Understand the structure of the stamen, the layers of the microsporangium, and the development of pollen grains.
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Pollen grains are practically indestructible.
Pollen grains are so tough they are preserved as fossils for millions of years! Their outer layer is made of sporopollenin, the most resistant organic material known, capable of withstanding extreme heat, strong acids, and alkalis.
The four wall layers of the microsporangium and their functions.
A typical angiosperm stamen has two parts: the long, slender stalk called the filament, and the terminal anther. The anther is generally bilobed, with each lobe having two theca (making it dithecous).
Inside the four-sided anther are four microsporangia located at the corners, which develop further into pollen sacs.
Visual of the anther wall layers and microspore mother cells.
Transverse section of a microsporangium showing the protective outer layers and the innermost nutritive tapetum.
Formation of pollen grains and their cellular structure.
Inside the microsporangium, each cell of the sporogenous tissue acts as a Pollen Mother Cell (PMC).
Through a process called microsporogenesis, the PMC undergoes meiosis to form a cluster of four cells known as a microspore tetrad. As the anther matures and dehydrates, these microspores separate and develop into independent pollen grains (the male gametophytes).
Stages of a microspore maturing into a 2-celled pollen grain.
Stages of a microspore maturing into a pollen grain with a vegetative and generative cell.
Key terms for the microsporangium and pollen.
Recall test on the microsporangium layers and pollen structure.
The development of pollen grains occurs within the microsporangium, which is surrounded by four wall layers. The innermost wall layer of the microsporangium is the , which nourishes the developing pollen grains. As the pollen grain matures, it develops a hard outer layer called the . This protective layer is made of the highly resistant organic material known as , which can withstand high temperatures and strong acids. The areas where this material is absent on the pollen surface are called , allowing for eventual pollen tube growth. Finally, in over 60 percent of angiosperms, the mature pollen grains are shed at the stage, containing a vegetative cell and a generative cell.