The moss Physcomitrella patens is a great model system for studying plant gene function using reverse genetic approaches. It undergoes efficient gene-targeting by homologous recombination, allowing the generation of specific gene knockout and tagging a gene at its endogenous locus. Additionally, RNAi is quite effective in P. patens, providing an effective tool for rapid gene silencing and phenotypic characterization. Taking advantage of these features, this dissertation described the establishment of a system to perform an unbiased gene-by-gene RNAi assay to screen for tip growth phenotypes in P. patens. A small set of RNAi constructs were tested, within them one gene was identified to be involved in the vesicle transport between ER and Golgi, which when silenced resulted in severe tip-growth phenotype. This showed that the pilot screen was a successful proof of concept for a large RNAi screen in the future. A more targeted approach was utilized to study the function of the class VIII myosin gene family. Analysis of the phenotypes of a myosin VIII quintuple knockout line revealed that there were defects in protonemal patterning and cell plate placement. Further, using cellular localization studies I demonstrated the involvement of actin and myosin VIII in cell division plane specification and phragmoplast guidance. Overall, this dissertation demonstrates two examples of what moss as a model system can achieve, both in screening for potentially interesting genes and in studying the basic molecular functions of a specific gene family. While the screen provided novel candidates for future studies of polarized growth in moss, the targeted approach has begun to answer a long-standing question in plant cell biology regarding the role of actin and myosin in plant cell division.