Abstract: This article conducted comparative anatomy of rachis of six species of Cycadaceae and two species of Zamiaceae, i .e ., Cycas taiwaniana , C. hainanensis, C. parvulus, C. simplicipinna , C. diannanensis, C. elongata , Macrozamia . longispina and Dioon edule in transverse section , in order to provide anatomical evidence for a better understanding of the systematic
position , taxonomy and to discuss its anatomy structure and the ecological adaption mechanism . The results showed that there were significant differences between Cycadaceae and Zamiaceae in epidermis, mechanical tissue , ground tissue, vascular tissue and ergastic substance and so on . The rachis structure of genus Dioon was also different from genus Zamia .
The 6 species of Cycadaceae are more similar to each other , particularly among C. taiwaniana , C. hainanensis, C. parvulus, C. diannanensis, C. simplicipinna , which manifests they are closely related to one another , but each species has its own structure characteristic . The arrangement and the process of the increase of vascular bundle number of C. elongata issimilar to the two species of Zamiaceae which implied that its status is comparatively derived in Cycas . The study discovered that there are assimilating tissue which like the shape of palisade tissue in adaxial surface of rachis in all species of Cycas . The rachis of these cycads had structure characteristic of xerophytes , i . e ., thicker cuticular membrane, developed mechanical tissue , the more secretion , crystal and secretory canal and more number of vascular bundles etc . These results provided evidence to study the evolution and relationship between different families and species of the Cycadopsida , and revealed the fact that primitive and ancient cycads can exist in the geological history is because cycads unique structure characteristics have been evolved so that they can adapt to various evironment such as drought , extreme heat , barren , and salt alkaloid and other harsh environment .

Key words:

Cycadaceae