Epiphyllous Plants

Translation of: Les Végétaux Épiphylles


Professor at the University of Brussels, Assistant at the Botanical Institute.

In the equatorial forest, where abundant rainfall, the regularity of the temperature and the calmness of the air maintain a constantly humid atmosphere, many small plants (algae, mushrooms, lichens, liverworts, mosses) have been able to leave the ground and establish themselves on the branches or even the leaves of trees. These epiphylls are sometimes species which, by chance, have found favorable conditions on some large leaf; such are various Orchidaceae and Ferns which usually live as epiphytes, but which in the humid and calm virgin forest can spend their whole existence on a leaf. Among the occasional epiphylls are Infusoria, Rotifera and Nematodes, which swim in the large spongy clumps formed on the leaves by mosses and hepatics.

The epiphylls do not inhabit all plants or all localities indiscriminately. Thus, in the Botanical Garden of Buitenzorg and in the groves around the city, one meets only Thallophytes. In the jungle of Depok and in the forest covering the Goenoeng Tjibodas in Tjampea, besides the thallophytes which are also very common, some bryophytes live on the leaves of the undergrowth. In the virgin forest of Tjibodas epiphyllous plants are extremely abundant, but the various groups do not mix much. On the leaves which occupy the tops of trees, there are hardly any but lichens. Near the ground, the leaves support numerous algae, liverworts and the large brown clumps of an Ephemeraceae. Finally, in the narrow and particularly humid gorges, the mosses are added to the liverworts, but the thallophytes disappear. There are few species in the forest whose leaves are completely safe from the invasion of epiphylls. I can only mention one: Trichomanes pallidum. Its leaves are covered with a waxy layer: raindrops roll off it without wetting it and the spores of the epiphylls cannot cling to it. However, if the other plants of the Tjibodas forest are all, without exception, infested with epiphyllous fungi, they are far from being so to the same degree. The ferns, Cyrtandra, Elettaria, and in general the rough or somewhat hairy leaves, are much more inhabited than the very smooth leaves like those of Musa and Curculigo.

The plants adapted to epiphyllous life are not uniformly distributed among all the systematic groups.

The Phanerogams and Pteridophytes contain only accidental epiphylls.

Schizophytes have very few truly epiphyllous species. Bacteria are probably common on old leaves, but they are probably common species, living everywhere. Schizophyceae are represented by Scytonema.

Among the algae, essentially aquatic organisms, only the Chroolepidae group contributes to colonize the leaf surface. In addition to many terrestrial and epiphytic species, the genus Trentepohlia also includes some epiphyllous species. Phycopeltis inhabit exclusively living leaves. Finally, the Cephaleuros, which have become parasitic, have gone beyond the stage of simple epiphylls.

It is probable that most of the parasitic fungi of leaves began by living on the surface of these organs. But the absence of assimilating pigments forced them to borrow food from their support. However, some genera of Pyrenomycetes (Meliola, Merina, Schneepia, etc.) have remained true epiphylls. The same is true of Fumago.

If the Thallophytes with holophytic or saprophytic nutrition are quite rare on leaves, it is not the same for lichens. Almost always the orange patches of Chroolepidea are partly lichenized. Other lichens are also very common, both pyrenolichens and discolichens.

The epiphyllous hepatics are also very varied, although they all belong to the group of acrogynous Jungermanniaceae, especially to the tribes of Stephaninoideae and Jubuloideae.

The mosses inhabit the living leaves only in very humid places; moreover, it is almost always found that the clumps of mosses are installed on the stems and that only a few branches reach the leaves and lie on them. The only species that is really epiphytic is an ephemeral.

Although the air is usually very still in the virgin forest, the leaves are nevertheless shaken violently from time to time by gales. Therefore, in order not to be blown off, the epiphylls must be firmly attached to their support. From the point of view of the shape of the body and the method of attachment, epiphylls can be divided into three groups: those which are filamentous, those which are disc-shaped, and finally those which consist of a leafy stem.

The filamentous epiphylls seem at first sight to be ill-suited to live on a surface as smooth as that of most leaves. In the case of Trentepohlia, this is not actually the case: most of the species which live on leaves have layered filaments, applied against the support, quite different from the erect filaments; in T. diffusa, there are, in addition, short filaments, serving as holdfasts [clamps]; finally, T. prostrata has only surface filaments.

The Ephemeraceae, so common in the Tjibodas forest, has an arrangement similar to that of T. diffusa: some branches of the persistent protonema are assimilative and spread out in the air, while others, shorter, are tightly clamped to the hospitable leaf.

We have just seen that T. prostrata has only horizontal branches. Let us suppose that the filaments, instead of being arranged in order, are regularly radiating and contiguous, and we have the discoid vegetative apparatus of a Phycopeltis. The thallus of Asterina is also formed of radiating filaments, arranged in a single layer next to each other.

It is probably the difficulty of fixation that excludes fruticose and foliaceous lichens from epiphyllous life. In any case, the crustaceous forms are the only ones that have been able to adopt this mode of existence; they constitute small rounded crusts, intimately attached to the leaf surface.

It is also necessary to attach to the group of discoid epiphylls a Jungermanniaceae (Metzgeriopsis pusilla) which remains during its whole life in an infantile stage. In fact, just as the Ephemeraceae retains its protonema, so this Hepatic retains its prothallus; this is applied by its entire lower surface against the living leaf, and is, so to speak, the only assimilating apparatus of the plant.

Except for the two aberrant forms just mentioned, the epiphyllous Bryophytes all have a leafy stem. Among the mosses, the pleurocarpic Bryaceae are the only ones that provide epiphylls. It is understandable that in order to avoid being blown away by the wind, which whips the leaves, the plant must be fixed over its entire length; this is what is achieved in the pleurocarpic mosses by the abundant production of rhizoids along the appressed stem. On the contrary, the other mosses, which have an upright stem, would be attached only at the base, and the wind would soon have blown them away.

In the Hepatics we also find epiphylls only among those forms which give numerous rhizoids on the ventral side. Rarely do rhizoids form on the stem itself. In the Lejeuneaceae, they usually arise from the basilar cells of the amphigastrum; sometimes the amphigastrum is even replaced entirely by a tuft of rhizoids. In the epiphyllous Stephaninoideae they are borne by the auricles of the leaves. It will be noted that the only Jungermanniaceae which have become epiphyllous are those whose leaves and amphigastrums were already in the process of being varied, and which were able, because of this variability, to bend more easily to the requirements of epiphyllous life. Let us add that the species of these tribes present still other advantageous devices: instead of having, like most of the other Jungermanniaceae, unicellular propagules, they produce discoid propagules whose fixation is easy; moreover the spores of these plants germinate in a small flat disc; finally, thanks to the presence of the auricles, they are able to reserve water for the days of dryness.

This last point is very important. Indeed, whatever the humidity of the atmosphere, there are certain hours of the day when the plants risk drying up.

There are no fleshy epiphylls with intracellular water reserves. On the other hand, Bryophytes accumulate large quantities of rainwater between their aerial organs, to the point that a whole small aquatic fauna has developed there. In Hepatics, when this external reserve is exhausted, another one remains in the auricles. Among the Thallophytes, only the Trentepohlia can collect water between the filaments, while the Phycopeltis, the Fungi and the Lichens probably have the ability to resist desiccation.

The epiphyllous plants constitute, as we see, a very interesting ethological group, in which most of the adaptations of epiphytic plants are found, but taken to a higher degree.

Translated with www.DeepL.com/Translator (free version) with minor corrections. [Rough translation only]

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