • joshua
    Which plant contain winged pollen grains?
    1 year ago
  • JOHN
    Is winged pollen grains found in cycas?
    1 year ago
  • stanley milne
    Does pollen grains of pinus have wings?
    1 year ago
  • HENRIK PAATELAINEN
    What is a pollen grain in gymnosperms?
    1 year ago
  • HAMID
    Why pollen grains are found in form of fossils?
    1 year ago
  • Louise
    How does the gymnosperm transport the pollen grain to the female?
    12 months ago
  • stephan
    When do pollen grains become completely developed in angiosperms?
    11 months ago
  • finlay davidson
    How are the pollen grains of angiosperms different from the pollen grains in a pine tree?
    10 months ago
  • Gerard
    Do pollen/cone morphologies of gymnosperms change with elevation?
    9 months ago
  • teodros
    Why would anyone care about the structures of the conifers pollen?
    6 months ago
  • folcard
    Which of the following gymnosperm possesses wing pollen grains?
    4 months ago
  • Razanur
    What is the type of pollen found in gymnosperm?
    25 days ago

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