MEDULLARY BUNDLES AND THE EVOLUTION OF CACTI

Medullary bundles are absent from the pith of the leafy, relictual cacti (genus Pereskia) but are present in most members of subfamily Cactoideae. They are absent only from tribes Hylocereeae, Rhipsalideae, and some members of Cacteae and Notocacteae. Presence of medullary bundles tends to be correlated with presence of a broad pith, but exceptions occur. Most medullary bundles are collateral, and in all genera phloem is produced and accumulates throughout the lifetime of the bundle. Xylem definitely accumulates as medullary bundles age in some groups, but it definitely does not accumulate in others, being produced only while the bundle is young. Pith can be broad (up to 75 mm in diameter), can constitute half the shoot volume, and is long-lived, remaining alive as long as the shoot is alive. Medullary bundles appear to be adaptive in allowing this large pith to be used for storage of water and starch. Medullary bundles have fewer, narrower tracheary elements than does the stele xylem in the same region; medullary bundles probably could not carry out significant long-distance transport if a major part of the stele becomes damaged.