Chapter 13: Sex and reproduction in non-seed plants
Chapter 13: Sex and reproduction in non-seed plants
Almost all plants are capable of reproduction without sex. Most commonly this happens as a consequence of the plant being severed into pieces and these pieces being able to regenerate the parts that were lost. Additionally, some species have developed pieces that are particularly prone to be broken off, e.g., the gemmae (singular of gemma) of some mosses and liverworts. It is relatively common for flowering plants to produce asexual propagules in the places where seeds normally develop, producing seed-like structures without the normal process of sex. Although reproduction without sex is common in plants and for some species it represents their sole means of reproduction, most plants rely on sexual reproduction, and two innovations, the seed and the flower, have been particularly significant to the evolution of plants.
With respect to sex, the plant kingdom illustrates wonderful patterns of unity and diversification. On the one hand, all plant life shows a fundamentally similar pattern of sex, on the other hand, plants illustrate remarkable diversification of this common theme.
TOPICS
- Alternation of generations
- Mosses, liverworts and hornworts
- Ferns
- Horsetails
- Clubmosses
All plants exhibit alternation of generation s, they produce two types of multicellular organisms: one diploid and derived from the development of a zygote, and one haploid and derived from the development of a haploid cell called a spore. ‘Development’ involves the proliferation of cells, the differentiation of cells and the formation specific structures with specific roles, i.e.. morphogenesis.
All plants exhibit alternation of generations and alternate between haploid and diploid organisms. To complete the sexual cycle the haploid organism must produce gametes that unite to form the zygote. That is, among all the haploid cells that are produced as a result of cell divisions of a haploid spore, some of the cells are endowed with special capabilities that allow them to encounter and interact with another gamete to form a zygote. Similarly, of all the diploid cells derived from the zygote, some, called a ‘spore mother cells’, are endowed with the ability to undergo meiosis to produce haploid cells, spores, that grow into haploid individuals. Note that meiosis does NOT produce gametes, rather it produces spores that are dispersed and develop into haploid plants. The diploid plant that produces spore mother cells and spores is called a ‘sporophyte’. The gamete-producing organism, which is haploid, is called a gametophyte. Thus, there is an ‘alternation of generations’ with a haploid form alternating with a diploid form (Figure 1). Often one of the two forms is challenging to appreciate, largely because they are often challenging to see — they are often small, short-lived, and may not live a separate existence from the previous generation. That is, the haploid and diploid forms may not spatially distinct from each other;