独叶草叶二叉分枝脉序中网结脉和盲脉的形态学研究

MORPHOLOGICAL STUDIES ON ANASTOMOSES AND BLIND VEINS IN DICHOTOMOUS VENATION OF THE LEAF IN KINGDONIA UNIFLORA

对独叶草营养叶二叉分枝脉序及其中的网结脉和盲脉的形态学研究表明:(1)网结脉中2条完全汇合的与靠近脉中完全分离的叶脉之间未发现任何形式的维管束汇合的中间类型及网结脉中具有不同程度的连接脉退化痕迹的事实表明,网结脉不可能由靠近脉产生,相反,由于网结脉中联结脉的退化而形成开放脉;(2)盲脉是通过伴随着齿退化的达齿脉的退化、网结脉中联结脉的间断、非网结脉由分枝处间断三种方式产生的;(3)越裂片脉的出现及其可以形成网结脉的现象表明独叶草营养叶可能曾具有较为复杂的脉序,这种叶脉也呈现出退化的趋势;(4)独叶草营养叶的二叉分枝脉序可能是一种退化性状,而网结脉的出现可能是这种退化过程中的残留痕迹。

The morphology of the dichotomous venation, anastomoses and blind veins in the venation of the leaf of Kingdonia uniflora were studied in the present work. The lamina is 5-divided. There are three veins at the base of the lamina, among which the middle one enters the base of the middle segment, and each lateral one dichotomyzes into two daughter veins. Each daughter vein enters the base of the lateral segments. The veins at the base of the segments dichotomize repeatedly 5~6 times forming the veins of different orders. Each vein of a particular order and its branches corresponde a particular lobe of the same order, and the ultimate vein terminates in the middle part of the umbo at the apex of a tooth. In some cases, a vein in a lobe or a lobule dichotomizes and one of the daughter veins enters the lobe or lobule, but the other enters the neighbouring lobe or lobule. This kind of dichotome is named lobes-overstepped vein. One or two of the daughter veins can be blind veins with different lengths. The daughter veins overstepping the other lobe or lobule can form an anastomosis. The existence of the lobes-overstepped vein and the anastomosis formed by the overstepped daughter vein indicate that the venation of this monotypic genus is more complex than it was considered. Sometimes the two neighbouring veins of two lobes or lobules can be extremely approached and this is the approximate vein. There are two types of anastomoses in the venation of this species. No intermediate forms of the fusion of the vascular bundles were observed between the two completely fused veins of the anastomoses and the two separate veins of the approximate veins. But the vestige of different degrees of the com-missural veins was observed. Therefore, we consider the formation of anastomosis from the approximate veins impossible. There are three types of blind veins in the venation of this species. The blind veins might have formed by the reduction of the vein terminating in a tooth accompanied by the reduction of the tooth, the interruption of the commissural veins of the anastomoses and the interruption of a veinlet from the branching point of a vein di-chotome. According to the present morphological studies, we conclude that the dichotomous venation of the leaf of Kingdonta uniflara is of a characteristic of reduction and that the anastomosis is the vestige of the reduction from the closed venation to the open one. The occurrence of this kind of venation in the leaf of this species and some ferns and gymnosperms is a result of convergent evolution.