ADVANCES IN THE STUDY OF ROOT-STEM TRANSITION REGION OF THE ANGIOSPERM
Gu An-gen, Wang Li-jun, Wang Mao, Gu Yi
1991, 11(3):
85-90.
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As everyone knows, hypocotyl, so-called root-stem transition region, was generally considered as an insurmountabale barrier in the study of the primary vascular system of Angiosperm. The order of the traditional discr-iption of the vascular system was from root apex through hypocot 1 to sho-ot apex (Eames and MacDaniels, 1947; Lenoir, 1920.). Though some rese-archers considered that the vascular system of the seedling was originally not continuous (Dangeard, 1889, 1913) or suggested that double origins (end-ogenous and exogenous) existed in the development of the vascular system of seedlings (Thoday, 1939), they did not pay attention to the existence of CNZ and its significance in the study of phylogeny of the primary vascular system of angiosperms. Therefore we do not agree to the viewpoint that the seedlings of angiosperms is divided into two parts:roothypocotyl-cotyleden and epicotylary shoot (Esaw, 1953). We maintain that the seedling of angiosperms should be divided into three parts:hypocotyl-root zone; CNZ and epicotylary shoot zone.#br#We think the research methods mentioned above is contrary to the fo-ssil plants discovered and the ontogenic process of the modern Angiosperm. From the phylogenic point of view, stem developed first and root after; form the ontogenic point of view, active shoot apex and inactive root primordi-um occured as early as the cordiform embryonic stege. When embryo axis was vissible after the further development of embryo, there existed a cons-ervative CNZ between shoot apex and root primordium. The haplostele in shape of the primitive protostele still remained in this zone. Later, hypoc-otyl developed from the lower part of CNZ and root from the end of hyp-ocotyl. Hypocotyl was the cotyleden node-root transition region. In the middle and upper parts of CNZ, namely, the part above the plate shape showed by the combination of two cotyleden traces, an meristimatic tissue-cotyleden node-stem transition region remained, where the differentiation of cells was slow. In R. japonicus, the embryonic tissue differentiated into the vascular tissue only when the growing tip changed from the vegetative shoot apex into the reproductive shoot apex after several foliage leaves developed on the growing tip. This vascular tissue connected with the vascular tissue of the stem above and the vascular tissues of the lower part of CNZ, hyp-ocotyl and root bellow to form the whole of the primary vascular system of the seedling. Thus it can be seen that the origin and evolution of the prim-ary vascular system of the seedling. Thus it can be seen that the origin and evolution of the primary vascular system of the stem do not relate to hyp-ocotyl and root. Therefore the traditionalmethod discribing and studying the primary vascular system of angiosperms from root through hpocotyl to stem should be discarded. The new method discribing and studying the primary vascular system from CNZ to both up and low ends (shoot apex and root apex) should be put int use. Studies showed that the evolution of the stele conformed to the telomic theory and related to the direction of the differ-entiation of metaxylem (Zimmermann, 1956).
