The New Perspective in the Halophytes

Abstract

A SUMMARY of the recent literature on the halophytes appeared in the Botanical Review for 1941 (Uphof), but it would seem that there is still need for a critical discussion of the more important recent papers, some of which have appeared since the review referred to above. The object of such a discussion would be first, the assessment of those phases in which progress has been most pronounced, and second, an indication of the direction along which future research might profitably proceed. Stocker (1928) has defined halophytes as "those plants which at any stage of their life are subjected to a concentration of salt [Salzwirkung, in the original] which is more than 'normal' glycophytic plants can bear without dying." (Glycophyte is a convenient term for all plants that cannot grow in places where the concentration of sodium chloride in the soil solution is more than 0.5 per cent. It must be understood that this is a purely arbitrary definition.) While it may be easy to distinguish between extreme halophytes and glycophytes it is by no means so easy to determine to which class a plant should belong at the lower concentrations of sodium chloride. Field observations and experimental work indicate that the boundary should be placed at about 0.5 per cent sodium chloride in the soil water. Among plants there appears to be a marked difference in tolerance above and below this concentration. At this point it is essential to define the terms that it is proposed to employ subsequently. Halophyte. A general term which includes all plants that are capable of growing in an environment where there is more than 0.5 per cent sodium chloride. Its use will not imply that the species is either common or rare in such habitats nor will the term involve the assumption that a plant cannot grow under any other conditions. , The Halophytes will not include plants that grow in places where the soil is characterized by an excess of a salt other than sodium chloride, e.g. inland areas where there is an excess of magnesium salts, etc. The flora of such places should be distinguished from halophytes, as defined above, and the plants should be placed in another category. In the past they have commonly been included in the halophytes. Eulhalopiyte. This term will be restricted to those plants whose optimal growth takes place in an environment in which there is more than 0.5 per cent sodium chloride. (It is impossible to define more accurately the term "optimal growth." We cannot say growth to maturity because plants may flower and set seed and yet be weakly or stunted. Whilst field observations may indicate whether growth is approaching optimum or not, experiment must ultimately be the decisive factor.) Euhalophytes will not grow unless there is an abnormal concentration of sodium chloride in the habitat. This class includes species of Salicornia, Rhizophora, and the 'submerged' halophytes, e.g. Zostera, Cymodocea, Posidonia. Miohalophyte. This will be applied to those plants which are to be found, either commonly or rarely, in habitats where there is more than 0.5 per cent sodium chloride; their optimal development occurs, either naturally or experimentally, in an