344 MESOPHYTES SECT, xvi 



type of leaf is perhaps of the ' laurel-form ', that is to say, it is a large, 

 glabrous, glossy, elliptical, or lanceolate leaf, of which an example is 

 provided by Ficus elastica. Glossy coriaceous leaves are in general 

 a strikingly characteristic feature of tropical forest ; whereas foliage in 

 the forests of northern Europe is matt and more translucent. Haberlandt x 

 estimates that entire leaves are more frequent than in northern Europe. 

 The leaves are often of huge dimensions, for instance in the humid coastal 

 forests of Brazil and in Amazonian forests. Moreover, they are of a darker 

 tint than in temperate climes, because they, and particularly the palisade 

 parenchyma, are thicker than in European trees. Other leaves, on the 

 contrary, especially in lower storeys of the forest, are very thin because 

 the light is weak and the air humid. 



Regulation of the amount of water in the plant. According to the investi- 

 gations of Haberlandt 2 and others, plants in Javanese rain-forest, and 

 possibly in the higher storeys of tropical rain-forest in general, are exposed 

 to conditions far more extreme than are experienced by European vegeta- 

 tion. From about 6 to 7 a.m. until about i p.m. the temperature rises, 

 while the atmosphere gradually becomes drier under the direct insolation, 

 and the relative atmospheric humidity finally sinks to 70 per cent. The 

 second diurnal period commences at about 2 to 3 p.m. with storms and 

 violent discharges of rain, and during this period the air is so charged 

 with moisture (93 to 95 per cent.) that transpiration is almost arrested. 

 Thus during two-thirds of the day the air approaches saturation-point. 

 During the course of the day the plants are therefore threatened from two 

 entirely different sides, and against the double danger, which particularly 

 concerns the process of assimilation, they guard themselves in various ways. 



The significance of hydathodes possessed by leaves has already been 

 discussed on p. 101. 



But the opposite danger arises from great atmospheric aridity and 

 from the consequent intense transpiration during the morning. It is true 

 that the transpiration as a whole is small (according to Haberlandt, 3 two 

 or three times less than in plants in Central Europe, but this estimate 

 is regarded by Stahl 4 as being too low, 5 yet in the morning it is intense 

 and brings with it the risk of fading, or, at least, of so material a diminu- 

 tion of turgidity as to inhibit photosynthesis. This explains the remark- 

 able fact that many plants in tropical rain-forests exhibit protective 

 devices against excessive transpiration similar to those displayed by 

 xerophytes. A thick-walled, strongly cuticularized epidermis, sunken 

 stomata, mucilage-cells, storage-tracheids, aqueous tissue, and so forth, 

 present themselves. The aqueous tissue of Ficus elastica is well known. 

 The leaves of a number of palms, and the large thin leaves of Scitamineae, 

 display aqueous tissue towards the upper face or sometimes towards both 

 faces ; and this tissue may be quite as voluminous as the chlorenchyma. 6 

 In Javanese rain-forest, according to Haberlandt, several species, including 

 Gonocaryum pyriforme and Anamirta Cocculus, contain in their chloren- 

 chyma mechanical cells, precisely as do certain xerophytes mentioned on 

 p. 128. And these cells have the same significance in both cases they 

 guard the chlorenchyma against shrinkage due to desiccation. 



1 Haberlandt, 1893. * Haberlandt, 1892, 1897. s Haberlandt, loc. cit. 



4 Stahl, 1894. * See also Giltay, 1897, 1898 ; Holtermann, 1902, 1907; 



Burgerstein, 1904. Figures are given by O. G. Petersen, 1893. 



