72 
REPORTS OF SOCIETIES. 
Mar., 1890 . 
knowledge has been at times turned to useful ends. The New 
Zealand streams clogged by the rampant growth of an introduced 
foreigner—watercress—were found to be cleared by planting willows 
along the banks. The white clover will turn out the objectionable 
knot grass. In the tropics the struggle is greatest, as growth goes on 
all the year, the only alternation of seasons being between wet and 
dry. Hence the frequent modification to their surroundings found in 
tropical plants, instanced, for example, by the Cacti and Euphorbiae 
with leaves and prickles modified, on the one hand to forbid the 
ravages of cattle, on the other, to prevent evaporation. Now to pass 
to the special subject. Apart from the question of fertilisation, there 
are certain plants whose existence entirely depends upon the existence 
of certain insects. These insects are ants, which in the tropics, are 
to be numbered by millions. Many of them are far larger and more 
ferocious than any in temperate climes. Among them is a genus of 
leaf-cutting ants, Atta, found in great profusion, marching backwards 
and forwards in long files between some forest tree or shrub and their 
nest, each returning ant bearing with it a piece shorn out of a leaf, 
bigger than a sixpence. The result of such constant spoliation, if 
unchecked, must be to the tree—weakening of the reproductive 
powers, decay, eventual death—as happens to the coffee plants in 
Ceylon, whose leaves are destroyed by a fungus. Many trees are so 
attacked by the ants. A good example is one of the Cecropias—a 
forest tree, allied to the Bread-fruit, like a gigantic candelabrum. 
What can the plant do to obviate the mischief ? The Cecropia sets to 
work in an apparently wasteful way. It maintains a standing army 
of ants of another species, small but ferocious, to keep off the ravaging 
species. For its allies the tree provides both house-room and food. 
Its stem is thick, hollow-jointed like bamboo, but with shorter joints. 
From each joint springs a leaf, and above the leaf is a hole. This 
hole is bitten bv the ant through a thin spot, apparently left for that 
purpose, and thus access is gained to the room which it inhabits 
in the hollow of the tree. Also the tree provides food for its 
garrison. Just above the hole is a triangular mark at the base of the 
leaf-stalk. Closely examined this is found to consist of small 
albuminous bodies which the ants feed on. The garrison ants earn 
their food and shelter. The wood-cutter’s task is a most objectionable 
one, carried on, as it is, under a rain of biting ants. The marauding 
Attas, spite of their size, are horribly afraid of their diminutive 
opponents, and beat a hasty retreat at sight, leaving the tree 
unassailed. An example was shown of an Acacia from the Botanic 
Gardens, similarly adapted to the garrison of defending ants. Here 
the tip of the huge hollow thorns furnished the weak spot through 
which the hole was gnawed for entrance, while the apex of each 
leaflet provided an oval-pointed body of albumen grains, and in the 
leaf stalk were little glands secreting honey. Honey glands, extra¬ 
floral nectaries, are found on many plants. An absurd explanation 
had been given that they are meant to attract the attention of insects 
from the nectary of the flower, i.e., by paying black mail to save the 
flower honey. Doubtless in many cases they are meant to attract 
ants. The first three opening leaves of the Aspen Poplar have them; 
ants ascend and keep off leaf-devouring insects. When the leaves 
grow tough the glands disappear. The Banksia Rose has them, and 
this is the only rose that is not infested by the leaf-cutting insect. 
The Rev. G. J. Burch exhibited and explained an ingenious contrivance 
for showing and calculating the rate of the motion of the cilia of 
Infusoria under flashing light, some account of which will appear 
hereafter in this magazine. 
