Fig. 11.3-8. Transverse section of cactus stem (Lepismium cruciforme; no common name). This is an epiphytic cactus and it grows only in wet habitats: hanging from the branches of large trees in rainforests. It too has cortical bundles, but these are unusual because they have primary phloem fiber caps. And the caps are much larger than the rest of the cortical bundle. It may be that these cortical bundle fibers help strengthen the dangling stems – the stems can be about a meter long, and must resist the flexing and bending caused by wind.
In this section, two cortical bundles are running parallel to each other, which is not very common.