Figure 1.

Diagram of a transverse section of pumpkin stem adapted from Crafts (1932). The fascicular phloem (i.e. phloem in vascular bundles) is bicollateral, with both internal and external sieve tubes. The extrafascicular phloem inside the sclerenchyma ring is composed of peripheral sieve tubes closely adjacent to the vascular bundles, entocyclic sieve tubes just inside the sclerenchyma ring, and commissural sieve tubes that run transversely, linking these various elements to each other and to the fascicular phloem. Ectocyclic sieve tubes are found outside the sclerenchyma ring.