Studies on Tracheary Element of Several Native Cycad Species in Australia and Two American Plants

Cycas angulata, Cycas armstrongii and Cycas conferta that are native species of Australia were collected in Darwin city of tropical region, Zamia erosa and angiosperm Guaiacum officinale of Zygophyllaceae which are introduced collected in the Darwin Botanic Gardens, and were carried out light microscope and electron microscope observed research to tracheary element and parenchyma tissue cells of leaflet, rachis and stem. The results showed that there are more vessel elements in their xylems;the length of vessel element of Cycadaceae with Zamiaceae are not obvious difference, the length of vessel element of angiosperm Guaiacum officinale is shorter, but their diameter is smaller, we thought that the characteristics which the length of vessel element is longer and the diameter is bigger are more evolutionary, because thus vessel, their number which interconnected points of possessed perforations’ end walls of vessel element is fewer, and the passageway space is larger so that the conduction speed and quantity are faster and more in unit length range and unit time. These vessels are annular vessels, spiral vessels, scalariform vessels, reticular vessels, pitted vessels and scalariform-pitted vessels, etc. In the transverse section, the vessel elements of Cycadaceae and Zamiaceae are circular, polygonous, more similar to vessel characteristics of leaf and stem of some angiosperms;however, the number of circular vessel of Guaiacum officinale is more, about occupy 35%, others are polygonous. The diameter of vessel is that annular vessel, spiral vessel is least, scalariform vessel or scalariform-reticular vessel is medium, reticular vessel and pitted vessel are the biggest, the characteristic is same as angiosperm. In Cycas conferta, etc. plants often can see several large perforations in the end walls, some species are several big scalariform or pitted perforations formed multiple perforation plates, in many vessel element lateral walls, there are some perforations formed a horizontal transport avenue of aqueous solution. The scale of diameter of Cycas vessel element is similar to that of Zamia, but the diameter of more vessel elements of Cycas angulata and Cycas conferta is bigger than the latter, and is bigger than that of Guaiacum officinale, although Cycas plants are more primitive than Zamia plants, and Guaiacum officinale is more evolutionary, but the scale of vessel diameter of Cycas plants is similar to Zamia or bigger than the latter, even larger than Guaiacum officinale, these showed that cycads have evolutionary and more developed characteristics in aqueous solution transport system. In Guaiacum officinale, more vessel element has been not seen the ridges in the different directions of side walls which can make vessel has more mechanical supportability, but some possessed the ridges;however, more vessel elements are twist shape and can twist together each other, thus the state can increase their mechanical supportability, and their synergy with fiber elements which is longer with a twist shape, therefore their capability of xylem conduction and the combine mechanical support capability with mechanical tissue are also stronger. In the respect of observed parenchyma tissue, any parenchyma tissue cells are without perforation, all cell walls are smoothed and like a membrane. Aimed at only one or two people pointed the query about Jeffery’s method, some scholars have made comparative researches, their research used as following methods: 1) The fresh materials were cut sections 1 - 2 mm thick by hand (the query person provided and thought right method), were examined with SEM. 2) The materials were treated by Jeffrey’s Fluid and observed with SEM and so on. The results showed that the pit membrane, the remnants in the perforation and the structural characteristics of perforations were not different;and the results of our comparative research in past time and this research all justified that Jeffrey’s method is reliable and reasonable. Meanwhile, our research results showed that only vessel element can form perforation, in other all parenchyma tissue cells cannot form perforation are also proved that the perforations of vessel element are naturally inherited character. Analyzed from the structural characteristics, although they distributed so far apart from each other, they are a well and unified taxonomy system;we thought that Australia’ most species or all species of Cycadaceae came from Asia, and it is impossible from Africa, because the latter only has one more evolutionary species and lack of fossil of Cycadaceae, these species of Cycadaceae are the descendants of ancestor species;America possessed more genera of Zamiaceae, this family is mutual with Africa and Australia, we thought that these genera of Zamiaceae in Australia and America came from Pangea before continental drift in Jurassic Period, this family is also maybe one of the proofs to Australia drifted from Pangea to present position after Jurassic Period. The characteristics of tracheary element of several different genera of cycads and angiosperm further showed that more primary species or taxon, their more tissue characteristics are not also more primary, because of the need to adapt the environment, they are also constantly evolving. The research is important significant and theory reference to understand cycads evolutionary aqueous solution transport system, and understand the ecological adaptation mechanism which why present cycads of 3 families in world major distributed in tropical or subtropical regions, even many drought and barren areas.