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Alternation Of Generations

Alternation of generations (also known as alternation of phases or metagenesis) is a term primarily used to describe the life cycle of plants (taken here to mean the Archaeplastida). A multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of npairs. A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n. The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces gametes by mitosis, which does not alter the number of chromosomes. Two gametes (originating from different organisms of the same species or from the same organism) fuse to produce a zygote, which develops into a diploid sporophyte. This cycle, from gametophyte to gametophyte (or equally from sporophyte to sporophyte), is the way in which all land plants and many algae undergo sexual reproduction.

The relationship between the sporophyte and gametophyte varies among different groups of plants.
  • In those algae which have alternation of generations, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance.
  • In liverwortsmosses and hornworts, the sporophyte is less well-developed than the gametophyte and is largely dependent on it. Although moss and hornwort sporophytes can photosynthesise, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen.
  • By contrast, in all modern vascular plants, the gametophyte is less well-developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity.
  • In ferns, the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte is briefly dependent for its nutrition.
  • In flowering plants, the reduction of the gametophyte is much more extreme. It consists of a few cells that grow entirely inside the sporophyte.
Alternation of generations in the lifecycle of bryophytes, pteridophytes, gymnosperms and angiosperms are given in Figure 2.2, Figure 2.3, Figure 2.4 and Figure 2.5, respectively.

Differences between monocots and dicots are shown in Table 2.2.

Figure 2.2 Schematic Representation of Life Cycle of Bryophytes


Figure 2.3 Schematic Representation of the Life Cycle of Pteridophytes


Figure 2.4 Schematic Representation of the Life Cycle of Angiosperms

Difference between Monocots and Dicots



  • The seeds of these plants bear only one cotyledon
  • Leaves have parallel venation
  • Roots are fibrous
  • Flowers bear three or multiples of three floral parts (trimerous)
  • Produce seeds containing embryos with two cotyledons
  • Leaves have reticulate venation
  • They have taproot system
  • Flowers have four or five (multiples of 4 or 5) floral parts
  • Vascular bundles are closed and scattered (i.e., cambium is not found in closed bundles) and hence no secondary growth
  • Examples: Maize (Zea mays), rice (Oryza sativa), sugarcane (Saccharum officinarum), wheat (Triticumaestivum), onion (Allium epa), coconut (Cocosnucifera), grasses and banana (Musa paradisiaca)
  • Vascular bundles are arranged in a ring and are open type, i.e., possess cambium which is responsible for secondary growth
  • Examples: Potato (Solanum tuberosum), pea (Pisumsativum), sunflower (Helianthus annuus), rose (Rosa indica), neem (Melia indica), apple (Malus sylvestris), banyan (Ficus benghalensis), etc.

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