Q13(i):

Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and

1. Biological Foundation of Haploid Cell Division

In most multicellular eukaryotes, mitosis typically occurs in diploid ($2n$) cells. However, in specific organisms—notably male Hymenoptera (e.g., honeybees, wasps, and ants) and certain lower plants like bryophytes—mitosis occurs within a haploid ($n$) genetic background. This phenomenon challenges the standard assumption that mitosis is restricted to diploid cells.

2. Analysis of Haploid Insects: The Haplodiploidy Mechanism

In the order Hymenoptera, sex determination follows the Haplodiploidy system:

• Unfertilized Eggs: Develop via parthenogenesis into haploid males (drones).
• Fertilized Eggs: Develop into diploid females (queens and workers).

Because the male drone originates from an unfertilized haploid egg, every somatic cell in his body contains only one set of chromosomes ($n$). Consequently, for the drone to grow and develop, all somatic growth must occur through mitosis in haploid cells. During spermatogenesis in these insects, meiosis is bypassed; instead, a modified mitosis occurs to produce haploid sperm [Per the principle of reproductive economy in haplodiploidy].

3. Analysis of Lower Plants: The Gametophytic Generation

Lower plants, such as mosses (Bryophyta), exhibit a distinct alternation of generations where the haploid gametophyte is the dominant, independent, and photosynthetic phase. The lifecycle involves:

In these organisms, the haploid cell undergoes repeated mitotic cycles to form the complex, multicellular gametophytic plant body (protonema and leafy shoots). Unlike higher plants, where the haploid stage is limited to pollen or embryo sacs, these lower plants rely entirely on the mitotic division of haploid cells for their vegetative survival.

4. Schematic Representation of Mitotic Division

5. Comparison of Ploidy Stability

The fundamental mechanism in both instances is the Equational Division. During the $S$-phase of the cell cycle, DNA replicates ($n \rightarrow 2n$ equivalents of DNA), and during the $M$-phase, the sister chromatids separate. Because there is no homologous pairing (as there is no homolog to pair with), these cells do not undergo meiosis, preserving the haploid integrity of the organism [By the logic of mitotic cell cycle regulation].

Final Solution: Haploid insects (specifically males of the order Hymenoptera) and lower plants (such as bryophytes) utilize mitosis in haploid ($n$) cells to sustain vegetative growth and development, effectively proving that mitosis is not exclusively a process confined to diploid ($2n$) genetic environments.