Angiosperms are further divided into monocots and dicots. Monocots have one seed leaf. Dicots have two seed leafs. There are at least , species of angiosperms ranging from small flowers to enormous wood trees. Pollination is accomplished by wind, insects, and other animals.
The male part is the pollen grain, and the female part is the ovary. The ovary goes through meiosis to produce an "egg", which is them fertilized by the "sperm" carried by the pollen. The sperm of the male part travels down the pollen tube in the style. Two sperm enter the micropyle of the ovary.
After the process of mitosis, it turns into a seed with an embryo. Cycad leaves : This Encephalartos ferox cycad has large cones and broad, fern-like leaves. The single surviving species of the gingkophytes group is the Gingko biloba. Its fan-shaped leaves, unique among seed plants because they feature a dichotomous venation pattern, turn yellow in autumn and fall from the tree.
For centuries, G. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are produced on separate plants. Typically, gardeners plant only male trees because the seeds produced by the female plant have an off-putting smell of rancid butter. Gingko biloba is the only surviving species of the phylum Gingkophyta.
Gnetophytes are the closest relative to modern angiosperms and include three dissimilar genera of plants: Ephedra , Gnetum , and Welwitschia. Like angiosperms, they have broad leaves.
In tropical and subtropical zones, gnetophytes are vines or small shrubs. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs. Like angiosperms, but unlike other gymnosperms, all gnetophytes possess vessel elements in their xylem. Privacy Policy. Skip to main content. Seed Plants. Search for:. Characteristics of Gymnosperms Gymnosperms are seed plants that have evolved cones to carry their reproductive structures.
Learning Objectives Discuss the type of seeds produced by gymnosperms. Key Takeaways Key Points Gymnosperms produce both male and female cones, each making the gametes needed for fertilization; this makes them heterosporous.
Megaspores made in cones develop into the female gametophytes inside the ovules of gymnosperms, while pollen grains develop from cones that produce microspores. Conifer sperm do not have flagella but rather move by way of a pollen tube once in contact with the ovule. Key Terms ovule : the structure in a plant that develops into a seed after fertilization; the megasporangium of a seed plant with its enclosing integuments sporophyll : the equivalent to a leaf in ferns and mosses that bears the sporangia heterosporous : producing both male and female gametophytes.
Life Cycle of a Conifer Conifers are monoecious plants that produce both male and female cones, each making the necessary gametes used for fertilization. Learning Objectives Describe the life cycle of a gymnosperm. Key Takeaways Key Points Male cones give rise to microspores, which produce pollen grains, while female cones give rise to megaspores, which produce ovules.
The gametophyte itself is surrounded by layers of sporangia and integument; all of these elements comprise an ovule, which is found on the surface of a female cone. Fertilization occurs when pollen grains male gametophytes are carried by the wind to the open end of an ovule, which contains the eggs, or female gametophyte.
There, the pollen grain develops an outgrowth called a pollen tube, which eventually penetrates to the egg cell within one of the archegonia.
The sperm cells within the pollen tube then vie to fertilize the egg. Once fertilization has occurred, the embryo develops within the female gametophyte, and the ovule becomes the seed, complete with a food source the gametophyte tissue and a seed coat the integument. This embryo, which will eventually become a new sporophyte, consists of two embryonic leaves, the epicotyl and hypocotyl.
The female reproductive organ of angiosperms is the pistil, located in the middle of the flower. Such fertilization fluids were probably found among many extinct plants such as ancient cycads and others with swimming sperm, but were subsequently lost upon the evolution of siphonogamy direct delivery of sperm to the egg by pollen tubes , as found in modern gnetophytes, conifers, and Pinaceae.
Pollination drops are discussed in terms of three major types of PCMs and the unique combinations of morphological and biochemical adaptations that define each. These include their amino acids, sugars, calcium, phosphate and proteins. The evolution of PCMs is also discussed with reference to fossil taxa. The plesiomorphic state of extant gymnosperms is a sugar-containing pollination drop functioning as a pollen capture surface, and an in ovulo pollen germination medium.
Additionally, these drops are involved in ovule defense, and provide nectar for pollinators. Pollination drops in anemophilous groups have low sugar concentrations that are too low to provide insects with a reward.
Instead, they appear to be optimized for defense and microgametophyte development. In insect-pollinated modern Gnetales a variety of tissues produce sexual fluids that bear the biochemical signature of nectar.
Complete absence of fluid secretions is restricted to a few, poorly studied modern conifers, and is presumably derived. Aspects of pollination drop dynamics, e.
Large gaps in our current knowledge include the composition of fertilization fluids, the pollination drops of Podocarpaceae, and the overall hydrodynamics of sexual fluids in general.
Fluids play major roles during reproduction of gymnosperms. Ovule-derived fluids are almost universally found in pollen capture mechanisms PCMs. In addition, early diverging gymnosperms are dependent on fluids for fertilization, not just for pollen capture.
Before looking at the nature and complexity of these aqueous fluids it is necessary to introduce some of the aspects of reproduction that are unique to gymnosperms, beginning with pollination and then proceeding to fertilization.
A critical feature of gymnosperm pollination is that in almost all species the primary capture surface for pollen is an ovular secretion Williams, Generally, this is called a pollination drop Singh, Some angiosperm ovules are able to secrete fluids that influence pollen tube behavior Franssen-Verheijen and Willemse, Ovules secrete a fluid that fills the micropyles, which attracts pollen tubes into the ovule where the pollen tube breaches the relatively thin nucellus before depositing male gametes into the embryo sac.
Angiosperm ovular secretions are relatively unknown compared to pollination drops of gymnosperms. These liquid-based interactions between ovule and pollen are likely to be of ancient origin. Pollination drops provide a number of conserved functions that are essential components of mechanisms involved in pollen capture, delivery, and germination. Pollination drops also provide ovule defense against microbes during reproduction Little et al.
Figure 1. Chronogram of the extant genera of gymnosperms based on Lu et al. Blue branches represent presence of pollination drops sensu lato i. Gray branches represent missing data. Yellow branches represent well-studied taxa that have been reported to lack nucellar ovular fluids in their pollination pollination drops, sensu lato. Green branches represent free-sporing sex, whether homo- or heterosporous.
Purple branch for angiosperms represents flower-based sex; the origin is based on one of the divergence times from Clarke et al. Note that Saxegothaea , and Araucariaceae lack drops. Extinct fossil seed plants not shown; the earliest plants with seed-like structures appear in the Upper Devonian. Figure 2. Pollination drops A Ginkgo biloba , B Ceratozamia hildae , C Tetraclinis articulata , D Pseudotsuga menziesii post-pollination prefertilization drop , E Taxus x media scanning electron micrograph by A.
Lunny , F Gnetum gnemon female, G G. A distinctive aspect of some gymnosperms, and one that we will develop further in this review, is that ovules are able to secrete pollination drops that also double as attractants to pollinators. Gymnosperms that are insect-pollinated fall into two types: those that are ambophilous, i. Although pollination drops mediate pollen capture in both types, among those that have obligate pollination mutualisms is a group of gnetalean species that reward pollinators with nectar produced by ovules Kato et al.
The evolution of nectar from pollination drops is unique to gymnosperms and will be discussed in greater depth. In addition to fluid produced during pollination, ovules may also produce fluids during fertilization. Fertilization fluids are common to archegoniate plants, e.
These plants reproduce by means of eggs that are found inside the archegonium, the female sex organ whose presence sets gymnosperms apart from angiosperms. The structure of an archegonium is simple. A well differentiated, relatively large egg is found at the base.
Above the egg, in the case of gymnosperms, is one cell; in the case of mosses and ferns, there are two cells. These cells are surrounded by neck cells, which are an adaptation to fluid-based reproduction. Upon wetting, neck cells part to allow the contents of the cells above the egg to be released. Sperm swim down this now fluid-filled passage to the egg where fertilization takes place. Whereas ferns and mosses need free water to reproduce, gymnosperms, such as Ginkgo and cycads, produce their own fluid.
In short, reproduction with archegonia requires an aqueous medium for sperm delivery. Eventually, gymnosperm groups evolved for which this fluid requirement was bypassed. Water is the most abundant molecule in a sexual fluid, and is important to both fertilization and to pollination in gymnosperms.
However, this water is mainly a solvent for compounds that influence microgametophyte-ovule interactions. As mentioned above, early diverging embryophytes, such as mosses and ferns, are entirely dependent on water for reproduction. Since their sperm need water in which to swim it would at first appear that they do not contribute sexual fluids to this process.
However, mosses and ferns release a fluid from their archegonia that is developmentally timed to assist in fertilization. When an egg ripens, the other cells within the archegonium and above the egg, i. The contents of these dead cells are released into the surrounding free water after the necks have separated. Contents of the dead cells further improve the chances of fertilization by creating the chemical gradients that set up sperm chemotaxis.
Moss sperm were thought to be attracted to archegonia by a gradient of released sucrose Ziegler et al. Recently, Ortiz-Ramirez et al. However, the specific ligand released by the archegonia that triggers this chemotactic response by the sperm remains unknown. Archegonial secretion of chemoattractants also occurs in some gymnosperms Figure 3. Gymnosperms such as cycads release fluids during fertilization Chamberlain, One such fluid is that released by megagametophyte tissues surrounding their archegonia Takaso et al.
This fluid fills the specialized fertilization chamber in which the archegonia are found Figure 3. Once this chamber is filled, sperm are released from the pollen tubes and the archegonial neck cells divide forming a four-celled neck apparatus, centrally open to the egg.
Archegonia release copious amounts of a white-colored substance that appears to play a role in chemotaxis Takaso et al. Swimming sperm delivery via a microgametophyte with haustorial pollen tubes is known as zooidogamy and is characteristic of earlier diverging gymnosperms Williams, , such as Ginkgo and cycads.
More derived gymnosperms produce gametes that are delivered by a linear pollen tube, but these gametes lack flagellae and, therefore, cannot swim. Instead, pollen tubes deliver the male gamete directly into the egg. This is called siphonogamy and occurs in all extant lineages of conifers and Gnetales. However, they sometimes still have archegonial chambers, albeit small ones, such as those found in Picea Runions and Owens, The neck cells and neighboring cells surrounding the archegonium secrete lipid into the chamber space.
These lipids are thought to be essential in signaling and directing pollen tubes to their destination Runions and Owens, Figure 3. Schematic of ovule tip at time of fertilization, showing layers of integument I , nucellus N , megagametophyte M , with two archegonia in white , of which one is fertilized f , the other unfertilized u.
Pollen tubes p have grown into the nucellus; the sulcus end of the tube hangs over the archegonial chamber ac. Some published accounts state that fluids from megagametophytes may be sufficient to fill the chamber blue , or may be much less abundant, having only the fluids of a few ruptured pollen tubes mixed with secretions from archegonia. In the plant, the orientation of the ovule is reversed, with the megagametophyte at the top. It is the purpose of this review to trace the evolution of sexual fluids in gymnosperms, to describe the aspects of their biochemistry that we currently understand, as well as to suggest future directions of investigation.
This review also has a particular emphasis, which is to trace the unique origins of gymnosperm nectar. Pollination drops are widespread among modern gymnosperms, archegonial chamber fluids less so. We will discuss archegonial chamber fluids first. Although their role in sexual reproduction is clear, details of their composition are the most poorly understood of all of the gymnosperm sexual fluids.
This fluid is mainly restricted to cycads and Ginkgo , the extant zooidogamous gymnosperms. Since the process of secretion takes place inside the ovule it is difficult to observe.
Accounts of events are mostly of a descriptive, rather than experimental nature. For thorough historical discussions, see Hori and Miyamura and Norstog and Nicholls There are conflicting views as to the origins of the fluid s. Three origins have been proposed. The first of these is the pollen tube.
In Dioon edule , as pollen tubes rupture during sperm release, they release a fluid that is of sufficient volume Figure 3 to provide a thin film in which the sperm are able to swim Chamberlain, If pollen tubes are numerous, they may even release enough fluid to fill the entire archegonial chamber Brough and Taylor, A second source is the megagametophyte.
In Cycas revoluta , fluids are released from megagametophyte cells lining the archegonial chamber Figure 3. Cells at the rim of the depression secrete first, followed by cells at the base Takaso et al.
A third source of fluid is from individual archegonia. In Ginkgo biloba , archegonial neck cells release fluid Wang et al. Combinations of fluids are also possible, e. Some experimental work provides evidence for the functions of these fluids. In a study of pollen tubes in different conditions, Takaso et al.
The possibility that there may be a degree of molecular interaction between secreted pollen proteins and ovules that could be considered as a form of a recognition system was first put forward by Pettitt in his study of cycads. He considered the context of these fluids, recognizing that the archegonial chamber fluids occur at the interface between the haploid megagametophytes and the surrounding diploid sporophytic ovule tissue. These genetically different tissues are separated from one another by a megaspore wall, which is a thick, complex structure composed of glycoproteins, cellulose, hemicellulose, and sporopollenin.
The sporophyte-gametophyte Bauplan of the ovule imposes communication constraints Williams, The physiological isolation that this wall imposes prevents interactions between the gametophyte and the sporophyte Pettitt, Unfortunately, no molecular studies of protein interactions during reproduction have been carried out since these papers appeared.
Even an initial analysis of archegonial chamber fluid composition has yet to be carried out. Detailed proteomic and metabolomic analysis of these fluids would add significant information to our understanding of the evolution of sperm-ovule interactions, from sperm discharge and chemotaxis through to ovule defense.
Archegonial secretions and neck canal secretions have been mainly studied by transmission electron microscopy. In both cycads with their large archegonial chambers Takaso et al. These lipids have never been isolated and analyzed. Among modern gymnosperm taxa, species have various pollination syndromes, i. These mechanisms make use of secretions, i.
However, in a small number of species there are mechanisms that do not use secretions as far as we know Gelbart and von Aderkas, Such mechanisms are restricted to the conifer family, Araucariaceae Eames, ; Haines et al. We will only touch on these mechanisms throughout; this review focuses on cases of sexual secretions and possible nectars. Pollen capture mechanisms have been classified in several ways in the past.
Here, we divide the modern variation known into three categories based on their primary pollen capture surface Figure 1. This liquid surface is the first contact that pollen has with the ovule. Some species have a drop that appears later and brings pollen into the ovule. Figure 4.
Schematic of longitudinal sections of portions of ovules at time of pollination illustrating the three types of pollen capture arranged clockwise. Nectar is pink, pollination drops are red, and lipid microdrops are blue. Pollen is yellow and is either round or saccate, depending on the mechanism.
The lowest pollen grain can be seen entering a depression in the nucellus known as the pollen chamber, which is formed by PCD.
The two uppermost pollen grains can be seen floating into a pollen chamber. This nucellus-based ovular fluid also performs a myriad of functions, which include primary pollen capture, pollen delivery into the ovule, pollen germination, and defense of the ovule against pathogens.
We present a synthesis using the well-sampled genera-level phylogeny of Lu et al. Additional divergence times and phylogenetic relationships come from Clarke et al. The presence of nucellar secretions at the pre-fertilization stage of the seed, i.
The presence of the drop among modern gymnosperm clades is widespread Figure 1. Regardless of possible future alternative phylogenetic hypotheses, it seems very likely that the foundational nature of ovular fluids will remain a robust inference.
This is due, in part to the prevalence of sexual fluids among the majority of modern gymnosperm groups and thus the ancestral condition of having a pollination drop would be similar among all major lineages given alternative topologies i. As secretion continues, a fluid balloons outward from the opening of the micropyle in a spherical drop. In these ovules, the surfaces surrounding the opening are waxy. Hydrophobic forces between the watery secretion and the surface cause the secretion to form into a sphere.
During secretion, these ovules have their micropyles facing horizontally or upward, i. The non-saccate pollen sinks through the drop, coming to rest inside the ovule Tomlinson et al. By the time pollen reaches the nucellus, it is ready to germinate. The pollen tube grows and penetrates the nucellus. This mechanism occurs in G. An advantage of this PCM is that, depending on species, it serves as a key adaptation in both wind and animal pollination syndromes.
Understanding the constituents of this most prevalent PCM among extant gymnosperms is key to understanding the variety of roles that pollination fluids play in the reproductive biology of gymnosperms. We will look at water, sugars, amino acids, proteins, calcium and phosphates, as well as their role as nectar, and in pollen capture, delivery, germination, and in ovule defense. Water not only captures and hydrates pollen, but in many species, e.
This is an important event prior to germination. In cupressaceous conifers, exine shedding is also functionally significant.
Removal of the hard-shelled exine layer, reveals the intine, which is a much more flexible layer. This rules out one of the first tempting ideas about pollination drops, namely that they replace simple rainwater. We can conclude that the first of the three functions of pollination drops—pollen capture—may be largely due to water, but the other functions, germination, pathogen defense, and nectar, depend on solutes.
The most universal and abundant solute in these watery drops is carbohydrate. The three most important sugars are glucose, fructose and sucrose. These include melezitose and xylose, as well as two sugar alcohols Nepi et al. Sugars in pollination drops are necessary for pollen germination and pollen tube nutrition Nygaard, , as well as for the nutrition of insect pollinators Kato and Inoue, When TSC is analyzed, ambophilous species can be easily separated from species that are either solely wind-pollinated or insect-pollinated.
Wind-pollinated species had a significantly lower TSC than ambophilous species. The universality of sugars in pollination drops implies that they were present among the ancestors of extant gymnosperms. Although analyses tend to report stable sugar compositions, in some species of Gnetum , sugar concentration can vary according to relative humidity.
This is due to the high relative water content of the surrounding atmosphere, e. All pollination drops have amino acids Chesnoy, These include serine, glutamic acid, glycine, histidine, alanine and proline Nepi et al. Just as sugar concentrations can be used to discriminate pollination drops of wind pollinated species from those of ambophilous species, the total amino acid content TAC of drops also proves to be a reliable predictor of pollination syndromes.
Wind pollinated species have higher TAC values than ambophilous species such as Gnetum gnemon. From a nectar standpoint, it is not just a low total TAC that is important, but among the low concentration amino acids the relative concentrations of certain types of amino acids are significant also.
One class of amino acids—non-protein amino acids—is characteristic of nectar. Chesnoy, ; Nepi et al. Proteins are found in all gymnosperm sexual fluids that have been analyzed to date.
Because proteins are large complex molecules, by definition, they represent a sporophytic investment in the pollination drop that is substantial. These proteins are thought to be active in the apoplast. However, some proteins are found in pollination drops as a consequence of cellular breakdown and are not normally found in the apoplast. The degradome can be composed of over a dozen proteins von Aderkas et al.
The most common proteins of the secretome include carbohydrate-modifying enzymes, such as glucanases, and defense proteins, such as anti-fungal enzymes, e.
These classes of proteins are nearly universal in pollination drops, which implies that they may have been there since the beginning of gymnosperm reproduction. As such, they represent a relatively well-preserved fraction of the functions of the pollination drop Wagner et al.
Recently, proteomic analysis of pollination drops, coupled to a transcriptomic analysis of nucellus, was carried out on Cephalotaxus koreana and C. Pollination drops of these species have rich secretomes with nearly 30 proteins, many of which are involved in defense, carbohydrate-modification, or pollen growth.
There are also a number of unique proteins that likely function in starch and callose degradation. In addition to such carbohydrate-modification and defense-related proteins just described, proteins have also been found that may play a role in regulating pollen growth and selection. Protein composition of pollination drops of cycads, Ginkgo and many groups of conifers have yet to be studied. In addition, protein profiles, comparing male and female nectars found in strobili of the Gnetales need to be analyzed, as they may show differences as seen in angiosperms Chatt et al.
Calcium is important for pollen germination Brewbaker and Kwack, Recent studies have shown it to be present in Ginkgo pollen intine Lu et al. Phosphate was identified long ago in pollination drops of T. Recently, we found evidence in a transcriptomic analysis of Cephalotaxus nucellus during pollination drop secretion of expression of a gene involved in eATP regulation — an apyrase Pirone-Davies et al.
Since phosphates, such as extracellular ATP eATP , have immunogenic functions, including regulation of responses to fungal invasion in seed plants Gust et al. The strongest evidence that differentiates nectar from non-nectar pollination drops comes from the recent Principal Component Analysis PCA of carbohydrates and amino acids of ovular fluids Nepi et al.
PCA effectively separates out ambophilous from wind-pollinated species. Ambophilous species overlap with flowering plant nectar Nepi et al. Nepi et al. Chemical analysis also yielded a surprise: profiles of G. In our opinion, there are not many. Later, or possibly very early on, this drop acquired another function — insect reward.
The diversity of modern nectar types has resulted in nectar terminology being beset by historical circumstance for discussion see Koptur, For example, angiosperm nectaries were the first to be divided into floral and extra-floral nectaries EFNs , which has led to fern nectaries being referred to as EFNs, since they lack flowers.
As Marazzi et al. Nectar secretion processes are diverse enough to defy simple categorization based on anatomy. Nectar, it turns out, does not always flow from a nectary. Nectar is simply a sweet apoplastic fluid available on a plant surface where it can attract some animal or other that consumes it as a reward. Since it is of uniquely ovular origin, there is probably no modern angiosperm homolog. The nectar definition resides on the ecological service provided, that is, the mutualism of which it is a part.
It is the considered view of some nectar experts that pollination drops are functionally equivalent to angiosperm nectar Bernardello, Gnetum spp. In contrast, wind-pollinated species of Ephedra lack nectar production on their male strobili Bolinder et al. In both the female and male strobili of Gnetum and Welwitschia , ovules produce drops that are sugar-rich and contain non-protein amino acids Nepi et al. The largest difference between males and females is that the ovules in the male strobili are non-functional, sterile structures, the only function of which appears to be secretion Haycraft and Carmichael, This is one of the unique aspects of nectar production among extant gymnosperms.
It would be interesting to investigate gene regulation of ovule development to see whether ovules in male strobili are indeed different from those in female strobili. Because turning an ovule to another purpose is not common among plants, it would be of interest to know whether ovule development is redirected only for the purpose of providing nectar to attract insects. Compositional differences also exist. Fertile ovule secretions had greater fructose concentrations than those of male secretions.
Higher concentrations of non-protein amino acids were found in fertile ovules than in male secretions. This is similar to results reported for male and female flowers of flowering plants. For example, in species of Cucurbita , male and female flowers of Cucurbita maxima ssp.
Big Max male and female nectars differ in both metabolome and proteome Chatt et al.
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