The Cactus Explorer ISSN 2048-0482 Number 13 December 2014 STRANGE STEM ARCHITECTURE OF PACHYCAULOUS MORNING GLORIES Root Gorelick tells us about the stem architecture in Ipomoea , speculating that weird stem architectures may tell us about the evolution of succulence. Dan Mahr (2012) highlighted the wonderful years, producing not only fleshy storage tissue Mexican succulent trees in the morning glory (as do leeks and onions), but also concentric genus Ipomoea in the family Convolvulaceae. layers of wood and inner bark. While unusual, Only some members of the genus are included concentric vascular cambia are found in in the Illustrated Handbook of Succulent several plant families (Carlquist 2007), Plants (Eggli 2002), namely the tuberous including some semi-succulent members of the rooted southern African species. This list of genus Cycas L. (albeit none of the western succulent Ipomoea species probably simply hemisphere cycads; Norstog and Nicholls reflects the expertise of Ernst van Jaarsveld, 1997), and are often associated with succulent who compiled that section of the Handbook. or semi-succulent stems. Liannas are also Instead of trying to extend this list of succulent renowned for successive cambia, such as in Ipomoea species, which I honestly could not do, Gnetum scandens Roxb. (Chamberlain 1935) and I will highlight the weird stem architecture of Santaloidella gilletii G. Schellenb. (Isnard and Mexican Ipomoea species, focusing on the giant Silk 2009). Many mesembs, family Aizoaceae, of the genus Ipomoea arborescens Sweet. have concentric vascular cambia (Carlquist Most trees of either flowering plants or 2007). See the online Xylem Database conifers grow taller via apical meristems and (Schweingruber and Landol 2005; simultaneously grow wider by producing new http://www.wsl.ch/dendropro/xylemdb/ ) for tissue from a cylindrical vascular cambium. beautiful images of stem cross sections, Vascular cambia produce xylem to the inside including of concentric vascular cambia in (centripedal), which transports water. Vascular Aizoaceae, such as in Aptenia cordifolia . (L. f.) cambia produce phloem to the outside Schwantes. Even stranger, some, but not all, of (centrifugal), which transports the products of the concentric vascular cambia of Ipomoea photosynthesis. Horticulturists know that arborescens produce xylem to the outside when grafting it is important that two stems (centrifugal) and phloem to the inside being joined have some overlap in their (centripedal) of the cambium, which is known vascular cambia so that the scion can continue as a ‘reverse cambium’ (Terrazas et al. 2011). I. growing. Many plant stems have a single arborescens sometimes even produces cambia vascular cambium throughout their entire from pith, which is somewhat unusual. Plus, lives, albeit a cambium that branches when the these vascular cambia that arise in pith only stem branches. Note use of the adjective produce phloem. ‘vascular’ because there also exist cork cambia, Vascular cambia produce more than simply which form a cylinder that only produces xylem and phloem, i.e. wood and inner bark tough water-resistant cells, so-called cork cells, include more than just xylem and phloem cells. that form outer bark. Vascular cambia also produce unspecialized Pachycaulous species of Ipomoea , however, thin-walled cells (parenchyma) that form rays do things a little differently. I. arborescens has between the spokes of xylem cells in wood and multiple concentric vascular cambia (Terrazas between the spokes of phloem cells in inner et al. 2011). A cross section of such a plant bark. When splitting wood, these rays are would look reminiscent of a leek or onion but, where you place an axe because the thin- unlike leeks or onions, the concentric layers of walled ray parenchyma cells offer little Ipomoea arborescens stems continue growing for resistance, at least compared with the thick- 88 Number 13 December 2014 ISSN 2048-0482 The Cactus Explorer walled and heavily-lignified xylem cells that do self-standing upright trees and shrubs. form wood. Ray cells are used in transporting Weird stem architectures may tell us about the and storing water and products of photo- evolution of succulence. synthesis, as well as other plant metabolites. References Thus, a plant with multiple concentric vascular CARLQUIST , S. (2007). Successive cambia cambia should, all else being equal, produce revisited: ontogeny, histology, diversity, more storage tissue and be more succulent. and functional significance. Journal of the Concentric cambia may help in storing water if Torrey Botanical Society 134 : 301–332. the vascular cambia produce disproportionally CHAMBERLAIN , C.J. (1935). Gymnosperms: more rays than wood. One of the best common structure and evolution . University of Chicago examples of concentric vascular cambia, some- Press, Chicago. times known as ‘successive cambia’, is in beets, EGGLI , U. (2002). Illustrated handbook of succulent Beta vulgaris . Cut one open and look at the plants : Dicotyledons . Springer-Verlag, Berlin fleshy concentric rings. In addition to concen- & Heidelberg. tric vascular cambia, Ipomoea arborescens also ISNARD , S. & S ILK , W.K. (2009). Moving with produces many short segments of vascular climbing plants from Charles Darwin's time cambium from undifferentiated parenchyma into the 21st century. American Journal of cells in the rays, usually only producing Botany 96 : 1205–1221. phloem and phloem rays, which are called LOWELL , C. & L UCANSKY , T.W. (1986). ‘included phloem’. Included phloem is fairly Vegetative anatomy and morphology of common in semi-succulent plants, e.g. jojoba Ipomoea hederifolia (Convolvulaceae). Bulletin (Simmondsia chinensis C.K. Schneid.). If a stem of the Torrey Botanical Club 113 : 382–397. has multiple concentric vascular cambia, then MAHR , D. (2012). CSSA Tour 2010: the its evolutionary lineage may possibly have had botanical riches of Oaxaca. Cactus and the freedom to experiment with a subset of Succulent Journal (US) 84 : 168–186. these cambia and produce xylem and phloem NORSTOG , K.J. & N ICHOLLS , T.J. (1997). The in the opposite directions, especially if these biology of cycads . Cornell University Press, reverse cambia primarily are used for storage Ithaca. vis-à-vis rays, and not for water or sugar RAJPUT , K.S., R AOLE , V.M., & G ANDHI , D. (2008). transport vis-à-vis xylem and phloem. There Radial secondary growth and formation of are indeed lots of interesting ways that nature successive cambia and their products in created succulent plants, including adding Ipomoea hederifolia L. (Convolvulaceae). extra layers of mitotically dividing cambial Botanical Journal of the Linnean Society 158 : cells (Robert et al. 2011) and maybe even 30–40. having some reverse cambia, as in the pachy- ROBERT , E.M.R., S CHMITZ , N., B OEREN , I., caulous Ipomoea arborescens . DRIESSENS , T., H ERREMANS , K., D E MEY , J., ET The peculiar stem architecture of producing AL . (2011). Successive cambia: a concentric vascular cambia and reverse developmental oddity or an adaptive cambia, however, is not unique to pachycaul- structure? PLoS One 6 : e16558. ous Ipomoea species. These are also found in SCHWEINGRUBER , F. & L ANDOL , W. (2005). The annual vining members of the genus, such as xylem database. URL: in I. hederifolia L. (Lowell & Lucansky 1986; http://www.wsl.ch/dendropro/xylemdb/ Rajput et al. 2008). Liannas are not hugely [accessed 18 November 2014]. constrained in having to build orderly wooden TERRAZAS , T., A GUILAR -R ODRíGUEZ , S., & scaffolds the way self-supporting upright trees OJANGUREN , C.T. (2011). Development of and shrubs are. Thus vines and liannas almost successive cambia, cambial activity, and always have peculiar stem anatomy (Isnard & their relationship to physiological traits in Silk 2009). If I might speculate, succulence may Ipomoea arborescens (Convolvulaceae) have originated from liannas, which have seedlings. American Journal of Botany 98 : 765– vastly more variation in stem architecture than 774. 89