Difference Between Allogamy and Xenogamy

Main Difference – Allogamy vs Xenogamy

There are two types of pollinations known as self pollination and cross pollination. Allogamy and xenogamy are the two types of cross pollinations that occur during the sexual reproduction of higher plants. Flower is the sexual organ of angiosperms. Stamen is the male part of the flower, which contains anther and filamentous. Pistil is the female part of the flower, which contains stigma, style, and ovary. Unisexual flowers contain pistil and the stamens in separate flowers, enhancing cross pollination. Bisexual flowers contain both pistil and stamens in the same flower, enhancing self pollination. The main difference between allogamy and xenogamy is that allogamy is the deposition of pollen grains from the anther of the one flower on the stigma of another flower, either in the same plant or a in a different plant of the same species whereas xenogamy is the deposition of pollen grains from the anther of one flower on the stigma of a genetically different flower of the same species.

1. What is Allogamy
      – Definition, Characteristics, Process, Examples
2. What is Xenogamy
      – Definition, Characteristics, Process, Examples
3. What is the difference between Allogamy and Xenogamy

What is Allogamy

Allogamy is the fertilization of a flower by the pollen grains of another flower. The deposition of the pollen grains of the anther of one flower on the stigma of another flower of the same plant is called geitonogamy. In contrast, the deposition of pollen grains of the anther of one flower on the stigma of another flower of a different plant in the same species is called xenogamy. Both geitonogamy and xenogamy belong to allogamy. Therefore, allogamy can be simply considered as a type of cross pollination. Physically, geitonogamy is considered as a across pollination method. But genetically, it is a method of self-pollination since gametes of the same flower are fused to form the zygote. In xenogamy, genetically modified offspring are produced by the fusion of genetically different gametes which belong to the same species. Geitonogamy often occurs in flowers that originate in the same stem.

Figure 1: Geitonogamy

Allogamy is achieved with the aid of external pollinating agents. Two types of external pollinating agents can be identified as abiotic and biotic agents. Abiotic agents are wind and water. Biotic agents are insects like bees and butterflies, and animals like snails and birds. Wind pollination is called anemophily and pollination by water is called hydrophily. Insect pollination is called entomophily; bird pollination is called omithophily and snail pollination is called malacophily. 

What is Xenogamy

Xenogamy is the fertilization a flower by the pollen grains of a genetically different flower of the same species. It is the typical cross pollination method where genetically modified offspring are formed. Xenogamy also occurs through external pollinating agents. Flowers which use hydrophily contains long stigma along with unwettable floral parts. In contrast, flowers which use anemophily are small in size and contain exerted stigma and anthers. Flowers which are pollinated by zoophily exhibit several characteristics such as brightly colored petals, nectar, and scents. Entomophily, omithophily and malacophily are types of zoophily. Xenogamy occurring by entomophily is shown in figure 2.  

Figure 2: Xenogamy

Several adaptations of the cross pollinating flower itself prevent self-pollination. In herkogamy, flowers possess mechanical barriers on the stigmatic surface like gynostegium and pollinia. Dichogamy is the differential maturation of pollen and stigma. Some flowers are incompatible with self-pollination. Some plants exhibit male sterility, where pollen grains of the plant are not functional, and only the cross pollination is capable of producing seeds. Heterostyly is the production of stamens and style in different lengths.

Difference Between Allogamy and Xenogamy

Definition

Allogamy: Allogamy is the fertilization of a flower by the pollen grains of another flower. Both geitonogamy and xenogamy belong to allogamy.

Xenogamy: Xenogamy is the fertilization a flower by the pollen grains of a genetically different flower of the same species.

Self/Cross pollination

Allogamy: Allogamy contains both self and cross pollination methods.

Xenogamy: Xenogamy is purely a cross pollination method.

Genetically Modified Offspring

Allogamy: Geitonogamy does not produce a genetically modified offspring.

Xenogamy: Xenogamy produces a genetically modified offspring.

Advantages

Allogamy: Geitonogamy can occur even without the assistance of external pollinating agents.

Xenogamy: Xenogamy is capable of producing beneficial offspring.

Disadvantages

Allogamy:  Genetic variations of the offspring is avoided in geitonogamy.

Xenogamy: Excess force should be generated in order to be pollinated by external pollinating agents.

Adaptations of the Flower

Allogamy: Several geitonogamy flowers are located on the same stem.

Xenogamy: Flowers contain brightly-colored petals, scents, and nectar for attracting the insects and animals.

Examples

Allogamy: Corn is the most common example of geitonogamy flowers.

Xenogamy: Squash, onions, broccoli, spinach, willows, grasses and olive trees are the examples of xenogamy.

Conclusion

Allogamy and xenogamy are two types of pollination methods found among flowers. Allogamy contains both geitonogamy and xenogamy. Geitonogamy is the deposition of pollen grains of one flower on another flower of the same plant. Hence, two plants are genetically similar, and no genetically modified offspring is produced. Geitonogamy is physically a cross pollination method, but genetically it is a self pollination method. Xenogamy is the deposition of pollen grains of one flower on the second flower in a different plant of the same species. Here, two plants are genetically different though they belong to the same species. Hence, genetically modified offspring is produced by xenogamy. Therefore, xenogamy is considered as more beneficial than self pollination. Most flowers bear adaptations to promote cross pollination. The main difference between allogamy and xenogamy is in the pollinating processes.

Reference:
1.”Pollination in Plants: Types, Advantages and Disadvantages.” YourArticleLibrary.com: The Next Generation Library. N.p., 22 Feb. 2014. Web. 02 may 2017.
2.”Types of Pollination.” Types of Pollination | Cross Pollination or Allogamy | [email protected] N.p., n.d. Web. 02 May 2017.

Image Courtesy:
1. “2126664” (Public Domain) via Pixabay
2. “Flower Orange Butterfly Pink Monarch Zinnia Red” (CC0) via Max Pixel

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