32 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



The deposition of dew is of very great importance in tracts where 

 there is but little rain ; many, especially subtropical tracts, would be 

 almost devoid of vegetation were it not for the strong deposition of dew 

 in the dry season. The deposition of dew is much greater in lower than 

 in higher latitudes. It plays a remarkable part in plant-life, for instance, 

 in the Egypto-Arabian desert ; ^ it must be the dew that in many places 

 evokes in spring-time the phenomena of plant-life, which take place 

 despite the fact that no rain has fallen for months.^ According to 

 Mez,^ some epiphytic Bromeliaceae, Tillandsia usneoides for example, 

 are adapted to take in dew especially by the aid of their loose, chaff-like, 

 scaly hairs ; when the dew-absorbing leaves have an aqueous tissue of 

 considerable extent, this is situated on the lower side of the leaf, but is 

 on the upper side in species adapted for the absorption of rain. More- 

 over, in European heath-moors dew is of the greatest importance to plant- 

 hfe, and especially to bog-mosses, indeed it is the solitary source of water 

 during the season of scanty rain. In temperate countries the deposition 

 of dew may be very considerable, but it is of significance not so much as a 

 source of water-supply as an influence depressing transpiration. 



It must be assumed that everywhere plants are adapted to the given 

 mean supply of water. But as regards this, great specific differences exist. 

 Wiesner's* researches have shown that many terrestrial plants are 

 adapted to a definite average amount of rain, which in general varies 

 with the species. He discovered the existence of two extreme kinds of 

 plants, which he termed respectively omhrophilous (rain-loving) and 

 onibrofhohous (rain-hating) plants, according to their power of enduring 

 without injury the action of rain for a long period (often several months) 

 or only for a short one. Xerophytes are mostly ombrophobous ; meso- 

 phytes are ombrophilous or ombrophobous. Ombrophoby is usually 

 associated with unwettability of the leaf-surface, ombrophily with wetta- 

 bility. 



Many features have been regarded as adaptations for the removal of 

 rain. Jungner and Stahl ^ have demonstrated in plants from rainy 

 climates several characteristic structural features which serve to conduct 

 rain rapidly from the leaves, so that transpiration may not be hindered 

 by the blocking of the stomata, the plant may not be overloaded, fungal 

 spores may be washed off, and so forth. Subserving this purpose are 

 drip-tips, which are abnormally long, sudden, apical attenuations especially 

 possessed by entire leaves of tropical plants, such as Ficus religiosa, 

 Theobroma Cacao, and species of Dioscorea ; such tips facilitate the rapid 

 removal of rain-water from the leaf. 



Whether certain other features to which Lundstrom ^ has directed 

 attention subserve the same purpose is perhaps dubious ; lines of hairs, 

 for example in Stellaria media and Veronica Chamaedrys, have been 

 interpreted as affording means of carrying away water, so likewise have 

 furrowed nerves and petioles of Lamium album, Humulus Lupulus, and 

 Ai uncus Silvester ' ; and likewise velvety leaves in the tropical forest.^ 



It has been believed that falling rain, and particularly violent torrents 



^ Volkens, 1887. ' Warming, 1892. ^ Mez, 1904. 



* Wiesner, 1894, 1897. ^ Jungner, 1891 ; Stahl, 1893, 1896. 

 Lundstrom, 1884. ' Stahl, 1893. 



* See Section XVI, p. 346. 



