Harmomegathic characters of Pteridophyta spores and Spermatophyta pollen

Abstract

Spore and pollen grain volume decrease after meiotic cleavage and starts to increase during exine and intine formation; with dehydration volume decreases again. In the atmosphere pollen volume accomodates according to relative humidity. The mechanisms leading to and regulating dehydration of the sporangia and anthers, the stationary phase during dispersal, and hydration are species specific. They vary between taxonomic groups and are influenced by the environment. Pollen adhesion occurs only in angiosperms and is a prerequisite for pollen hydration; the opposite is true for gymnosperms. The environment of the landing site determines physical recognition which enables proper hydration to take place. Another instance of physical recognition is found in fern spores before germination, and involves temperature and light quality. Chemical recognition appears only in Spermatophyta. The water for rehydration is provided in different was in pteridophytes, gymnosperms and angiosperms, and the molarity of the hydrating solutions varies. In vitro hydration tests have demonstrated that Pteridophyta spores achieve maximum size in water, whereas the optimum media for gymnosperms and angiosperms have sucrose concentrations of 5–50 g/l. The volume of Spermatophyta pollen grains decreases during germination but that of Pteridophyta spores increases. The increase percentages in volume from the dry to the hydrated state vary between species and depend on the hydration site and the spore/grain itself. The internal factors are: hydration of the intine, selective permeability of the plasma membrane, hydration state of the cytoplasm, and accomodating power of the walls. Different devices regulate these volumetric changes in angiosperm pollen grains, e.g., furrows and apertures. Harmomegathic devices are less variable in gymnosperm that in angiosperm pollen, and even less variable in the pteridophytes.