Tubular Elements Of The Base Of The The Angiosperms Key Groups

Tracheary elements, including vessel elements, tracheids and fibres, are important members of secondary xylem in vascular plants. It is considered that tracheids are the most primitive tracheary elements, that evolved into vessel and fibre phylogenetically. The origin and evolution of vessel is a key factor for plants removing from wet areas to seasonal dry areas and is closely related to the evolution of vascular plants. So, the morphology characters of vessel elements bear important signification in plant taxonomic and phylogenetic researches and the vessel evolution reflects the evolution level of vascular plants.Basal angiosperms are considered as the key groups for the origin and evolution researches of angiosperms, have many ancestral morphology, therefore, attracted much attention of botanists. In recent years, molecular phylogenetic analysis based on multiple genes showed that angiosperms could be grouped into basal angiosperms, monocots and eudicots. Basal angiosperms contain ANITA group, Ceratophyllales, Chloranthaceae, Winterales, Piperales, Laurales, and Magnoliales. In this research we selected 18 species of 13 families in basal angiosperms as representatives for studying their tracheary elements with SEM. We wished to uncover the morphologic changes of perforation plates on end walls, pit membrane remnants and lateral wall pitting in the evolution of vessel elements, and to discuss the relationship of vessel evolution and phylogenetic progress of basal angiosperms. Here we show that:Tracheary elements in secondary xylem of Amborella (Amborellaceae) have the tendency of transformation from tracheids to vessel elements, and have already formed primitive vessel elements. Secondary xylem in Drimys wintera (Winteraceae) are even more primitive without really vessel elements present. Primitive vessel elements have presented in Chloranthus and Sarcandra of Chloranthaceae. Tracheary elements in these three groups show that transition from tracheids to vessel elements is a continuous progress. Vessel elements are formed by losting pit membranes in perforations on end walls, and congregating and enlarging of the perforations to form scalariform perforation plates on end walls of tracheary elements. The difference between perforation plates on end walls and lateral walls is the key feature for vessel elements origin.The evolution levels of vessel elements in basal angiosperms are various. There are incipient vessel elements with scalariform ends in Amborella; Vessel elements have long scalariform perforations plates with numerous bars in Illicium, have scalariform plates with less bars in Austrobaileya and Monimiaceae, and have formed simple perforation plates on end walls in Calycanthaceae, Lauraceae and Piperaceae. It shows the tendency of transformation from scalariform perforation plate to simple perforation plate. We could estimate the evolution rate of tracheary elements in various basal angiosperms based on different evolution levels of tracheary elements in different plants families and the origin times of these plants families. In basal angiosperms, the common ancestor of various groups bear primitive tracheary elements as tracheid type, so the vessel in different branches were not unitary origin and evolved independently. After origin of plant groups, their tracheary elements evolved parallelly with different evolutionary rate.Tracheary elements in Winteraceae, Amborellaceae and Chloranthaceae have various degree of pit membrane remnants in end walls. Present of pit membrane remnants in these three groups show the transition from tracheids to vessel elements is a continuous progress and presence or absence of pit membranes in end walls is used to differentiate the two cell types. Some of vessel elements in Illiciaceae, Schicandraceae, Eupomatiaceae and Monimiaceae have various degree of pit membrane present in scalariform perforations plates, which couldn't account for transition from tracheids to vessel elements in these families, but show primitiveness of the vessel elements.According to the evolution level of vessel perforation plates and the phylogenetic position of plants groups, we can presume that the small circular pits scattering or arranging in single row on lateral walls of vessel elements could be primitive type. As the diameter of vessel elements increasing in evolution progress, pits on lateral walls enlarge transversely to form scalariform or increase in quantity, arrange in different patterns to form opposite or alternate. There are transitional types between scalariform, opposite and alternate lateral pitting. Pitting between vessels and rays are much larger than intervascular pitting, maybe it is for more effective in transverse water transport.Helical sculptures on vessel walls are not extensive in basal angiosperms, there are helical thickenings in Illicium henryi and Chimonanthus praecox, and grooves interconnecting pit apertures in Litsea tsinlingensis. Maybe this feature is related to ecological factors.Reticulate perforation plates are not extensive in basal angiosperms, there are several primitive reticulate perforations and forked or interconnected bars in scalariform perforation plates in Drimys winteri, Amborella trichopoda, Chloranthus multistachys, Chloranthus elatior and Illicium henryi, and typical reticulate perforations in Eupomatia laurina, Schisandra sphenanthera, showing that several rows of small circular perforations on end walls enlarged to form reticulate perforations, and the bars between paratactic reticulate perforations degenerated and clear away to form scalariform perforation.