Harmony and Conflict in the Living World - Alexander F. Skutch [43]
Most aggressive of all are the strangling fig trees, which from seeds deposited by fig-eating birds and other animals on high limbs start life as epiphytes but send roots to the ground along the trunk of the host tree. As they thicken, these roots meet and coalesce, until the supporting trunk is enveloped by a massive network. After the strangled trunk dies and decays, the fig tree remains standing on a high cylinder of fused roots, through the gaps in which one can look. Some of these usurping figs are among the giants of tropical forests.
The exploitation of plants by plants reaches a more advanced stage in the half-parasites or water-parasites, which might be described as green epiphytes that synthesize their own food but draw water and minerals from the host plant's vessels by means of haustoria that penetrate its tissues. Best-known and most abundant of the water-parasites are the mistletoes of the family Loranthaceae, with about one thousand three hundred species distributed over most of Earth, chiefly in the tropics and subtropics. A few make colorful floral displays on high boughs, but a heavy infestation may kill the supporting tree. Their fruits are eaten by many birds and are a principal food of several species of both the Old World and the New. After digesting off the pulp, the birds void the seeds surrounded by mucilage that attaches them to branches on which they happen to fall, where they germinate and grow.
Among flowering plants, full parasites are rare. Some are inconspicuous plants of humid forests, where they grow upon roots of other plants in the dark undergrowth. Their leaves, reduced to scales, are devoid of chlorophyll, making them wholly dependent upon their hosts, or associated fungi, for nourishment. The parasite may be white, yellowish, purplish, brownish, or dark red. Their flowers may be small and inconspicuous or very big, like the yard-broad flower of Rafflesia arnoldii of Sumatra, reputed to be the largest in the world. Much more numerous than fully parasitic flowering plants are parasitic fungi, which attack agricultural plants of many
Page 83
Aerial roots of an epiphytic fig tree
strangling trunk of the host tree
Page 84
kinds as well as forest trees, often causing heavy losses if not combated with fungicides that help to pollute the environment. Parasitic fungi are apparently derived from saprophytic forms that play an indispensable role in nature's economy by decomposing dead vegetation of all kinds, returning its mineral contents to the soil, where the minerals are picked up by the roots of living plants. Without these saprophytes, woodlands would become impenetrable mazes of fallen trunks and branches because termites and other insects would hardly suffice to reduce to humus all the debris of thriving forests.
A wide survey of exploitation in the vegetable kingdom reveals that it is caused chiefly by severe competition for living space and sunlight. Many plants, the vines and epiphytes, solve their problem by climbing over other plants or perching upon them. Those that grow upon others, often far above the ground, sometimes find it expedient to draw sap from their hosts; instead of sending roots down to the soil, they become half-parasites. A different set of plants, mostly growing in deep shade, have become full parasites, often on the roots of their hosts. None of these dependent plants has anything to gain by killing its host; the liana or epiphyte falls with the tree that supports it; the half-parasite or full parasite dies with the plant on which it grows. Aside from parasitic fungi, the chief vegetable enemies of trees are the lianas that weigh them down, smother them, or constrict their trunks. In contrast to the situation among animals, exploitation of plants by plants has no psychic effects upon them, or none detectable by us. It is so slow and silent that it hardly distresses the most sensitive onlooker.