What is the defining feature of pollen grains under microscopic examination?

Pollen grains are primarily characterized by their intricate and varied outer structures, which often display features such as spherical shapes that possess distinctive cell walls. Under microscopic examination, a defining feature of pollen grains is the presence of concentric circles or patterns on their surface, which can vary widely among different species of plants, making them identifiable and significant in botanical studies.

The spherical shape coupled with the complex structure is key to their function in reproduction, as pollen grains contain the male gametes for flowering plants. This structural characteristic allows for the efficient transfer of genetic materials during the pollination process, highlighting the evolutionary adaptations of pollen grains for survival and reproduction in diverse environments. Understanding these features is crucial for the identification and study of plant species in various ecological and agricultural contexts.