The Resistance to Desiccation of Ten Species of Tropical Mosses

Abstract

Ten species of tropical mosses were exposed to a range of humidities from 76% to 10% for four hours and then allowed 24 hours to recover. These mosses fell into two groups; those with high resistance to desiccation and those with low resistance. Those of the first group may be found in ecological niches of low humidity in the tropical forest. Certain cryptogamic plants have considerable resistance to desiccation, and their survival depends both on the physiological state of the plant as well as the conditions during drying (Fogg, 1969). Fritsch and Haines (1923) noted that there was great variation between the cells of Pleurococcus, Prasiola, and Zygogonium in permeability to stains and resistance to plasmolysis and desiccation. Cells which successfully survived desiccation had a diminished tendency to plasmolyse in hypertonic solutions, a more rigid and viscous protoplasm, and more abundant granules. For the survival of terrestrial algae the importance of zygospores and other resting spores has been greatly exaggerated (Strom, 1924). Certain algae appear to be able to survive prolonged exposure as modified vegetative cells with thickened walls but in others there is no detectable change in the vegetative cells under such conditions (Evans, 1958). Algal cells which survive may have one or more of the following: i) accumulation of fat in the cells, ii) thickening of the cell wall, and iii) accumulation of mucilage (Evans, 1959). The viscosity of the protoplasm in Bryum and Mnium during desiccation was investigated by Northen (1943), who used a centrifuge method to show that during drying the structural viscosity decreased, returning to normal after a recovery period. The osmotic values of cells of mosses living under different environmental conditions are higher in more extreme habitats such as rock surfaces, sunny places, and bark of trees (Ochi, 1952a). Such xerophilous mosses do not readily reduce their water level below the quantity of their own minimum hydrability when they are air-dried but are capable of imbibing water vapor in air even if it is not saturated with water (Ochi, 1952b). There is, in general, an increase in osmotic values of the same species parallel to the increase in the upper limits of their vertical range on trees. Epiphytic mosses die almost immediately when exposed to relative humidity of 0%. At higher humidities they exhibit three degrees of desiccation resistance according to their position on the tree trunk (Hosokawa & Kubota, 1957). The survival value of the vegetative plant in mosses may be exceedingly important since, unlike those of terrestrial algae, the spores of mosses have a very short survival period (Bernstein, 1928). The respiration of algae and mosses is normally dependent on their associated water (Fraymouth, 1928). In Hypnum cupressiforme the respiration 1 Department of Biology, Nanyang University, Jurong Road, Singapore 22, Malaysia. This content downloaded from 157.55.39.15 on Mon, 19 Sep 2016 06:46:35 UTC All use subject to http://about.jstor.org/terms 1970] JOHNSON & KOKILA: DESICCATION OF TROPICAL MOSSES 683 of the hydrated moss is 250% of the dry moss. There is an increase in respiration as hydration proceeds until excessive soaking impairs the rate of respiration. Algae, such as Trebouxia, may reach a maximum respiration of 30% of their water content. In the marine alga Scytothamnus australis respiration is depressed by rapid drying before submersion. While photosynthesis occurs in a moist chamber kept at 100% humidity, at a lower humidity no photosynthesis is observed even under adequate lighting conditions (Brown & Johnson, 1964). Parallel metabolic activity in bryophytes