PLANTS GROWING ON LIVING LEAVES
Maria I. Moralesost epiphyllous or foliicolous (leaf-inhabit-
ing) plants in Monteverde are foliose liver-
worts belonging to the family Lejeuneaceae,
a group that comprises about 90 genera and hundreds
of species worldwide. Foliicolous mosses are few;
most also grow on twigs and spread to leaves. Mosses
appear unable to establish themselves on leaves as
young plants (Richards 1984). Until recently, foliico-
lous bryophytes were documented in Central America
by a single study from El Salvador, which reported
one moss species and 63 epiphyllous liverwort spe-
cies (Winkler 1967). The epiphyllous bryophyte flora
in Braulio Carrillo National Park, Costa Rica, revealed
83 bryophyte species, the highest species number thus
far recorded from a single locality (Lucking 1995).
Epiphyllous species are found mostly near the
ground, but some colonize the forest canopy if it sup-
ports high humidity. Epiphylls colonize many species
of higher plants, but "prefer" those with smooth leaf
surfaces, which promote the establishment of repro-
ductive devices with adherent cells. Colonized leaves
are frequently long-lived, so the epiphyllae are able
to complete their life cycles before the leaf falls
(Winkler 1967). Epiphyllae have developed means of
rapid vegetative reproduction such as plant fragments
and gemmae (multicellular structures that are able to
develop new plants).
Although many genera of epiphylls can occur on
a single leaf and many species within a genus can
occur in a small area, the most common colonizers
in Monteverde belong to the following genera: (1)
Cyclolejeunea, which produces rounded gemmae on
the leaf border; (2) Ceratolejeunea, a brown shiny liv-
erwort with perianths (female structures) that have
four small horns at the top; (3) Odontolejeunea, with
dentate leaves; and (4) Metzgeria, a thallose form.
There are also very small species, for example, Dre-
panolejeunea and the tiny Aphanolejeunea, invisible
to the naked eye.Rhizoids of the epiphyllous liverwort Radulaflac-
cida penetrate the leaf cuticle and insert themselves
between the cells of the upper epidermis (Berrie and
Eze 1975). Water from the host plant may help epi-
phyllous liverworts survive the dry season. Epiphylls
absorb minerals from their hosts and are thus semi-
parasites (Eze and Berrie 1977).
Is shading of the host leaf by epiphylls sufficient
to have an effect on the photosynthetic rate of the
host? A coating of epiphyllous Radula flaccida inter-
cepts a maximum of 2% of the incident light (Eze and
Berrie 1977), a seemingly insignificant amount.
Richards (1984) pointed out that the most heavily
colonized leaves are the oldest ones, which probably
no longer make an important contribution to the en-
ergy balance of the plant. On the other hand, coloni-
zation by epiphylls may benefit the host plant, be-
cause epiphylls provide a suitable environment for
nitogen-fixing bacteria and cyanobacteria (Richards
1984). The relationship between some plants and their
epiphylls may thus be mutualistlc.
There are many unanswered questions about epi-
phylly. Are foliicolous liverworts obligate epiphyllae?
Are they specific to one species of leaf? The latter is
unlikely, because epiphylls do not seem to receive
organic compounds from the host (Eze and Berrie
1977). Host leaves may exude substances that are
needed for growth of some foliicolous species.
Epiphylls only rarely colonize some leaves (e.g.,
Costus and Dieffenbachia), An explanation is the irregu-
lar surface of the former and the presence of an irritat-
ing substance in the latter. Texture or other leaf charac-
teristics and the incidence and distribution of light are
considered the most important factors in controlling
colonization by epiphyllous plants (Monge-Najera
1989). Based on a cluster analysis, Lucking (1995) con-
cluded that characteristics of the host plants are respon-
sible for epiphyll composition on different leaves and
that microclimate conditions are less important.80 Plants and VegetationM