SHOOT ARCHITECTURE

Mature plants of Pothos display an interesting and at times bewildering range of shoot architecture. To date very few field observations have been made but from what has been done it is clear that some potentially useful systematic characters are present. Preliminary investigations of Pothos shoot architecture can be found in Boyce & Poulsen (1994), Boyce & Nguyen (1995) and Boyce & Hay (1998).
For P. scandens and P. curtisii, almost the only species for which tolerably comprehensive shoot architecture observations exist, the following summary can be made. On germination both species produce an eocaul, a thread-like, cataphyll-bearing but otherwise leafless, skototropic (shade seeking, see Strong & Ray, 1975), physiognomically monopodial (see Boyce, 1998) shoot. The eocaul is capable of extending for a considerable distance along the forest floor and, at least in the initial stages, although the green stem is presumably capable of photosynthesis, it appears to depend partly on food reserves in the large seed. Once a suitable vertical surface is encountered, the shoot alters its mode of growth and attaches itself to the substrate by means of short clasping roots arising from either the nodes and internodes; it can therefore be termed a root-climber (see Schimper, 1903). At this stage the juvenile shoot also begins to produce foliage leaves. In P. scandens (subgen. Pothos) these are similar to adult leaves in appearance, although more congested and smaller; the plant is thus homeophyllous. Pothos curtisii (subgen. AlloPothos) produces a juvenile root-climber with more-or-less orbicular imbricating leaves arranged in the manner of the tiles or shingles of a roof (‘shingle-climber’) and of very different appearance to leaves produced later in the life cycle; that species is thus heterophyllous.
Initially all branches produced are adherent. In P. scandens growth can continue in this manner for a considerable time, the adherent shoots climbing high into the canopy where conditions permit and giving rise to branches from older lower portions but usually not branching distally unless the shoot tip is damaged. These adherent stems are referred to as ‘mature sterile’ in the descriptions to follow. By the time the plant has reached two or three metres non-adherent irregularly sympodial (i.e. terminating without flowering) side branches have usually begun to develop. These branches are plagiotropic, often repeatedly branching to form extensive curtains of foliage pendent under their own weight and are referred to below as fertile shoots. It is from the leaf axils of these plagiotropic branches that the flowering shoots arise. The juvenile stage of P. curtisii behaves similarly to that of P. scandens, the major difference being that the shingle growth is of limited duration (usually climbing to no more than 3 meters, often considerably less) before the plant abruptly begins producing leaves of the adult form. The alteration to the adult form is often accompanied by extensive branching with both adherent and free shoots arising, and the plant often forming a mass of interlacing branches (‘hammock-forming’).
Inflorescences of P. scandens are solitary and born terminally on non-reiterating short lateral flowering shoots bearing cataphylls but no foliage leaves. In other species (e.g. P. curtisii) these lateral shoots can be elaborated by sympodial branching into leafless, sometimes highly complex, compact or lax synflorescences (see Mayo et al., 1997) bearing two to many inflorescences simultaneously or sometimes single inflorescences in series (e.g. P. lorispathus, P. lancifolius). Synflorescences are usually borne along or at the end of leafy branches or, more rarely on older leafless parts of the stem, sometimes arising from there (e.g. Bornean and Philippine (Palawan) P. insignis Engl.; see Boyce & Poulsen, 1994). Very occasionally inflorescences are solitary and terminal on leafy branches (e.g. P. kingii).
A feature of P. curtisii shared by a number of other species is the production of flagelliform, leafless (cataphyll-bearing), skototropic, ‘foraging’ shoots whose function appears to be to vegetively propagate the individual by searching for and colonizing suitable climbing surfaces. These foraging shoots can arise in a variety of positions but are most often seen at the tips of free or adherent branches.
Certain species (e.g., P. repens, P. chinensis) occasionally produce enormously robust reiteration shoots from older, usually leafless, parts of the plant. These shoots are notable not only for their size but also for the way in which the leaves are directed forwards and tightly imbricated and also in the mass of adherent roots that arise from them. Based on observations of plants in Vietnam, Boyce & Nguyen (1995) speculated that these reiteration shoots might serve as a means to rejuvenate ageing plants in which the quantity of high-climbing stems had become too great for the functioning root mass.