87 
certain Tropical Trees . 
movement enables it to rise up daily to a fully horizontal 
position ; and, in the absence of the sun’s direct influence, after 
performing its daily swing, the final plane of the leaflet is 
horizontal. Thus the shaded leaflet makes the most of its 
leaf-surface, the sun-exposed leaflet adjusts itself doubtless 
in that position in respect to the sun in which the various 
functions of the leaf are best fulfilled ; in which transpiration 
is not too excessive, and quite probably also in which assimi- 
lation and translocation in the leaflet are most favourably 
affected by the sun’s light- and heat-rays. For Costerus 1 has 
quite recently shown, or at least gone far towards showing, 
that leaves of trees are markedly influenced by the sun’s rays, 
in the rate of their assimilation and translocation. In connexion 
with the ultimate position assumed by the leaves of Brownea 
grandiceps , another factor which may make itself felt is the 
directive influence of light, for, like other dorsi-ventral leaves, 
those of this tree are dia-heliotropic. Thus three or four seed- 
lings, which had been grown in the diffuse light and whose 
leaflets were horizontally disposed, were so placed in a room 
as to be unilaterally illuminated. At the end of fourteen days 
the leaflets had all arranged their upper surfaces at right 
angles to the light. To do this not a few had been compelled 
to rise up so that their apices pointed upwards to such an 
extent that the plane of the leaf made but a very small angle 
with the vertical. 
The effect of this dia-heliotropism is often well marked in 
leaves of Brownea grandiceps trees growing in shady places. 
In such cases not only do the petioles of the most shaded 
pinnate leaves rise up more than those exposed to the light, 
but the leaflets themselves assume positions quite like those 
taken up by the leaves of such plants as Vero7iica Tr aver si, 
Lamium album 2 , &c., viz. each pair of leaflets lies in one plane 
at right angles to the incident light. 
Consider now to what conditions of insolation these trees of 
the genera Amherstia , Brownea , and Humboldtia , whose leaves 
1 Costerus, Ann. Jard. Buitenzorg, XII. i. 
2 Cf. Physiology of Plants, Darwin and Acton, Fig. 23. 
