The Kampong, National Tropical Botanical Garden, 4013 Douglas Road, Miami, FL 33133, USA.
Am J Bot. 2013 Mar;100(3):461-4. doi: 10.3732/ajb.1200550. Epub 2013 Mar 1.
Woody stems that have completed some maturation of metaxylem elements should not be capable of further axial extension ("secondary stem lengthening"). However, this mechanism has been claimed by Renninger and Phillips (American Journal of Botany 99: 607-613) to be a feature of the palm Iriartea deltoidea. In response, we describe structural features of palm stems based on extensive known features of their anatomy and development. In addition to the inability of metaxylem vessels to extend after they are mature, fully differentiated fibers of the vascular bundle sheath, which would exist at the time of proposed stem elongation would not be capable of belated extension. "Vessel spirals" claimed by these authors to be capable of stretching to accommodate secondary stem lengthening does not refer to well-established features of the course of vascular bundles. The approach adopted by Renninger and Phillips simply measures stems of different sizes as an implied developmental series. Consequently, results do not take into account changes in the development of the palm stem as it ages. The existence of secondary stem lengthening in the palm Iriartea deltoidea, something never before observed in any tree, cannot occur because it would indeed disrupt mature metaxylem vessels and would also require the secondary extension of mature lignified fibers.
已经完成木质部后生木质部元素部分成熟过程的木茎不应能够进一步轴向延伸（“次生茎伸长”）。然而，雷宁格和菲利普斯（《美国植物学杂志》99：607-613）声称这种机制是掌状 Iriartea deltoidea 的特征。作为回应，我们根据对其解剖结构和发育的广泛了解，描述了掌状茎的结构特征。除了后生木质部导管在成熟后不能延伸之外，血管束鞘的完全分化纤维，在提出的茎伸长时就已经存在，也不可能迟滞延伸。这些作者声称能够拉伸以适应次生茎伸长的“导管螺旋”，并不是指血管束的既定特征。雷宁格和菲利普斯采用的方法只是将不同大小的茎作为隐含的发育系列来测量。因此，结果没有考虑到随着掌状茎的老化，其发育的变化。掌状 Iriartea deltoidea 中的次生茎伸长，这是以前从未在任何树木中观察到的，不可能发生，因为这确实会破坏成熟的木质部导管，并且还需要成熟木质化纤维的次生延伸。
