Plant Life Cycles: Alternation of Generations, Haplontic Life Cycle, Practice Problems and FAQs

You all like Thor (God of thunder) from the Marvel comic series called ‘Avengers’. You know that he is immortal, that means he will never die. But what about us? We all have different phases in life like the juvenile phase, reproductive phase, breeding phase and phase of senescence. Once we reach the phase of senescence, our body organs will show degradation and finally we will die. But what about plants? 

Have you ever wondered about the life cycle of plants? Do you think algae, bryophytes, pteridophytes, gymnosperms and angiosperms show the same type of life cycle? Do they have some other differences apart from their morphological, anatomical and physiological characteristics?

Fig: Plant life cycle

Yes! The query is worth it! Plants differ from each other with respect to their life cycles. The difference is not only in the number of years they live but also about the type of generation dominant in their life cycle. They show alternation of generation that means they show haploid and diploid phases in their life cycle. Let’s take a deep dive into the world of life cycles of various plant groups and alternation of generation occurring in them.

Table of contents

• Alternation of generation
• Patterns of life cycle
• Haplontic life cycle
• Practice Problems
• FAQs

Alternation of generation

The diploid phase of a plant is sporophyte (2n) and the haploid phase is gametophyte (n). The type of life cycle in which sporophyte alternates with gametophyte is known as alternation of generations. 

Process of alternation of generation

The gametophyte (haploid plant body) possesses sex organs which produce haploid gametes by mitosis. Gametes fuse to form the zygote (2n) which is diploid. It undergoes mitosis and develops into a sporophyte. Sporophyte (2n) produces spores (n) by meiosis which develop into the gametophyte (n). This cycle continues. 

Fig: Alternation of generation

Patterns of life cycle

Based on the prevalence of the haploid and diploid phases, there are three patterns of life cycle in plants as follows:

Haplontic 

The haploid phase called gametophyte is the dominant photosynthetic phase in this life cycle. Examples include Volvox, Spirogyra etc. 

Diplontic

The diploid phase called sporophyte is the dominant photosynthetic independent phase in this life cycle. Examples include gymnosperms and angiosperms. 

Haplodiplontic

Both haploid and diploid phases are equally prevalent in this life cycle. Both gametophyte and sporophyte are multicellular. Examples include bryophytes and pteridophytes. 

Fig: Life cycle patterns

Haplontic life cycle

In this life cycle, the sporophytic generation is represented by one-celled zygote and free-living sporophytes (2n) are absent. The zygote undergoes meiosis which results in the formation of haploid spores (n). The haploid spores divide mitotically and form the gametophyte (n) which is the free-living dominant phase. Gametophyte produces gametes by mitosis which fuse to form zygotes (2n) and the cycle is repeated. Examples include algae such as Volvox, Spirogyra, some species of Chlamydomonas.

Fig: Haplontic life cycle

Practice Problems

Q1. A plant has no free-living sporophyte but has a free-living gametophyte which is the dominant, photosynthetic phase. We are talking about ____________.

A. Haplontic life cycle as shown in Volvox
B. Diplontic life cycle as shown in angiosperms and gymnosperms
C. Diplontic life cycle as shown in bryophytes and pteridophytes
D. Haplo-diplontic life cycle as shown in Ectocarpus

Solution: In algae like Volvox the haploid gametophyte is predominant and free-living. It forms the main photosynthetic, vegetative plant body and produces male and female gametes which fuse to form a diploid zygote. The sporophytic phase is represented by this diploid zygote which is not free-living and immediately undergoes meiosis to form haploid spores that germinate into haploid gametophytic plant bodies. As the photosynthetic haploid gametophyte represents the dominant phase of the life cycle, the life cycle is said to be haplontic. Thus, the correct option is a.

Q2. Identify the plant in which sporophytic generation can be represented by zygote? 

A. Liverworts
B. Selaginella
C. Pinus
D. Ulothrix

Solution: The sporophytic generation is short-lived and represented by the diploid zygote in plants that exhibit a haplontic life cycle. This zygote undergoes immediate meiosis to form haploid spores that germinate into haploid gametophytes that represent the dominant, free-living, photosynthetic plant body. Most algae such as Spirogyra, Ulothrix, etc., have a haplontic life cycle. Thus, the correct answer is option d.

Q3. If the zygote of an algae with haplontic life cycle has 20 chromosomes then the chromosome number of the dominant plant body will be ________________.

A. 20
B. 10
C. 40
D. 60

Solution: The haplontic life cycle is characterised by a dominant haploid gametophyte which forms the main plant body and a short-lived diploid sporophyte which is represented by the zygote only. This zygote undergoes meiosis to form haploid spores which develop into haploid gametophytes. Thus, if the diploid zygote has 20 chromosomes, the gametophyte will be haploid and have 10 chromosomes only. The correct option is b.

Q4. Study the following diagram and choose the option representing the labels correctly.

A. A - gametophyte, B - mitosis, C - gametes, D - zygote
B. A - zygote, B - meiosis, C - spores, D - gametophyte
C. A - zygote, B - mitosis, C - spores, D - sporophyte
D. A - gametophyte, B - meiosis, C - spores, D - zygote

Solution: The given figure represents the haplontic life cycle in which -

• The diploid zygote (A) represents the short-lived sporophyte and undergoes meiosis (B) to form haploid spores (C).
• The spores germinate into the haploid gametophyte (D) which represents the dominant phase. It produces gametes by mitosis which fuse to form the zygote. Hence the correct option is b. 

Fig: Haplontic life cycle

Q5. The dominant phase of life cycle in most algae is ________________. 

A. haploid
B. diploid
C. both haploid and diploid
D. isomorphic

Solution: In most algae, the dominant phase is the haploid gametophyte. A free-living diploid sporophyte is absent. The sporophytic generation is short-lived and represented by a diploid zygote only. Meiosis occurs in the zygote and forms haploid spores which divide mitotically to form new gametophytes. In such a life cycle, the dominant phase is haploid and the life cycle is said to be haplontic. Hence the correct option is a.

Q6. Zygotic meiosis is considered as a characteristic feature of ________________.

A. Ectocarpus
B. Fucus
C. Chlamydomonas 
D. Funaria

Solution: Chlamydomonas is a unicellular algae with a haplontic life cycle. The main plant body is a haploid gametophyte that forms gametes through mitosis, which on fusion forms a diploid zygote. Under favourable conditions, this diploid zygote undergoes meiosis to produce haploid spores which would then germinate into haploid plant bodies. Hence the correct option is c.

Q7. Name some organisms in which haplontic life cycle is seen?
Answer : In haplontic life cycle, the sporophytic generation is represented by one-celled zygote and free-living sporophytes are absent. The zygote undergoes meiosis which results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte which is the free-living dominant phase. Gametophyte produces gametes by mitosis which fuse to form zygotes (2n) and the cycle is repeated. Examples include algae such as Volvox, Spirogyra, some species of Chlamydomonas.

FAQs

Q1. Do haploid cells undergo meiosis?
Answer: No, the haploid cells do not undergo meiosis but they are formed through the process of meiosis.

Q2. Which process is responsible for altering the ploidy of a cell?
Answer: Meiosis is a type of cell division that reduces the chromosome number to half. Therefore, it alters the ploidy of a cell.

Q3. Why do organisms like plants need a life cycle?
Answer: All living beings have a life cycle, whether it is plant, animal or bacteria. Life always has a beginning point, followed by that there is a growth phase, reproductive phase and phase of senescence. This life cycle of plants is important to agriculture. A farmer can easily plan their cultivation time, fertilising time and harvesting time according to this life cycle. 

Q4. What would be the ploidy of an organism if it has two sets of chromosomes?
Answer: The ploidy of an organism will be 2n if the cell has two sets of chromosomes. 

Q5. Why is the haplontic life cycle considered primitive?
Answer: Haplontic life cycle is the primitive type of life cycle. It is characterised by dominant gametophyte and zygotic meiosis. Here, a free-living sporophyte is absent. The sporophyte generation is represented by the one-celled zygote. This zygote undergoes meiosis and forms haploid spores.

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