244 TROPICAL NATURE Ir 
surrounding trees, in which a few ultimately prevail and fill 
up the space vacated by their predecessor. Yet beneath this 
second set of medium-sized forest trees there is often a third 
undergrowth of small trees, from six to ten feet high, of dwarf 
palms, of tree-ferns, and of gigantic herbaceous ferns. Yet 
lower, on the surface of the ground itself, we find much variety. 
Sometimes the earth is completely baré, a mass of decaying 
leaves and twigs and fallen fruts. More frequently it is 
covered with a dense carpet of selaginella or other lycopodi- 
ace, and these sometimes give place to a variety of herba- 
ceous plants, sometimes with pretty, but rarely with very 
conspicuous flowers. 
Flowering Trunks and their Probable Cause 
Among the minor but not unimportant peculiarities that 
characterise these lofty forests is the curious way in which 
many of the smaller trees have their flowers situated on the 
main trunk or larger branches instead of on the upper part 
of the tree. The cacao-tree is a well-known example of this 
peculiarity, which is not uncommon in tropical forests; and 
some of the smaller trunks are occasionally almost hidden by 
the quantity of fruit produced on them. One of the most 
beautiful examples of this mode of flowering is a small tree 
of the genus Polyalthea, belonging to the family of the 
custard-apples, not uncommon in the forests of north-western 
Borneo. Its slender trunk, about fifteen or twenty feet high, 
was completely covered with star-shaped flowers, three inches 
across and of a rich orange-red colour, making the trees look 
as if they had been artificially decorated with brilliant gar- 
lands. The recent discoveries as to the important part played 
by insects in the fertilisation of flowers offers a very probable 
explanation of this peculiarity. Bees and butterflies are the 
greatest flower-haunters. The former love the sun and fre- 
quent open grounds or the flowery tops of the lofty forest 
trees fully exposed to the sun and air. The forest shades are 
frequented by thousands of butterflies, but these mostly keep 
near the ground, where they have a free passage among the 
tree-trunks and visit the flowering shrubs and herbaceous 
plants. To attract these it is necessary that flowers should 
be low down and conspicuous. If they grew in the usual 
