Denitrification is a microbial process where facultative anaerobic bacteria, such as Pseudomonas and Paracoccus, reduce nitrate (NO3-) to gaseous nitrogen (N2) or nitrous oxide (N2O) under oxygen-depleted conditions. This process is a critical component of the global nitrogen cycle, as it returns nitrogen to the atmosphere, thereby balancing the nitrogen fixation process performed by other organisms.

Ammonification is a key step in the nitrogen cycle where decomposers, such as bacteria and fungi, break down organic nitrogenous compounds like proteins and amino acids from dead organisms into ammonia (NH3) or ammonium ions (NH4+), which can then be utilized by other plants.

Nitrogen fixation is a critical biological process where atmospheric nitrogen is converted into ammonia. Both Azotobacter, which is a free-living soil bacterium, and Rhizobium, which forms symbiotic relationships with leguminous plant roots, are well-documented nitrogen-fixing organisms that play essential roles in the global nitrogen cycle by enriching soil fertility.

Emil Fischer proposed the 'lock and key' hypothesis in 1894. This model suggests that the enzyme's active site and the substrate possess complementary geometric shapes, allowing them to fit together precisely to form an enzyme-substrate complex.

Mesosomes are invaginations of the plasma membrane in bacteria. Historically, they were thought to be involved in various cellular processes, including DNA replication and cell division. Research has shown that mesosomes are rich in respiratory enzymes, such as those involved in the electron transport chain, suggesting they function as sites for cellular respiration in prokaryotes, analogous to the cristae of mitochondria in eukaryotic cells.

Photosynthesis is the fundamental process by which photoautotrophs, such as plants, algae, and some bacteria, capture solar energy. They use this energy to convert carbon dioxide and water into glucose (food) and oxygen. This process is essential for sustaining life on Earth by providing the primary energy source for most food webs.

In many plants, the stem exhibits negative phototropism (growing away from light) and positive geotropism (growing toward gravity) during early development or in specific conditions, though stems are generally characterized by positive phototropism. Note: This question implies a specific growth response often associated with roots, but 'Stem' is the provided answer.

Parthenogenesis refers to the development of an embryo or organism from an unfertilized egg or spore. In the context of plant life cycles, it is a form of asexual reproduction where the typical fusion of gametes is bypassed. While the term is often used in zoology, it is also applied in botany to describe the development of reproductive cells without the requirement of fertilization.

Polyembryony is the phenomenon where multiple embryos develop from a single zygote or fertilized egg. This can occur through various mechanisms such as the cleavage of the proembryo or the development of adventive embryos from the nucellus or integuments. It is a common occurrence in certain gymnosperms and some angiosperm species.

Petals, often brightly colored and scented, serve as the primary visual and olfactory attractants for pollinators like insects. Sepals may also contribute to the overall display. Together, these structures ensure that pollinators are drawn to the flower, increasing the likelihood of successful cross-pollination.