Physiological plant studies in South Africa Part II : transpiration of grasses and other plants under arid conditions

Abstract

From the locus of writing this article and the companion paper on “Wilting,” it is impossible to consult the extensive literature on transpiration available in European libraries, and for many points of reference Burgerstein's textbook (1904 and 1920) had to be relied upon. Although a modicum of the most recent literature was procured from Europe, not all journals were available, and in considering the Armoedsvlakte experiments attention must be confined, for the time being at least, to certain recent papers which seem to have a special hearing upon their significance. Iljin (1915 and 1916) and Maximow (1923 and 1924) were probably the first authors who treated the Xerophytic plant from a point of view entirely different to that of the ordinary textbooks. Although the South African data now recorded do not altogether bear out Maximow's views, the experiments themselves were carried out with due regard to his standpoint. Huber (1924), whose excellent critical paper was only published towards the conclusion of the Bechuanaland work, actually points out that results different from Maximow's might possibly be obtained in the extreme drought areas of Africa, but apart from this he subscribes almost completely to the general views of the Russian author. He considers it characteristic of a Xerophyte (p. 49) that it can continue transpiration with open stomata even when the soil is drying out and the atmospheric saturation deficit is large, and that in arid areas it can retain its natural ventilation without danger to its water balance. The reduced transpiration as a distinction from Mesophytes loses its significance (p. 96) when Xerophytic characteristics occur, until finally, in extremely dry soil, it is only the osmotic forces which decide the retention of water balance. The more xeromorphic a plant the less important becomes the absolute saving effected by decreased transpiration. These views of Huber, expressed so recently as last year, render it necessary to emphasize again the totally contradictory behaviour of the flora of Bechuanaland. Not one of the investigated plants is capable of extensive dehydration without vital changes in its metabolism, although these cannot be observed without analysing the plant itself, and may proceed even if the plant can still recover its turgescense through rain. In the area studied, an admirable place for Xerophytes, the only plants which keep the stomata open when the temperature is high, the relative humidity and the soil moisture low, are the grasses. All other plants close their stomata, and some even adopt protective positions. At the end of 1922, after a rainless period of ten months, when the country suggested the appearance of a real desert, Elephantorrhiza, Bouchea, Cassia, and Stachys were found with closed stomata; and the two Leguminosae adopted their characteristic protective positions (photographs in Part I on “Wilting”). Should these plants be refused the name of Xerophyte because they cease assimilation in the drought-stricken soil and desiccated air? Is it not wonderful that under such extreme conditions the plants were able to shoot new leaves, continue the metabolism for a few hours each day, but close their stomata for the rest of the time? During this period, of course, no grasses could be found, but in no sense should they be classed as Xerophytes. Huber emphasizes the fact that different plants by no means employ the same means of protection against drought. In South Africa diminished transpiration plays a relatively dominant role, perhaps because some of the most drought-resistant plants have very thin leaves which cannot stand alternate drying out and recovery. It seems probable, however, that the subterranean parts of all these plants are drought resistant in the sense of Maximow and Huber, and that it is this, together with their high suction force, which enables them to shoot in an extremely dry soil. The grasses, which should be separated altogether from the Xerophytes, do behave somewhat like plants which Maximow includes under that term. They continue, at least to a limited extent, their gaseous metabolism right up to the point at which their continued transpiration of water results in permanent withering. Before this point their metabolism is not reduced to a few specific hours each day, but continues all day long in a disturbed fashion. The nature of the disturbance can of course only be revealed by chemical analysis, in which direction certain data are being now accumulated. When the grasses suffer only temporary wilting they may recover completely after rain, so far as external appearance goes; but their metabolism is not restored to normal at the same rate. It is not known at present, in how far the drying out of the plants in Maximow's experiments affects their metabolism, but from the researches of Iljin (1923) some influence would he expected. It seems that the metabolic consequences of alternate dehydration and recovery will have to be investigated before any definite conclusion can be arrived at, concerning the relative advantages to the plant of a capacity to decrease transpiration, and a capacity to lose water at one period which it can regain at another.