As with most vining
plants, new growth on your Philodendron is always at the ends of the stems. Invariably, vining
plants reach a point where they can no longer support additional leaves. Rather than stop
growing, they continue to add new leaves at the ends of the stems, but they sacrifice the older leaves. Eventually you end up with long, bare strings with leaves only at the ends.
The only remedy is to prune back very long stems to within a few inches of the soil. That way the new growth will emerge close to the pot and fill out the
plant.