224 ZONES AND REGIONS [Pt. Ill, Sect. I 



foliage appear to have been acquired with the special object of saving 

 the chlorophyll, or at any rate they subserve that end ; among them are 

 movements of the leaflets of pinnate leaves, strong reflection of light by 

 the foliar surfaces, prolonged retention of a non-turgid condition and 

 the consequent flaccid drooping of young leaves, foldings, tomentum, and 

 so forth l . In spite of all such protective devices, the destruction of 

 chlorophyll by the intense tropical light is a very conspicuous and wide- 

 spread phenomenon. Thus the leaves of Pisonia alba, which is frequently 

 grown as an ornamental tree, are vertical and sap-green when young ; 

 later on they arrange themselves at right angles to the direction of the 

 most intense daylight, and suffer such a complete destruction of their 

 chlorophyll as to become almost pure white 2 . In sunny situations within 

 the tropics yellowish discolorations of the foliage are very common. 



The greater intensity of tropical light also renders possible a more 

 luxuriant development of the shade-flora than in higher latitudes. At 

 the same time, the property plants possess of demanding less light at 

 higher temperatures also operates in the same direction. Wiesner has 

 actually seen tropical plants thriving under such a feeble illumination 

 as with us would entirely exclude all green vegetation. It would be 

 extremely useful to investigate more closely the vegetation in the shade 

 of a virgin forest from Wiesner's points of view and in accordance with 

 his methods. 



Thus, Wiesner found a Javanese grass, Orthopogon Wiesneri, Schifmer, growing 

 in patches in the shade of Myristica moschata with L = j^g (I maximum = o^oi6) 3 , 

 but not anywhere in the shade of the densest palm-thickets, where L = y^ 

 (I maximum =o-oi i, I mean = o-oo3). Of all herbaceous non-epiphytic Dicotyledones 

 Wiesner found Geophila reniformis, Don., penetrating most deeply into the shade. 

 It continues to blossom with L = ^ (I maximum = o-o26 ; I mean = c-on). Without 

 flowering, it bears almost as great a deprivation of light as Orthopogon Wiesneri. 



The intensity of insolation in the tropics is so great that many cultivated plants, 

 including coffee and cocoa, at least when young, require shade. For this purpose 

 trees with a thin crown are used, especially those with pinnate leaves, for instance 

 Albizzia moluccana, Miq., also Cedrela serrulata, Miq., Cedrela odorata, Linn., 

 Pithecolobium Saman, Benth., in Java. In tropical America species of Erythrina 

 are usually employed for the same purpose. Probably, it is less a question of keep- 

 ing off the luminous and chemical rays than the heat-rays, which would cause too 

 great heating and consequently too great transpiration. 



Wiesner made some measurements of the intensity of light under the shading of 

 trees in the experimental garden at Tjikomoh near Buitenzorg, and he found : — 



Albizzia moluccana, Miq ^q 



Cedrela serrulata, Miq. ^g 



Cedrela odorata, Linn. ...... 577 



Pithecolobium Saman, Benth xr-I 



1 See Wiesner, Johow, Haberlandt. 2 Wiesner. 3 See p. 55. 



