Fig. 11.4-5. Transverse section of cactus stem (Morawetzia doelziana; no common name). The low magnification micrograph shows the pith (and a little bit of the surrounding wood on the right). There are numerous vascular bundles in the pith (vascular bundles in the pith are known as medullary bundles), but at this low magnification they are difficult to see. The high magnification micrograph shows one medullary bundle: it is collateral, having both xylem and phloem.
Most cacti have
both cortical bundles and medullary bundles. And of course they all have the
ordinary bundles that make up the central ring of vascular bundles.
Consequently, their stems have three sets of vascular tissues, and they are all
interconnected: medullary bundles and cortical bundles both attach to the
central ring of bundles, and medullary bundles can run through the parenchyma
between the main bundles and connect directly with cortical bundles. By having
these two extra sets of vascular tissues, it is feasible for the cortex and pith
of cacti to evolve to be much larger than the corresponding tissues in ordinary