324 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 \ 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-. In sunny situations within 

 the tropics yellowish discolorations of the foliage are very common. 



The greater intensit)' of tropical light also renders possible a niort 

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

 the same time, the property plants possess of demanding less light a1 

 higher temperatures also operates in the same direction. Wiesner has 

 actually seen tropical plants thriving under such a feeble illuminatior 

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

 extremely useful to investigate more closely the vegetation in the shadi 

 of a virgin forest from Wiesners points of view and in accordance witlj 

 his methods. i 



Thus, Wiesner found a Javanese grass, Orthopogon Wiesneri, Schiffner, growin. 

 in patches in the shade of Myristica moschata with L = yj^j (I maximum =o-oi6)' 

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

 (I maximum=o.oii, I mean=o.oo3). Of all herbaceous non-epiphytic Dicotyledone 

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

 It conthiues to blossom with L = ^^ (I maximum =o-026 ; I mean = o-oii). Withoi 

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



The intensity of insolation in the tropics is so great that manj' cultivated plant 

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

 trees with a thin crown are used, especially those with pinnate leaves, lor instam 

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

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

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

 ing off the luminous and chemical rays than the heat-raj's, which would cause t( 

 great heating and consequently too great transpiration. 



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

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



Albizzia moluccana, Miq. . . . ' . . . ^^1^ 



Cedrela senulata, Miq. ^ 



Cedrela odorata, Linn. jtj- 



Pithecolobiimi Saman, Benth. 



1 



' See Wiesner, Johow, Haberlandt. - Wiesner. ' See p. 55. 



