Examination of leaf and stem tissues in a broad range
of genera resulted in the discovery and analysis of a novel tracheid type
termed Wide-Band Tracheids (WBTs; the term is derived from the comparatively
wide secondary wall) in derived genera of Aizoaceae, Cactaceae and Portulacaeae
(Caryophyllales). In Aizoaceae, WBTs are only found in genera of Ruschioideae;
in Cactaceae, WBTs are found in xylem of leaves and stems in genera of
Opuntioideae and Cactaceae. However, in the genus Anacampseros
(Portulacaceae), WBTs are found in leaf xylem, but not as part of the
xylem of the stems and instead, WBTs are found in piths and rays. It was
hypothesized that the wide secondary wall prevents primary wall contact
during extreme water stress and thus WBTs were thought to differentiate
in response to water-stress. In order to determine what factors cause
WBT initiation and differentiation, seedlings of Anacamperos rufescens
(Portulacaceae) were exposed to varying light intensities that mimicked
spring and summer light levels found in southern Africa. In this experiment,
results show that WBTs are generally formed in advance of probable water-stress
event times. Furthermore, the number of WBTs are directly correlated to
the intensity of light received as a seedling; however, the mean WBT size
remained relatively unchanged, presumably due to a rigid genetic control.
Results suggest that, in Anacampseros rufescens, the later a seedling
germinates, the greater the number of WBTs differentiate, which pre-adapts
the plant for future water-stress events.
James Victor Landrum , 2008. Wide-Band Tracheids from a Southern African Succulent and Their Responses to Varying Light Intensities: A Pre-Adaptation for Future Water Stress?. International Journal of Botany, 4: 99-103.