Sexual Reproduction

Concepts

• Benefits of sexual reproduction relate to increased fitness - specifically, this includes the production of genetically variable offspring (often expressed as hybrid vigour) that allows at least some offspring to survive in a heterogeneous environment.

• Costs of sexual reproduction include disrupting well-adapted genotypes and the cost of producing reproductive structures.

• Plants can have complex combinations of gender expression; for example, they can be exclusively male or female, or both male and female at the same time.

• Pollination by animals is more accurate, but more energetically expensive because the floral structures are elaborate; wind pollination requires less initial resource commitment because the floral structures are small; since it is less accurate, a lot of resources used to produce pollen and ovules can be wasted.

• Self-compatibility guarantees some degree of mating success by ensuring pollination occurs even when only one individual is present.

• Self-incompatibility prevents inbreeding depression by ensuring that ovules are fertilized by pollen from genetically different individuals.

Introduction

Plants have two general means by which they reproduce: asexually and sexually. Since humans only reproduce sexually, asexual reproduction (= vegetative reproduction) is not as familiar to us, but it is rather common in plants. Asexual reproduction involves the replication of chromosomes without the production of gametes or the need for sex. Asexual reproduction pro duces offspring that are genetically identical to their parents. Typical examples of this form of reproduction are the stolons ('runners') produced by strawberries (Fragaria species), and root sprouting ('suckering') by aspens (Populus species). We will discuss asexual reproduction in the next chapter; in this chapter, we focus on sexual reproduction.

As in any organism, plant sexual reproduction requires the fusion of two gametes (a

© 2003 CAB International. Weed Ecology in Natural and Agricultural Systems (B.D. Booth, S.D. Murphy and C.J. Swanton)

sperm and ovum) to form a zygote. Each gamete normally contains one set of chromosomes and the zygote will normally have two sets of chromosomes: one from each parent. Therefore, sexually produced offspring possess a unique recombination of their parents' genes and are genetically different from parents. In flowering plants, sexual reproduction is facilitated by pollination. Pollination occurs when pollen is transported to the stigma on a flower. Once on a compatible stigma, pollen produces a pollen tube that delivers the sperm to the female gametes (ova) and, ultimately, a seed will be produced. Through sexual reproduction, there are many ways to successfully transmit at least some portion of an individual's genotype (= all the genes of an individual) to its offspring, for example:

• Plants may express different combinations of gender, e.g. individuals can be either genetically programmed to be only male or female ('diclinous') or be both male and female at some time during their lifespan ('monclinous') (Table 4.1).

• Because they cannot move, plants package sperm inside specialized protective

Table 4.1. Gender expression in plants. Term Description

Monocliny Each individual is genetically capable of expressing both genders.

Whether both genders are actually expressed can be influenced by genetic and environmental factors: Sequential monocliny Each individual expresses only one gender at a given time. Gender changes over a growing season or from year to year, e.g. saltbushes (Atriplex) (Freeman and McArthur, 1984) Simultaneous monocliny An individual expresses both genders at a given time, but not all flowers necessarily express both genders, at the same time or ever Sexual monomorphism Male and female gender will be expressed in the same flower, though not necessarily at the same time: Male gender expressed before female gender, e.g. wild carrot,

(Daucus carota) (Dale, 1974) Female gender expressed before male gender, e.g. common mullein

(Verbascum thapsus) (Gross and Werner, 1977) Both genders expressed at same time in same flower, e.g. sow thistles

(Sonchus) (Hutchinson et al., 1984) In at least some flowers, only one gender is ever expressed: Female flowers and perfect flowers exist, e.g. plantains (Plantago) (de

Haan et al., 1997) Male flowers and perfect flowers exist, e.g. horsenettle (Solanum carolinense) (Steven et al., 1999) All flowers are either male or female; no perfect flowers exist, e.g. nettles (Urtica) (Bassett et al., 1977)

Each individual is genetically capable of expressing only one gender during its existence. However, species or populations are not always totally diclinous:

Dioecy (true dicliny) All individuals are either entirely male or entirely female, e.g. poison ivy

(Rhus radicans) (Mulligan and Junkins, 1977) Gynodioecy Some individuals are entirely female; others are monoclinous, e.g. viper's bugloss (Echium vulgare) (Klinkhamer et al., 1994) Androdioecy Some individuals are entirely male; others are monoclinous, e.g. annual mercury (Mercurialis annua) (Pannell, 1997)

Note that some terms apply to the gender expression of the individual as a whole, while others refer to gender expression within the flowers. Cruden and Lloyd (1995) give alternative terminology for gender expression.

Protandry

Protogyny

True monomorphism

Monoecy Gynomonoecy

Andromonoecy

True monoecy

Dicliny tissue ('pollen') that can be sent across short and long distances via gravity, water, wind and animals. • Plants can mate with themselves or with another individual.

We will discuss these in detail, but first we will examine the benefits and costs of reproducing sexually.

The Benefits and Costs of Sex

Sex costs resources - this encompasses everything a plant uses and produces, e.g. fats, proteins, carbohydrates and water. Resources used in sexual reproduction will not be available for anything else, like making leaves that will increase photosynthesis and sugar production to feed the plant. Therefore, in order for sex to exist, its benefits must outweigh its costs. Benefits include producing genetically variable genotypes that are both adaptable and less likely to suffer from genetically related 'medical' problems that are contained in a mobile unit (a seed) that can escape the parental environment. Costs include disrupting well-adapted genotypes and having to use resources to produce floral structures to facilitate sexual reproduction. In general, this 'trade-off' between benefits and costs of sex relate to the concept of fitness.

Fitness

Fitness provides a relative measure of how well an individual succeeds at continuing its lineage. Individuals that are 'fit' to their environment are ones that can survive and reproduce successfully. In any population, the genotype with the highest relative fitness is the one that produces the most offspring that will survive and reproduce themselves. Relative fitness is often measured by testing for significant effects of any phenomenon (e.g. low nitrogen concentrations in the soil) on specific fitness components. These components are usually tangible traits of plants that can be measured empirically, for example:

• number and mass of seeds produced;

• success of seed dispersal;

• amount and rate of seed germination;

• mass, height and growth rate of seedlings;

• resource allocation to roots, shoots and flowers (measured by examining their relative masses);

• pollination and fertilization success.

The effective measure of relative fitness itself can be thought of as how much of the original parental genotype survives from generation to generation, both in terms of the genetic composition of direct descendants and how much of the population eventually contains some portion of a parental genotype. If the environment generally remains constant, then individuals continue to produce offspring that are very close copies of themselves, i.e. the most fit offspring genotype will be those that are most similar to the parental genotype. This is what happens with agricultural weeds when farming practices do not change over time - weeds adapt to these specific practices and produce many similar offspring because these offspring encounter an equally favourable and unchanged environment. When environmental conditions change, however, the fitness of these weeds may decrease.

This same principle applies when plants are introduced to new environments. Effectively, the environment for the plant has 'changed' since it encounters a new location and habitat. If the physical environment of the originating habitat is similar, but the biological environment differs, then the plant often has greater fitness than existing plants. One example in North America is the introduction of garlic mustard (Alliaria petiolata). Garlic mustard originates from Eurasia, where the physical environment is reasonably similar to northeastern North America. Unlike Eurasia, garlic mustard in North America does not appear to have any effective natural enemies (e.g. pathogens, herbivores). Also, garlic mustard prefers forest edge habitats; many species native to the forest understories of North America are not adapted to this type of environment. With forest fragmentation increasing rapidly, garlic mustard can outcompete native species because of its high growth rate and fecundity in its new environment of forest edges without natural enemies (Anderson et al., 1996; Nuzzo, 1999).

Benefits of sexual reproduction

New, better fit and better adaptable genotypes

The main benefits of sexual reproduction are the potential for genetic combinations that may be better fit to the current or new environments and, concomitantly, producing genetically variable offspring that can adapt to changing or new environments. Each seed is a unique genotype containing different alleles ('versions') of different genes, hence whatever type of environment the offspring encounter, there is a high probability that some of them will survive to reproduce. Formally, we sometimes refer to genetic variation as 'hybrid vigour' (= 'heterosis'). Intraspecific ('within species') hybrids are very common in sexual organisms (including diverse organisms like garlic mustard and humans). Intraspecific hybrids are less likely to express deleterious combinations of alleles, i.e. 'bad' products from 'bad' versions of genes that reduce the ability of an individual to germinate, grow and survive to reproduce. The reason that sex avoids this problem is because when two individuals of the same species ('conspecifics') that have dissimilar genotypes mate, genetic recombination occurs so their offspring are less likely to receive a copy of the same deleterious allele from each parent.

In plants, there is another reasonably common form of hybridization: interspecific ('between species'). Again, this is less familiar to humans because it does not happen with us. However, in plants, interspecific hybrids are formed when individuals from two different species mate. Not all species can mate to produce viable hybrids and, in fact, hybrids usually form from mating between closely related species because their genomes must be similar enough to successfully produce offspring capable of reproducing themselves. Interspecific hybrids may have higher fitness because of new genetic combinations. They usually have characteristics that are intermediate to their parents (Table 4.2) (Bailey et al., 1995; Clements et al., 1999). New species with weedy characteristics can arise through hybridization either when two weeds hybridize or when a weed and native species hybridize (Briggs and Walters, 1984). Examples of some hybrids (indicated by the 'X' below) between weed species are:

• bitter yellow dock (Rumex X crispo-obtusifolius = R. crispus X R. obtusi-folius);

• false leafy spurge (Euphorbia X pseudo-esula = E. cyparissias X E. esula);

Table 4.2. Characteristics of giant knotweed (Fallopia sachalinesis) and Japanese knotweed (Fallopia japónica var. japónica) and their hybrid Fallopia x bohémica. (Adapted from Bailey et al., 1995).

Characteristic

Giant knotweed

Hybrid

Japanese knotweed

2n

44

44 or 66

88

Height

Gigantic plant, up to

Intermediate in size,

Large plant 2-3 m tall

4 m tall

2.5-4 m tall

Leaf size

Up to 40 cm long by

Up to 23 cm long by

10-15 cm long

22 cm wide

19 cm wide

Leaf length:width ratio

Approx 1.5

1.1-1.8

1-1.5

Leaf underside

Scattered, long flexible

Larger leaves have

Smooth

hairs

many short, stout hairs

Floral sex expression

Male-fertile flowers and

Male-fertile flowers and

Flowers usually

male-sterile flowers

male-sterile flowers

male-sterile

borne on separate plants

borne on separate plants

• tall cat-tail (Typha X glauca = T. angusti-folia X T. latifolia);

• goat's-bladder (Tragopogon X mirus = T. dubius X T. porrifolius);

• hybrid goat's-beard (Tragopogon X mis-cellus = T. dubius X T. pratensis).

In some cases a hybrid can be found beyond the distribution of the parent species. For example, the range of hybrid goat's-beard (T. X miscellus) has increased substantially beyond the range of at least one of the parent species in Washington state (Novak et al., 1991). Thus, this species may have a greater ecological amplitude than its parents.

Hybridization between a native species and a related weed species can be more serious than hybridization between two weed species, because it can cause extinction of the native species if the hybrid species has greater fitness. For example, Freas and Murphy (1988) determined that the widespread Australian saltbush (Atriplex serenana) appeared to be hybridizing with the one remaining population of Bakersfield saltbush (Atriplex tularen-sis). Several native sunflowers (Helianthus species) in the southern USA are vulnerable to extirpation or extinction because of hybridization with the introduced annual sunflower (Helianthus annuus) (Rhymer and Simberloff, 1996).

Getting away from your parents: the mobility of offspring

Sexual reproduction generally has an ancillary benefit of producing mobile offspring, i.e. seeds or seeds inside fruits. Sexually produced offspring are usually dispersed away from the maternal parent so there is less chance of competing with their parents, siblings or other relatives. When the environment is not favourable to the parent (and hence the offspring are also likely to suffer), dispersal away from the parent is important. The benefits of dispersal are discussed in more detail in Chapter 6 but it is useful to keep in mind that dispersal is an indirect benefit of sex.

Costs of sexual reproduction

Sex disrupts well-adapted genotypes

Plants that reproduce sexually risk breaking up well-adapted genotypes, because it results in genetic recombination. In a relatively unchanging environment, offspring that are similar to the maternal genotype are usually better adapted than ones with recombined genotypes. This fitness disadvantage of a recombined genotype is called 'outbreeding depression' (Waser and Price, 1989, 1993; Parker, 1992). The offspring lose the complex genetic structure that made their parents so successful in a local environment.

Cost of producing reproductive structures

To reproduce sexually, plants must allocate resources to produce sexual organs, and floral structures that increase the chances of pollen dispersal. These structures can be quite resource expensive. Milkweed, for example, allocates 37% of its photosynthate to nectar production (Southwich, 1984). Plants that reproduce only once in their life span ('monocarpic') must maximize reproductive output per unit of resource expended. Even in plants that have repeated reproductive events in their life span (polycarpic), the costs of sexual reproduction are important because it may result in resources being directed away from growth and maintenance. Sexually reproducing plants often have to commit resources to reproduction early in the growing season. This increases the risk associated with sexual reproduction because if the weather prevents pollination or if seeds are destroyed, the plant may not have enough resources left to survive.

Ecology of Flowers and Flowering

Gender expression

In most animals, an individual is either male or female. Defining gender in a plant, however, is complicated. In plants, gender can apply to individual flowers or to the individual as a whole (Table 4.1). The rea son for this complexity is related to sexual selection, i.e. the factors that influence the relative ability of individuals to obtain mates and reproduce offspring (Willson, 1994). Generally, the more options plants have in expressing gender, the more likely they are to reproduce (sexually) successfully, no matter what environment they encounter. We will also show, however, that there can be risks for an individual to express many combinations of gender and, consequently, there can be benefits of expressing only one or few combinations of gender.

Gender based on flowers of individuals

Most people recognize that flowers can be male and female at the same time because we are taught to recognize the basic structures of a typical flower, i.e. petals, sepals, stamens and carpels (Fig. 4.1). However, some or all flowers on an individual may express only one gender. The sexual expression of a flower also can be separated in time with male structures (anthers and pollen) maturing first and then the female structures (e.g. stigma) becoming receptive, or vice versa. There are many complex variations of this with equally complex terminology (Table 4.1).

Gender based on the individual

The gender of an individual plant can be controlled genetically, environmentally or both. Plants often have the genetic ability to be male and female but the relative expression of male and female traits varies with the short-term environmental conditions and perhaps long-term selection pressures (Barrett, 1998; Campbell, 2000). Plants where individuals are (genetically) one sex are called 'diclinous'; plants that (genetically) can express more than one sex are called monoclinous (Table 4.1). Humans would considered to be 'diclinous', using this terminology. Like its flowers, an entire individual plant can be: male, female, both male and female at the same time, male and then female, female and then male, continually changing from female to male or vice versa.

Allocation strategies for expressing genders in flowers and individuals

Generally, environmental stress tends to increase the expression of male gender in plants (Freeman et al., 1980; Escarre and Thompson, 1991). In a resource-poor environment, it is better to be male than female.

Monocliny Flower
Fig. 4.1. Drawing of archetypal flower.

This is because male structures (like pollen) require fewer resources to develop, whereas female structures (like ovules) are where offspring develop and they require allocation of more resources to be nurtured and dispersed. Conversely, in resource-rich environments, it may be advantageous to express more female gender as there is no question that any offspring produced are, in part, carrying the female parent's genotype (as we shall see, males have little control of their reproductive success as pollen can go astray). Since plants cannot predict their future environment, any allocation and gender expression strategy is risky and generally depends on whatever previous and current selection pressures favour. Some plants try to use 'bet-hedging' by allocating equal amounts of resources to both male and female genders; however, even this may reduce fitness if the environment currently or eventually changes to favour the expression of one gender rather than both. Further complicating gender expression is the fact that all of the resources allocated may be wasted because the process of mating is rather risky in plants as they rely on a 'third party' to facilitate sex. The 'third party' relates to pollination mechanisms, (i.e. what carries the pollen from male to female).

Pollination mechanisms

For plants to reproduce sexually, there must be ample pollen available to carry sperm that will fertilize ova. Because of its micro-

Table 4.3. Suites of floral traits associated with pollination syndromes (adapted from Howe and Westley, 1977).

Pollinating agent

Anthesis

Colour

Odour

Flower shape

Insect pollination Beetles

Carrion or dung flies Bees

Butterflies

Vertebrate pollination Bats

Birds

Abiotic pollination Wind

Water

Day and night

Day and night

Day and night or diurnal

Day and night or diurnal

Night

Dull

Purple-brown or greenish Variable, but not pure red

Fruity or aminoid Decaying protein Usually sweet

Variable; pink Sweet very common

Drab, pale, Musty often green

Vivid, often red none

Day or night Drab, green None

Variable

Variable None

Flat or bowl-shaped; radial symmetry Flat or deep; radial symmetry; often traps Flat to broad tube; bilateral or radial symmetry; may be closed Upright; radial symmetry; deep or with spur

Flat 'shaving brush' or deep tube; radial symmetry; much pollen; often upright, hanging outside foliage, or borne on trunk or branch Tubular, sometimes curved; radial or bilateral symmetry, robust corolla; often hanging

Small; sepals and petals absent or reduced; large stigmata; much pollen; often catkins Minute; sepals and petals absent or reduced; entire male flower may be released scopic size (micrometers), pollen is usually produced in large quantities in order to increase the chance of reaching non-mobile ova. For a plant to mate successfully, pollen must be transferred from the anther to a genetically compatible stigma, style and ovum. Pollen generally is delivered via three mechanisms: by animals (zoophily), wind (anemophily) or water (hydrophily).

Animal pollination (zoophily)

Animal-pollinated species must allocate resources to create floral morphologies that attract animals and these can be very resource expensive (Harder and Barrett, 1995). Floral morphology varies with the type of animal pollinator, as do the pigments used to colour flowers, the height and breadth of the inflorescence, and the provision of nectar (Table 4.3) (Wyatt, 1983).

Plants vary in their pollination strategy, i.e. whether to use many types of pollinators or very specialized pollinators (Johnson and Steiner, 2000), but some general trends do exist. Of most relevance here, weeds tend not to need elaborate floral morphologies because they are not usually co-adapted with their pollinators or use abiotic vectors for pollination (Baker, 1974). For example, wild carrot (Daucus carota) has an open flat inflorescence that enables a variety of insects to access pollen (Dale, 1974); many weeds use a similar strategy. Regardless, there are no guarantees of successful pollination because the inflorescence can be eaten, pathogens or parasites can infest the flowers, or animals can rob nectar without transferring pollen. Plants may increase the likelihood of successful pollination by:

• deceiving pollinators (using chemicals that resemble nectar to lure them);

• trapping pollinators in a flower to ensure they are covered in pollen;

• forcing pollinators to specialize by hiding rewards like nectar or having specialized flowers that require structures like uniquely shaped proboscises;

• flowering only when other species are not flowering.

Wind pollination (anemophily)

Wind-pollinated flowers are more drab, have small or absent petals and no nectar. They are less showy (though not necessarily less elaborate) but often are less energetically expensive (Whitehead, 1983). Wind-pollinated plants must produce vast quantities of pollen to ensure success. Wind pollination presents risks because most of the pollen does not reach the proper stigma, and successful pollination depends on appropriate environmental conditions such as precipitation, temperature, relative humidity and wind direction (Whitehead, 1983; Murphy, 1999). Examples of wind-pollinated weeds include ragweeds (Ambrosia species), quackgrass (and other weeds in the grass family (Poaceae)), and Monterey pine (Pinus rigida). It may be advantageous for weeds and other colonizing species to be windpollinated to avoid reliance on other organisms to ensure successful pollination.

Water pollination (hydrophily)

Water pollination is unique to submergent aquatic plants (see Les, 1988, for an extensive review). Submergent weeds that are water pollinated include horned pondweeds (Zannichellia) and pondweeds (Najas). Generally, water pollination is inefficient since the pollen (or sometimes the entire male parts of a flower) must float on the water or be transported in the water to reach stigmas. We emphasize that many familiar aquatic weeds actually are not water pollinated. Emergent aquatic plants like cattails (Typha species) are wind pollinated, while other emergents (pickerelweed, Pontederia cordata) and floating plants (water hyacinth, Eichhornia crassipies) are animal pollinated.

Pollination problems

Earlier, we discussed the concept that interspecific hybrids can be quite fit. However, we emphasized that not all species can mate with each other. Improper pollen transfer refers to situations where pollen from an individual of a different species ('heterospe-cific') lands on a stigma and does not pro duce any hybrid offspring. This is a problem for the pollen donor because a lot of pollen is therefore 'wasted' on individuals where fertilization will not occur. The pollen recipient is also affected if heterospecific pollen contains toxins ('allelochemicals'), pathogens or parasites (Murphy, 1999). For both donors and recipients, the result is lower pollination success and production of fewer viable seeds. Plants also suffer reduced pollination success and seed set if pollen or pollinator limitation exists (e.g. Lalonde and Roitberg, 1994; Collevatti et al., 1997) because:

• few compatible mates are nearby;

• compatible individuals produce low numbers of flowers, pollen or ovules because of genetic defects or poor environmental conditions;

• the weather is poor for wind pollination or animal pollinators;

• there are few appropriate animal pollinators in the community.

Self-compatibility and self-incompatibility

Some individuals can successfully mate with themselves if pollen is transferred from stigma to style because they have: (i) viable male and female flowers open simultaneously, or (ii) their flowers have both male and female reproductive parts that are viable simultaneously. This is called 'self-compatibility'.

The benefits and costs of self-compatibility

Self-compatibility can be important to colonizing species because it means a single individual can invade a site, and be able to self-fertilize and produce seed. With this advantage, it is not surprising that many exotic or native plants considered weedy are self-compatible (Mulligan and Findlay, 1970; Baker, 1974; Barrett, 1992). A second advantage of self-compatibility is that it can be less costly if resource allocation to floral structures is reduced, as pollinators may not be necessary or as important if the indi vidual simply uses gravity to collect pollen from its anthers to land on its stigma.

Though self-compatibility might be advantageous, not all weeds use this, e.g. jimsonweed (Datura stramonium) (Motten and Antonovics, 1992). This is because self-compatibility has costs as well as benefits. Consequently, many individuals are 'self-incompatible', i.e. they cannot mate with themselves. Self-incompatible plants can avoid some costs of self-mating. The main costs avoided are the otherwise increased chances of accumulating harmful alleles and decreased adaptability to new or changing environments. You have seen these explanations given in the discussion of the benefits and costs of sex. The only novel aspect here is that the reduced fitness caused by mating with a close relative and accumulating multiple copies of deleterious alleles is formally termed 'inbreeding depression'. Even self-incompatible plants have no guarantees of avoiding costs since their genetically recombined genotypes may not be adapted to the environment. Additionally, a self-incompatible individual still may mate with a close relative since its likely mates (close neighbours) often are close relatives (see Madden, 1995; Lefol et al, 1996; Nunez-Farfan et al, 1996; Guttieri et al, 1998; Sun and Ritland, 1998; see Stanton, 1994; Wilson and Payne, 1994, for discussion of mate selection to avoid this problem).

How self-incompatibility is enforced

If an individual is to avoid inbreeding depression in its most extreme form (self-mating), there must be mechanisms available to block self-pollen from eventually producing offspring. Some individuals use a mechanism described as 'histochemical incompatibility'; this is a bit like a pollen grain causing an allergic reaction in the female so that the tissues change and fertilization cannot occur. The basis for histo-chemical incompatibility is a class of compounds called 'glycoproteins' expressed in pollen, stigma and style. The glycoproteins are signals that identify incompatible mates, usually relatives and self-pollen. The histo-chemical incompatibility reaction can occur

Fig. 4.2. Illustration of tristyly in purple loosestrife (Lythrum salicaria). The three floral morphs are: long-styled (with short and mid anthers), mid-styled (with long and short anthers) and short-styled (with long and mid anthers). Petals and calyx on the close side are removed to reveal flower parts. Arrows show direction that pollen must travel from anther to stigma to ensure full fertilization (Darwin 1877).

Diagram For Dicliny

Fig. 4.2. Illustration of tristyly in purple loosestrife (Lythrum salicaria). The three floral morphs are: long-styled (with short and mid anthers), mid-styled (with long and short anthers) and short-styled (with long and mid anthers). Petals and calyx on the close side are removed to reveal flower parts. Arrows show direction that pollen must travel from anther to stigma to ensure full fertilization (Darwin 1877).

at different parts of the flower and at different times of its life cycle (Sims, 1993). Sporophytic incompatibility occurs when pollen is on the stigma; the glycoproteins signal the stigma not to exude the water needed for pollen to germinate. Gametophytic incompatibility usually occurs as the pollen tube is trying to grow in the style towards the embryo sac with the ovules.

Structurally, plants may avoid incompatible mates by having pollen that cannot physically adhere to certain stigmas, i.e. it is too big, too small, the wrong shape or the wrong texture. Additionally, the stigmas may be located above the pollen-bearing anthers so pollen cannot fall on top of the stigma. In some species, 'heterostyly' occurs where different types of flowers have stamens and styles of distinct lengths. Figure 4.2 illustrates heterostyly in purple loosestrife (Lythrum salicaria). The subsequent physical separation of stigmas from anthers (with self-pollen) enforces outcrossing.

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Responses

  • max
    How does weeds reproduce sexually?
    2 years ago
  • Valentin
    Why are all pollen tube of not equal length wich grow on stigma?
    2 years ago
  • lobelia
    How weeds survive in sexual reproduction?
    2 years ago
  • mafalda
    How is a hedge plants asexual or sexual?
    11 months ago
  • TARQUINIA
    How sexual reproduction in weed influence weeds management?
    10 months ago
  • Dwayne Vargas
    How to know if the weeds are asexual or sexual?
    5 months ago
  • esmeralda
    How do weeds reproduced?
    5 months ago
  • imogen hughes
    What are the impilcation of weed sexual reporduction ?
    3 months ago
  • Petteri
    How would sexual sexual reproduction in weeds influence their environment?
    3 months ago

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