Is it possible for sepal leaves to get chromosome number n, or are they always 2n?
Chromosomes in plants
DNA material is placed in the nucleus of most living creatures, including plants. Prior cell division, DNA will organize and form dense structures called chromosomes. Number of chromosomes is species-specific, and usually all somatic cells have two times higher amount of DNA compared to the reproductive cells. Reason is simple. Sexual reproduction results in fertilized egg or the beginning of new life. Pollen and ovules contain reduced number of chromosomes (n) in order to reach required number of chromosomes (2n) during fertilization. Once genetic information is completed, zygote has all “recipes” that it needs for successful development. Sexual reproduction leads to genetically different organisms with unique qualities, inherited partially from their mother, and partially from their father. High genetic variability results in stronger and better adapted organisms.
Besides sexual reproduction, plant could reproduce asexually. Buds on different parts of their body (root, stem, leaves…) will develop in new, daughter plants. New individuals will have the same genetic material as their mother. They will grow by increasing the number of already diploid cells through simple cell division. Unlike sexual reproduction, asexual reproduction doesn’t increase genetic variability.
Some plant species have 2 different generations and form distinct shapes in a single life cycle. Life of these plants consists of two phase, haploid and diploid, that appear alternatively. Moss is typical example of a plant with two generations. Haploid generation is more dominant. When haploid spores are released from the sporangium, moss will develop. Unlike other organisms, it doesn’t require another set of chromosomes to accomplish successful development. From the spore, it will turn into haploid male or female gametophytes (plants). Next generation will start when sperm cells produced in the male gametophyte reach an egg of the female gametophyte. Diploid zygote will develop in sporophyte or diploid plant. This generation will form capsule or sporangium where haploid spores will be formed during meiotic division. From these cells (spores), haploid generation will develop again.
Here are few facts about number of chromosomes in different plants:
Ophioglossum reticulatum or adder’s tongue lives in tropic and subtropical areas. This plant has the highest number of chromosomes in the world. Diploid plant has 1440 chromosomes.
Cotton is a plant that forms soft and fluffy fiber balls around the seeds. Exploatation of these cellulose fibers began early, ~5000 years BC. Modern (cultivated) cotton is allotetraploid. That means that one plant has 4 complete sets of chromosomes that originate from different species. Allopolyploids are result of hybridization of plants that are not related. Chromosomes that belong to different species can’t bind with each other to form a fully functional diploid number, but if each set of the chromosomes duplicate itself, new plant, with duplicated 2 or more sets of chromosomes will emerge. Some polyploid organisms are infertile, but that is not always the case. If chromosomal sets manage to duplicate, newly created plants could become more resistant to the environmental conditions.
Besides the cotton, there is a long list of polyploid plant that are widely used by human. Typical example is Triticum turgidum (wheat). That is a hexaploid plant, used for production of pastas and bread. It has 6 complete chromosomal sets, 4 from wheat and 2 sets from rye. Tobacco and potato are tetraploids; their DNA material consists of 4 different chromosomal sets.
Various species in the cabbage family have the same genetic material. Broccoli, cabbage, kale, kohlrabi, brussels sprouts, and cauliflower maybe don’t look like closest relatives at first glance, but they all have the same number and type of chromosomes, 2n=18.
Estimated number of plants on the Earth, including mosses, ferns, algae, gymnospems, dicotyledons and monocotyledones overcomes 290 000 different species. Plant show great variety of shapes, structures and adaptations that helped them survive in various areas. They also have wide arsenal of reproduction techniques that are used either as a normal part of their life cycle or as a way to overcome negative environmental conditions. They have more flexible genetic material (at least I never heard for a polyploid monkey), but basic principles associated with duplication and dissemination of the genetic material from one generation to another is the same. In sexually reproducing plants, only pollen and ovules have haploid number of chromosomes. No matter on the genome size.