I. 



Rainfall and the Forms of Leaves. 



THE following notes are taken from an admirable paper by 

 E. Stahl in the Annales die Javdin Botaniqiie de Buitemorg, vol. xi., 

 pp. 98-182, on leaf- forms in relation to rainfall, chiefly based on 

 observations of tropical plants at Buitenzorg. On arriving in Java 

 (November, 1889), he was impressed with the great humidity of the 

 atmosphere as well as with the extraordinary rainfall, and he wa^ led 

 to ask how the plants met such conditions. In the tropical forests 

 they are not only subjected to daily thunder showers during the rainy 

 season, but the sun's rays scarcely penetrate through the thick foliage, 

 and the air is saturated with moisture which causes a perpetual drip 

 from the leaves. An examination of desert forms, with their arrange- 

 ments for economising their scanty water-supply, has thrown so much 

 light on the peculiarities of certain plants of our temperate climates, 

 that plant adaptations to opposite conditions on an equally large scale 

 promised a rich and suggestive field of study. 



Leaves, as we know, differ greatly in their behaviour towards 

 atmospheric water ; from waxy leaves the drops roll off, leaving no 

 perceptible moisture behind, while other leaves, without this provision, 

 tend to retain the moisture on the leaf-surface, and are weighed down 

 and often broken off, in the tropics, by the added pressure on the 

 brittle leaf stalk. The retention of moisture on the leaf after the 

 rain is over interferes too with the transpiration, with which the rise 

 of water in the stem with the supply of mineral salts to the growing 

 and assimilating organs is connected. This function is provided for, 

 and the difficulty so far met, by the great leaf-surface, but there are 

 also various methods by which plants with easily moistened leaves 

 dispose as speedily as may be of their superabundant water-supply. 

 One of these methods is the adoption of the sleeping position by 

 leaves, such as those of the sensitive plant, that possess this power of 

 movement. Jungner,' a previous observer, says: " x\s soon as rain 

 falls the horizontal leaves bend upwards as in the sleeping condition, 

 and the rain-drops easily run off by the base of the leaf." 



The most frequent way of disposing of the excessive moisture, 

 however, is by draining it off by means of long points to the leaves. 



1 " Anpassungen der Pflanzen an das Klima in den Gegenden der regnerischen 

 Kamerungebirge." Botanisches Ceniralblutt, 1891, p. 358. 



