Abstract:

To identify the cell types and analyze the transport mechanism of vascular tissue and explore the cell evolution during tree evolution，a comprehensive anatomical study was carried out on a kind of special tracheids of Tetracentron sinense. After slicing and dissociating， the distribution position and morphological characteristics of the special tracheids of the 34-year-old T. sinense were observed by using optical microscope and scanning electron microscope. The results showed that the special tracheids were arranged from the inside to the outside radially in the horizontal of the tree， and ran through the growth ring boundary. Most of them were in one row， and few were in two rows. And each special tracheid was connected with wood ray cells on one or both sides. In the longitudinal direction， the special tracheids were connected individually or with several cells terminally. There were three main types of special tracheids： fusiform without end wall， fusiform with one sloping end wall or with two sloping end walls. The average length of special tracheids was 286.44 μm. Its cross section was quadrilateral. The average tangential width of special tracheids were 55.22 μm respectively. The average wall thickness of special tracheids was 1.53 μm. The two ends of the special tracheids were closed without perforation. The number of pits on the side wall of special tracheids was large and the pit membrane was clearly visible. There were a large number of bordered pits on the wall in tangential section， which were arranged in opposite， alternated and scalariform pitting. The single pits on the radial wall were rounded or ellipse. There were 2-10 pits， arranged in 1-4 rows in per region. There were no pits between the special tracheids and the normal tracheids on the radial wall. The distribution of special tracheids has certain regularity. Compared with normal tracheids， the special tracheids were much shorter in length， and the tangential width were slightly larger. The type and arrangement of the pits were similar to those of the pits on the wall of general hardwood species. The shape of the special tracheid was different from the general vessel in the hardwood， and it was in an intermediate state of the evolution from tracheid to vessel， indicated a certain level of evolution of this type of cell.

Key words: Tetracentron sinense, wood anatomy, special tracheid, morphological characteristics