Several cycad species have entered the horticulture industry with Cycas revoluta Thunb. (Fig. 1A) represented more than any other taxa. All species of this group of dioecious gymnosperms are of value to collectors because of their ancient position in spermatophyte evolution and their unique morphological features (Norstog and Nicholls, 1997). The columnar stem of arborescent cycad species (Fig. 2A) lacks thin twigs and is therefore classified as pachycaulous (Stevenson, 1980). Cycad stems have concentric cylinders of vascular tissue that contain soft, parenchymatous xylem tissue and little lignified tissue with each new additional cylinder differentiating at the base of the stem between the cortex and the youngest pre-existing cylinder (Bork, 1990; Chamberlain, 1935; Terrazas, 1991). Persistent parenchymatous pith and cortex are not replaced by wood or phloem like in more typical woody plants. Active cambial activity occurs within every vascular cylinder regardless of plant or vascular cylinder age, and parenchymatous rays connect the cortex and pith tissue (Terrazas, 1991).
Cycas male cones are striking organs that add horticultural appeal (Fig. 2B). The genus Cycas differs from other cycad genera in having loosely arranged megasporophylls rather than a true female cone. Structure of the female reproductive structure varies among Cycas species with megasporophylls opening up in some species to eventually radiate out and then down from the stem apex (Fig. 2C) or remaining upright in other species to retain a globular shape (Fig. 2D). Both male cones and female structures are conspicuous, are important for taxonomic classifications, and add interest in any horticultural application.
More than half of the described cycad taxa are threatened or endangered (Donaldson, 2003), so very few studies have been conducted that require destructive sampling techniques. Of those that have been conducted, most of the focus has been on seedlings because they are relatively less valuable (Norstog and Nicholls, 1997). We recently studied the use of high-pressure stem injection of insecticides for use on six Cycas species to evaluate efficacy for control of Aulacaspis yasumatsui Takagi and resolve how the pachycaulous stem would respond to the treatment. We determined that the stem's succulent nature was highly susceptible to secondary infection whenever the protective external tissues were removed or the internal tissues were injured (Fisher et al., 2009).
We used this rare opportunity of access to large Cycas specimens to determine the size relations of various tissue categories within these stems that were necessarily sacrificed. We included C. revoluta, which is the most hearty Cycas species in horticulture, and C. edentata de Laub. and C. macrocarpa Griff., which have reputations for being highly susceptible to injuries that occur during various horticulture activities. We additionally included C. elongata (Leandri) D. Yue Wang, C. hainanensis C.J. Chen, and C. pectinata Griff. because they are intermediate in susceptibility to injury. The general appearance of large pinnately compound leaves radiating from the apex of a robust columnar stem is fairly similar among these six arborescent Cycas species (Fig. 1).
Understanding the characteristics of the cycad stem is crucial in horticulture, because this structure is transplanted and even marketed devoid of leaves and lateral roots (Fig. 2A). Our primary objective here was to determine how differences in stem structure may explain the differences in susceptibility to injury among the species. This information may explain why C. revoluta and other species are easy to grow and help improve management techniques for the species that are difficult to manage.
Chamberlain, C.J. 1935 Gymnosperms: Structure and evolution University of Chicago Press Chicago, IL [1966 reprint; Dover Publications, New York, NY].
Donaldson J.S. 2003 Cycads: Status survey and conservation action plan. IUCN/SSC Cycad Specialist Group International Union for Conservation of Nature Gland, Switzerland
Fisher, J.B., Lindström, A. & Marler, T. 2009 Tissue responses and solution movement after stem wounding in six Cycas species HortScience 44 848 851