The Growth Forms of Developing Fern Gametophytes: a Theoretical Consideration of Form and Function Relationships

This paper examines the hypothesis that the primary physiologicalroles of the different structures of fern gametophytes haveacted as significant evolutionary constraints on the growthforms of these structures. Geometric models of absorptive andphotoreceptive structures were constructed to describe the spherical,tip-growing, and planar growth forms, which are actually orcould hypothetically be adopted by each gametophytic structure.The surface-to-volume ratios calculated for the different absorptivestructures suggest that the tip-growing form of the fern rhizoidprovides the most favorable geometric circumstance for waterand nutrient absorption; thus, the selection pressure to optimizeabsorptive processes has apparently influenced the growth formof the absorptive structure. In contrast, similar calculationsfor photoreceptive structures demonstrate that these forms havealmost identical surface-to-volume relationships; thus, thepresumptive need to increase photoreceptive efficiency has notacted as a selection pressure for a specific photoreceptiveform. This result is interpreted in terms of how different growthforms may offer specific advantages for accomplishing otheraspects of the photosynthetic process, e.g. phototropic growthand assimilate transport, which are unrelated to photoreceptionper se. Fern gametophyte, photoreception, absorption, optimal shape, spore, protonema, prothallus