The formation of roots is in some respects one of the least
fundamentally understood of all plant functions. Propagation by cuttings
is the aspect that will occur first to most gardeners and
horticulturists, and it is certainly the most useful application. But
any observant traveller in the tropics can notice that some trees have
the habit of forming roots in the air. Climbers like Cissus bear long
fine strings of roots hanging down. Pandanus trees tend to have stout
aerial roots issuing from the bases of the long branches, while the
tangle of roots around the trunk of many of the Ficus species is
characteristic. In Ficus bengalensis, in particular, stout cylindrical
roots firmly embedded in the ground from a height of 3 to 5 meters give
support to the long horizontal branches, enabling them to spread still
further. In the big old specimen at Adyar near Madras, the spread of
these branches all around the tree, each with a strong root growing out
every few meters, makes a shaded area under which meetings of almost
5000 people are sometimes held. The history of how the formation of
roots on stem cuttings was found to be under hormonal control is worth
repeating here.