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Plant Life Cycles: Diplontic Life Cycle, Haplo-diplontic Life Cycle, Practice Problems and FAQs

Plant Life Cycles: Diplontic Life Cycle, Haplo-diplontic Life Cycle, Practice Problems and FAQs

Life is so precious. We all pass through different stages or phases of life like the juvenile phase, reproductive phase, breeding phase and phase of decline. Do you think just like animals, all plants pass through the same phases in their life cycle? 

Do you know that all algae cannot follow the same pattern of life cycle? Compared to humans, this is quite unusual and fairly interesting, isn’t it? You must be thinking, didn’t we just learn that algae have a haplontic life cycle with a more prevalent haploid phase? Well, yes we did but there are a few exceptions. 

Algae such as Fucus have a diplontic life cycle with a more prevalent diploid phase whereas Ectocarpus, Polysiphonia and kelps have a haplo-diplontic life cycle with equally prevalent haploid and diploid phases. 



GIF: Kelp

You will be amazed to know that algae are not the only plants to exhibit the diplontic and haplo-diplontic life cycles. Sounds intriguing right? Let us dig deeper into these life cycle patterns in this article.

Table of contents

  • Diplontic life cycle
  • Haplo-diplontic life cycle
  • Difference between haplontic and diplontic life cycle
  • Practice Problems
  • FAQs

Diplontic life cycle

In this life cycle, the diploid sporophyte is the dominant, photosynthetic, and independent phase of the plant which produces spores by meiosis. Spores develop into gametophytes. The gametophytic phase (n) is represented by single to few-celled haploid gametophytes (n).

Megaspore produces female gametophytes called embryo sacs. Microspore produces male gametophytes called pollen grains. The production of male and female gametes occurs inside the pollen grain and embryo sac respectively. Fusion of gametes results in the formation of a zygote (2n) which develops into a sporophyte. Examples include Fucus, gymnosperms and angiosperms.



Fig: Diplontic life cycle

Haplo-diplontic life cycle

In this life cycle, either haploid gametophyte or diploid sporophyte is slightly dominant over the other. Both gametophytes and sporophytes can be multicellular and free-living. The haploid gametophyte (n) undergoes mitosis to produce gametes which fuse to form the diploid zygote (2n). Zygote develops into a diploid sporophyte which produces haploid spores by meiosis. The spores develop into the gametophyte (n). This cycle continues. Examples include Ectocarpus, Polysiphonia, kelps, bryophytes and pteridophytes. 

Haplo-diplontic life cycle in bryophytes

In bryophytes, haploid gametophytes are dominant, independent, photosynthetic, thalloid or erect. The Diploid sporophyte is short-lived and depends totally or partially on the gametophyte for anchorage and nutrition. Examples include Funaria



Fig: Haplo-diplontic life cycle in bryophytes

Haplo-diplontic life cycle in pteridophytes

In pteridophytes, diploid sporophytes are dominant, independent, photosynthetic with a vascular plant body. It alternates with short-lived haploid gametophytes.



Fig: Haplo-diplontic life cycle in pteridophytes

Difference between haplontic and diplontic life cycle

Haplontic life cycle

Diplontic life cycle

The dominant phase is gametophyte which is independent, free-living and undergoes photosynthesis

The dominant phase is the sporophyte which is independent, free-living and can do photosynthesis

The haploid cells divide mitotically and produce new cells

The zygote divide mitotically and produce a new plant

The meiosis occurs in zygote to form haploid cells

Gametes are produced through meiosis

Examples include Volvox and Spirogyra

Examples include Fucus and Cladophora

Practice Problems

Q1. Study the given diagram and identify the life cycle pattern it represents.

A. Haplontic life cycle in Fucus
B. Diplontic life cycle in angiosperms
C. Haplo-diplontic life cycle in gymnosperms
D. Diplontic life cycle in Spirogyra



Solution: In the given diagram, the dominant phase of the life cycle is the diploid sporophytic generation. The haploid gametophytic generation is short-lived and represented by unicellular or few celled gametophytes that give rise to haploid gametes which fuse to form diploid zygotes that develop into the dominant sporophytes. Thus, it represents a diplontic life cycle as is seen in angiosperms and gymnosperms. Hence, the correct option is b.

Q2. The dominant haploid gametophyte is represented by an independent, photosynthetic, thalloid phase. This alternates with the short-lived multicellular sporophyte which is totally or partially dependent on the gametophyte for existence. This type of pattern is present in _________________.

A. bryophytes 
B. pteridophytes
C. gymnosperms
D. most of the algae

Solution: The life cycle of bryophytes is divided into haploid gametophyte and diploid sporophyte phases. The gametophytic phase is dominant, haploid, photosynthetic and free living. The gametophyte is an erect thalloid (not differentiated into true stems, leaves or roots). The sporophytic phase is multicellular, diploid, short-lived and dependent on the gametophyte for nutrition and anchorage. Thus, the correct answer is option a.

Q3. Match the columns A and B, and choose the correct combination from the options given.

Column A

Column B

A. Diplontic life cycle 

i. Gymnosperms and angiosperms

B. Haplontic life cycle

ii. Spirogyra

C. Haplo-diplontic life cycle 

iii. Bryophytes and pteridophytes

A.  A - i, B - ii, C - iii
B. A - ii, B - i, C - iii
C. A - iii, B - i, C - ii
D. A - ii, B - iii, C - i

Solution:  Gymnosperms and angiosperms usually exhibit a diplontic life cycle with dominant diploid sporophytes and a short-lived haploid gametophyte. Algae such as Spirogyra exhibit a haplontic life cycle with dominant haploid gametophyte phase and a short-lived diploid sporophyte represented by the zygote only. Bryophytes and pteridophytes have a haplodiplontic life cycle with multicellular gametophytic and sporophytic phases both of which have equal predominance. Thus, the correct answer is a.

Q4. Both male and female gametophytes are independent as well as free-living in ______________.

A. Pinus
B. Mustard
C. Cycas
D. Sphagnum

Solution: Free-living gametophytes are found in members of Bryophyta and Pteridophyta. Sphagnum belongs to Bryophyta, showing that it has independent gametophytes. Hence, the correct option is d.

Q5. Which of the following options correctly describe the life cycle of Ectocarpus and Fucus, respectively?

A. Haplontic, Diplontic
B. Haplo-diplontic, Diplontic 
C. Diplontic, Haplo-diplontic
D. Haplo-diplontic, Haplontic

Solution: The life cycle of any sexually reproducing plant is characterised by alternation of generations. The haploid gametophyte which produces gametes alternates with the diploid sporophyte that produces spores. The life cycle patterns are of three types: Haplontic, diplontic and haplo-diplontic.

  • Fucus exhibits a diplontic life cycle in which the diploid sporophytic body is dominant.
  • Ectocarpus has a haplo-diplontic life cycle with almost equally prevalent sporophytic and gametophytic phases. Hence, the correct option is b.

Q6. Gametophytes are not free-living and independent in _________________.

A. Pteridophytes
B. Algae
C. Angiosperms
D. Bryophytes

Solution: Plants show an alternation of generation between gamete-producing haploid gametophyte and spore-producing diploid sporophyte. In gymnosperms and angiosperms, the gametophytes are not free-living and depend upon the sporophyte for existence. Thus, the male and female gametophytes do not have an independent existence in gymnosperms and angiosperms. Hence, the correct option is c.

Q7. Which of the following divisions of the plant kingdom shows all the three different patterns of life cycle?

A.  Angiosperms
B. Pteridophytes
C. Bryophytes
D. Algae

Solution: In the life cycle of plants, there is an alternation of generation between gamete producing haploid gametophyte and spore producing diploid sporophyte. Based on the prevalence of the sporophyte and gametophyte stages, the life cycle of plants can be of three types as follows:

  • Haplontic: The haploid gametophyte phase is dominant.
  • Diplontic: The diploid sporophyte phase is dominant.
  • Haplo-diplontic: In this life cycle both the diploid and haploid phases are equally prevalent.

Algae shows all three types of life cycle patterns -

  • Haplontic: Examples include Spirogyra, Volvox and some Chlamydomonas species
  • Diplontic: Examples include Fucus
  • Haplo-diplontic: Examples include Ectocarpus, Polysiphonia, kelps. Hence, the correct option is d.

Q8. How is the sporophyte of a bryophyte different from that of a pteridophyte?

Answer:

Bryophyte

Pteridophyte

Sporophyte is short-lived and depends totally or partially on the gametophyte for anchorage and nutrition.

Sporophytes are dominant, independent, photosynthetic with a well-differentiated vascular plant body.

FAQs

Q1. Which type of gametes does Fucus have?
Answer: Oogamous fusion occurs when one large, non-motile (static) female gamete fuses with a smaller, motile male gamete. Thus, the Fucus have oogamous types of gametes.

Q2. Which stage is considered as the dominant stage in the life cycle of a tree?
Answer: The sporophyte is considered as the dominant generation in the life cycle of a tree. For example, the mango tree is a sporophyte. Embryo sac and pollen grain are the gametophytes. These multicellular male and female gametophytes are present within the flowers of the sporophyte.

Q3. In the life cycle of pteridophytes, which event is considered as an important step in evolution?
Answer: Heterospory ( two kinds of spores are borne by the same plant) is considered as an important step in evolution in the life cycle of pteridophytes. It is the precursor to the seed habit. In pteridophytes such as Selaginella and Salvinia, heterospory evolved first.

Q4. Why are pteridophytes called incomplete land plants?
Answer: Pteridophytes are considered as incomplete land plants because they require water for primary root formation and fertilisation.

Related Topics

Plant life cycles: Alternation of generations, Haplontic Life Cycle, Practice Problems and FAQs 

Bryophytes: Life cycle, Classification, Economic importance, Differences between liverworts and mosses, Practice Problems and FAQs 

Pteridophytes: Alternation of generations, Reproduction, Life cycle, Classification, Economic Importance, Practice Problems and FAQs 

Angiosperms: Life cycle, Classification, Economic Importance, Practice Problems and FAQs 

Gymnosperms: Alternation of generations, Reproduction, Classification and Economic importance, Practice Problems and FAQs 

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