COMPANION CELLS IN BAST OF GNETUM AND 



ANGIOSPERMS 

 W. p. Thompson 



(with seven figures) 



The presence of companion cells in the bast of angiosperms 

 is one of the constant anatomical features which distinguish that 

 great group of plants from the gymnosperms. On account of 

 the technical difficulties in the study of bast tissue in general, 

 little emphasis has been attached to this distinction, although 

 it is really quite as valuable as the familiar one based on the pres- 

 ence of vessels in angiospermic wood. The elements in question 

 are designated companion cells, because almost invariably one 

 of them is associated with each sieve tube. They are small, 

 vertically elongated, parenchymatous cells which have special 

 characteristics that will be described later, and which have no 

 counterpart in the bast of gymnosperms. 



In a study of the anatomy of the Gnetales (3, 5), it became 

 clear that the same elements or ones remarkably similar are to 

 be found in the bast of Gnetum. Another clear-cut characteristic 

 is therefore added to the long list of features in which Gnetum 

 departs from gymnospermic structure and resembles angiosperms. 

 It has been shown (5), however, that one of the most striking of 

 these resemblances, the possession of vessels, is not the result 

 of a genetic connection between Gnetum and angiosperms, because 

 the vessels have been evolved in entirely different ways in the two 

 groups. It becomes necessary, therefore, to examine and compare 

 the angiospermic and Gnetalean companion cells both as to 

 structure and as to development. 



Companion cells of angiosperms 



Angiospermic bast as found in Aristolochia macro phylla (Lam.) 

 is represented in fig. i. Two chief kinds of elements are visible: 

 the clear, irregularly shaped sieve tubes, and the richly proto- 

 plasmic parenchymatous cells (in most sections of bast there are 



451] [Botanical Gazette, vol. 68 



