![]() The reductive evolution of liverwort and hornwort spores entailed the loss of perine in both groups and the aperture in liverworts. Based on the current phylogenies, the ancestral plant spore contained an aperture, exine, intine, and perine. In addition to providing a site for water intake and germination, the elastic aperture is likely involved in desiccation tolerance. This complex aperture is elastic and enables the proximal spore surface to cycle between being compressed (concave) and expanded (rounded). Across the proximal surface of the polar spores, an aperture begins formation at the onset of spore development and consists of an expanded intine, an annulus, and a central pad with radiating fibers. The exine is at least partially extrasporal in origin, while the perine is derived exclusively from outside the spore. On the distal spore surface, the intine forms internally, while the spiny perine ornamentation is assembled. Dense globules assemble evenly throughout the locule, and these are incorporated progressively onto the spore surface to form the perine external to the exine. As they expand and separate, a band of exine is produced external to a thin foundation layer of tripartite lamellae. Nascent spores are enclosed in a second matrix that is surrounded by a thin SMC wall and suspended in the locular material. During capsule expansion, the diploid cells of the capsule, including spore mother cells (SMCs), inner capsule wall layer (spore sac), and columella, contribute a locular fibrillar matrix that contains the machinery and nutrients for spore ontogeny. Both diploid (sporophytic) and haploid (spores) cells contribute to the development and maturation of spores. patens sporogenesis is detailed from capsule expansion to mature spore formation, with emphasis on the construction of the complex spore wall and proximal aperture. In this study, the complete process of P. ![]() 2Department of Chemistry and Biochemistry, University of Regina, Regina, SK, CanadaĪlthough the evolution of spores was critical to the diversification of plants on land, sporogenesis is incompletely characterized for model plants such as Physcomitrium patens.1Department of Plant Biology, Southern Illinois University, Carbondale, IL, United States.
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