THOMPSON AND BAILEY: TETRACENTRON, ETC. 29 



of our observations upon the xylem of Tetracentron, Trochodendron, 

 and Drimys. 



Fig. I is a photomicrograph of a transverse section of a two 

 year old stem of Ginkgo biloba L., and iUustrates the typical 

 vesselless xylem of Gymnosperms, which is in marked contrast to 

 that of the Angiosperms, shown in FIG. 5, a transverse section of a 

 young stem of Schizandra chinensis Koch. This difference in 

 xylem structure is clearly illustrated in figs. 6 and 10, more 

 highly magnified transverse sections of Pinus palustris Mill, and 

 Swietenia Mahagoni J acq. 



Fig. 4 is a photomicrograph of a transverse section of a young 

 stem of Tetracentron sinense Oliver. The xylem of the central 

 cylinder and leaf traces is composed of radial rows of tracheids, 

 and vessels are entirely absent. Similar xylem, which is even 

 more typically coniferous, is shown in fig. 2, a cross section of a 

 young root of the same species. 



Fig. 3 illustrates the characteristically coniferous structure of 

 a seven year old stem of Trochodendron aralioides Sieb. & Zucc. 

 and FIG. 7 shows, under higher magnification, part of an annual 

 ring of the mature wood of this species. The differentiation of the 

 xylem into thin-walled "spring" tracheids and thick-walled 

 "summer" tracheids closely resembles that of many Conifers 

 (compare fig. 6). 



The entire absence of vessels in Tetracentron and Trochodendron, 

 in regions^ that are supposed to be conservative of ancestral 

 characters, is clearly illustrated in figs. 2, 3, 4, 8, 11, 12. In 

 fact, the writers have been unable to find vessels or vestiges of 

 vessels in Tetracentron, Trochodendro?i, Drimys Winteri Forst., 

 D. colorata Raoul, and D. axillaris Forst. 



Not only are vessels absent in the xylem of these plants, but the 

 tracheids are typically coniferous in form, structure, and general 

 arrangement. Thus the bordered pits, as in the case of the 

 Gymnosperms, vary considerably in form and arrangement. 

 Scattered circular pits, the commonest coniferous type, occur in 

 the thick-walled tracheids of the summerwood of Tetracentron 

 and Trochodendron, and in many of the tracheids of Drimys. 

 In the large thin-walled spring tracheids of the former genera 

 and in the tracheids of certain species of Drimys the bordered pits 



1 Root, seedling, young stem, node, leaf, etc. 



