Unter welchen Bedingungen haben Blätter sogenannte „Träufelspitzen”? )

Abstract

Many woody and herbaceous plant species in different families have leaves with so-called “drip tips” (German: “Träufelspitzen” Fig. 1). These are most frequently found in the humid parts of the warmer zones (Fig. 2) but even here not at all everywhere. Since nearly 100 years the prominent tips often were supposed to promote the transpiration of the leaves and to prevent the growth of epiphyllic algae, lichens and mosses by draining off the water from the surface. However field observations as well as experiments with leaves or leaf models gave rise to doubts into the teleologic interpretation. Morphologically the tip can be a “forerunner tip” (Vorläuferspitze) i. e. a special organ developing before the lamina. Rarely it may be an “after-running tip” (Nachläuferspitze) which continues to grow after the lamina has reached its definite shape. But in most cases leaves that are not included in a well protected bud simply begin their development with the tip which therefore is exposed to the atmospheric conditions days (or weeks) before the rest of the leaf stretches and broadens. This process ends up with a curved transition of the leaf edge into the tip. As it is often difficult to distinguish between these three cases the author proposes the neutral and unencumbered term “salient tip” (Vorspitze, in Latin apicula). In the tropical forest and woodland formations of Peru for example leaves with salient tips are more or less frequent depending on the altitudinal and humidity gradient (Fig. 3). This is illustrated in detail by “leaf form diagrams” showing the average shape of the leaves of all plant species (except for the rare ones) within a 10 × 10 m plot. Species covering less than 5% of the plot surface are represented with a simple drawing. For species covering 5–25 % the right contour is doubled etc. (25–50% = 3 × 50–75% = 4 × more than 75% = 5×). Evergreen hard leaves are hatched closely, softer ones more widely. Rain-green leaves are dotted (or marked with dotted lines when only abundant in wet years) and summer-green leaves (of cold-deciduous species) left without signature. The leaves are drawn to scale and the salient tip is marked with a thick dot for clarity. The species have been grouped according to the highest layer in which they occur (Ü = very tall trees, B1 and B2 = tree layers, L = lianas, S = shrub layer, K = herb layer; the moss layer and epiphytes are not depicted). Fig. 4 corresponds to a normal tropical lowland rain forest, Fig. 5 to a flood plain forest under similar conditions and Fig. 6 to a montane cloud forest on an eastern slope of the Andes which is extremely humid all over the year but much colder than the preceding examples. Fig. 7 represents an upper montane woodland in the semi-humid climate of the Andean “altiplano” which reminds a mediterranean hard-leaved wood. Fig. 8 corresponds to a cloud forest of the western Andes climatically intermediate between Figs. 6 and 7, Fig. 9 to a submontane mostly deciduous forest on the western Andean foothills, i.e. in a semi-humid warm climate. The lowland wood in Fig. 10 is situated west of the Andes in a semi-arid climate; but it was studied in an extraordinarily wet year (1957) during which soft-leaved annual (or geophytic) vines became overwhelming. Fig. 11 represents a “loma” woodland on the coastal mountains in southern Peru against which the fog is drifting during five winter months whereas rainfall is nearly lacking. For comparison Fig. 14 gives a leaf form diagram of a beech forest in Central Europe. Arranging these and other examples in relation to the average number of humid months and the mean air temperature Fig. 15 shows the percentage of species with salient leaf tips: These are concentrated in regions with more than 6 wet months and over 15 centigrades, i.e. in warm climates in which at least the vegetation period is rather long and humid. Under these conditions the woody plants do not need a strong bud protection and the freely developing leaves broaden quickly after having differenciated their tip. In a very rainy and foggy but cold climate where a “drip tip” would be most urgently needed (if this function were important!) most species have no such tips. When one and the same species is occurring in all altitudinal levels (Fig. 13) its leaves become smaller and the salient tip disappears with dropping warmth. The same phenomenon can be produced experimentally, the effect becoming even more striking when also the air humidity is lowered (Fig. 12).