Microspore are the spores of ground plants that develop into male gametophytes , while megaspores develop into female gametophytes. [1] The male gametophyte gives rise to sperm cells, which are used to form a zygote for fertilization of the egg cell . Megaspores are structures that are part of the alternation of generations in many seedless vascular cryptogams , all gymnosperms , and all angiosperms . Heterospores using microspores and megaspores in many plant groups during the Devonian period Plants with life cycles arose independently. Microspores are haploid , andarise from diploid microsporocytes by meiosis .
Morphology
Microspores have three different types of wall layers. The outer layer is called the perispore, next to it is the exospore, and the inner layer is the endospore . Perispore is the thickest of the three layers while exospore and endospore are relatively similar in width.
Seedless vascular plants
In heterosporous seedless vascular plants, modified leaves called microsporophylls bear microsporangia that undergo meiosis containing many microsporocytes , each producing four microspores. Each microspore can develop into a male gametophyte which consists of a somewhat spherical antheridium within the microspore wall . Each antheridium produces 128 or 256 sperm cells with flagella. [3] Only heterosporous ferns are aquatic or semi-aquatic, including the genera Marsilea , Regnellidium , Pilularia , Salvinia , and Azolla, Heterospory is also known in the lycopod genus Selaginella and the quillwort genus Isoëtes .
Types of Seedless Vascular Plants:
- Ferns
- spikemosses
- quillworts
Gymnosperm
In seed plants , microspores develop into pollen grains each containing a reduced, multicellular male gametophyte. [5] Megaspores, in turn, develop into fewer female gametophytes that produce egg cells, which, once fertilized, develop into seeds. pollen cone or microstrobiliUp to 50 or more develop toward the tips of the lower branches, usually in clusters. The microsporangia of gymnosperms develop in pairs towards the base of the scales, which are therefore called microsporophylls. In microsporangia each of the microsporocytes undergoes meiosis, forming four haploid microspores. These develop into pollen grains, each containing four cells and a pair of external air sacs. The air sac gives the pollen grains extra buoyancy which helps in the dispersal of the wind. [3]
Types of Gymnosperms:
- conifers
- Pines
- Ginkgo
- Shrink
- Gnetophytes
Angiosperm
As the anther of a flowering plant develops, four patches of tissue separate from the main mass of cells. These patches of tissue contain many diploid microsporocyte cells, each of which undergoes meiosis and produces a quartet of microspores. Four cells (pollen sacs) lined with nutritive tapetal cells are visible until the microspores are produced. After meiosis, haploid microspores undergo several changes:
- The microspore divides by mitosis producing two cells. The first of the cells (generative cell) is smaller and the second is formed inside the larger cell (tube cell).
- The members of each part of the microspores are different from each other.
- Then a double layer wall develops around each microspore.
Embryogenesis
Although it is not the normal route of a microspore, this process is the most effective method of yielding haploid and double haploid plants through the use of male sex hormones. [6] Under certain stressors, such as heat or starvation, plants select for microspore embryogenesis. It was found that more than 250 different species of angiosperms responded in this way. [6] In the anther, after a microspore undergoes microsporogenesis, it can deviate toward embryogenesis and become star-like microspores. The microspore can then go one of four ways: become an embryogenic microspore, from callogenesis to organogenesis (haploid/double haploid plant), become a pollen-like structure, or die.
