igig] THOMPSON— COMPANION CELLS 459 



pendent evolution in Gnetum and angiosperms, the inference is 

 natural that other resemblances may be in the same category. 



Summary 



1. Companion cells resembling those of angiosperms in size, in 

 their association with sieve tubes, in their usual location in the 

 angles of the sieve tubes, and in their vertical elongation, are 

 present in the bast of some species of Gnetum. 



2. The development of these companion cells, however, is 

 quite different from that found in angiosperms. Whereas, in the 

 latter, each sieve tube and its companion cell are derived from two 

 successive cells in a single row of cambial products, in Gnetum 

 sieve tubes and companion cells are produced from different rows 

 of cambial cells. 



3. Although the completed forms of companion cell in the 

 two groups are similar, they have probably been independently 

 evolved. 



4. Primitive conditions in which companion cells are lacking, 

 or in which continuous rows of companion cells are present, are 

 found in certain regions of some species. 



5. The parench)Tna of the wood is formed by those cambial 

 cells which form companion cells; the distribution of the wood 

 parenchyma is consequently in radial bands, which frequently 

 become interrupted by the expansion of vessels and fibers. 



University of Saskatchewan 

 Saskatoon, Sask. 



LITERATURE CITED 



1. DeBary, a., Comparative anatomy of Phanerogams and Ferns. Oxford 

 University Press. 1884. 



2. Strasburger, Noll, Schenk, and Karsten, Textbook of Botany. 

 Macmillan. 1912. 



3. Thompson, W. P., The anatomy and relationships of the Gnetales. i. 

 The genus Ephedra. Ann. Botany 27:1077-1102. 1912. 



4. , The morphology and affinities of Gnetum. Amer. Jour. Bot. 



4:135-184. 1916. 



5. . Independent evolution of vessels in Gnetales and Angiosperms. 



BoT. Gaz. 65:83-90. 1918. 



