Mosses and alternative adaptation to life on land



It is easy to dismiss bryophytes as ‘lower’ plants, mere primitive precursors long since left behind in the evolutionary race, and of only rather esoteric and incidental biological interest. But this is to let oneself be led astray by a simplistic image of a tidy evolutionary tree – an image that served Darwin well a century and a half ago (Desmond & Moore, 1992), but which we should now see as an intricately branched evolutionary bush with innumerable shoots reaching out from all depths to the growing apices that represent the present day. The earliest land plants may indeed have been at a bryophyte level of organization, but modern bryophytes, no less than vascular plants, are the product of some 450 million years’ evolution since that time (Edwards et al., 1998). Raven (1977, 1984) has emphasized the importance of the evolution of supracellular transport systems in the origin of vascular land plants. Bryophytes, on the other hand, evolved desiccation tolerance and represent an alternative strategy of adaptation to life on land, photosynthesizing and growing when water is available, and suspending metabolism when it is not. They are limited by their mode of life, but also liberated : they are prominent on hard substrates such as rock and bark, which are impenetrable to roots and untenable to vascular plants. Bryophytes (in species numbers the second biggest group of green land plants) may be seen as the mobile phones, notebook computers and diverse other rechargeable battery-powered devices of the plant world – not direct competitors for their mains-based equivalents, but a lively and sophisticated complement to them.