4. P. ser. Pachycaules, ser. nov.
TYPE: P. basii Matuda
Caudex succulenta; internodia brevia, crassus; cataphylla 2-costata, persistentia intacta; petiolus teres; lamina ovato-cordata, costae postice desnuda secus sinum ad 4 cm; pistila 4-6 locularia; locula 4-6 ovulati.
Etymology: pachy = thick; caule = stem, refering to the thick, succulent stems.
Philodendron ser. Pachycaules is represented by a single species and is characterized by its very stout succulent stems with intact persistent cataphylls, terete petioles, ovate-cordate blades with the posterior rib naked along the sinus to 4 cm. The pistil has a type D style and is 4-6 locular with 4-6 ovules per locule. It is believed that the succulent stems evolved to store water, allowing the plant to survive the long dry season in western Mexico. The series is represented only by P. basii from western Mexico.