EXUDATION OF DROPS OF WATER FROM LEAVES. 



479 



protected from active evaporation, a pressure of 10-20 centimetres of mercury 

 generally suffices to render drops of water visible on them after ten minutes, or an 

 hour, or longer, at any time of the day ; and with branches of Alchemilla alpina, the 

 common Vine, and others, the excretion of drops in this manner continues for eight 

 or ten days, so that several hundred cubic centimetres of water drop away. That 

 , the water forced in reaches the leaves through the wood can be made evident by 

 removing a ring of cortex. 



It is generally at specially organised places on the leaves that the drops 

 appear. These are often the so-called water-stomata (cf. Lecture VIII. p. 119); 

 in other cases, however, they are ordinary stomata, and occasionally, in Grasses for 

 instance, mere iissures in the epidermis ; or, finally, as Moll has shown ^, the drops 

 occur even at places on the margins or on the surfaces of the leaves where no 

 special openings are to be perceived. In Aroideae, Fuchsia, Tropaolum, Helleborus 

 niger. Primula sinensis, and others, the drops exude through water-stomata; and 

 here the water-stomata are usually at the margins of the leaf, and especially at 

 the upper side of the apex of the teeth, 

 several or many existing together. But 

 in some plants, such as Platanus, Ulmus, 

 and VHis, according to Moll, the excre- 

 tion of drops may take place on the 

 under side of the teeth, although they 

 possess water-stomata on the upper side. 

 In Cesirum roseutn. Datura, &c., indeed, 

 he found drops excreted at places where 

 no stomata at all exist. In general, 

 according to Moll, younger leaves are 

 more apt to exude drops than older 

 ones, where the organs of exit not rarely 

 become impassable. If in such cases 

 water is forced into the shoot-axes, it 

 penetrates into and fills the inter-cellular 

 spaces of the mesophyll, and the leaf 

 becomes injected; this does not directly 

 injure the leaves, however, and they re- 

 main living when the injected water has 



disappeared from the intercellular spaces by evaporation. A peculiar adaptation 

 to these processes is found in the leaves of the Aroideae mentioned. Following 

 the mjargins, in company with the vascular bundles, are certain wide canals, 

 which conduct .the water up to the extraordinarily large water-stomata at the 

 apex of the leaf From these as many as eighty-five drops per minute are 

 said to be excreted at night; and one observer even collected 22-6 grammes 



FIG. 216.— This apparatus, already described above (p. 244), 

 •may be employed to demonstrate the extrusion of drops from 

 leaves by forcing in water at the cut surface of the shooL 



' I have demonstrated the phenomenon of the excretion of drops on the leaves of cut shoots by 

 pressing water into them from below, in my lectures since 1869, and occasionally given instruction 

 in my laboratory on the same subject. Moll has published more detailed researches on this subject 

 in ' Bot. Zeitg.' (1880, p. 49) and in ' Mitlkeilungen der Kgl. Akad. der Wiss. in Holland' 

 (Amsterdam, R. II. Th. 15). 



