I9I9] 



THOMPSON— COMPANION CELLS 



453 



indication of relationship between sieve tube and companion cell. 

 The relative positions of these two elements is such as to indicate 

 that they are formed from two successive cells in a row, and that 

 they are products of the same mother cambial cell. This relation- 

 ship was mentioned long ago by DeBary (i): ''Both from their 

 arrangement in cross-section and on tracing them in the longi- 

 tudinal direction, it often has the appearance as if the cambiform 

 cells arose with the elements of the sieve tubes from one mother 







WM^BS^^^^X^ \B2M^^ t<»r^ v-^'-^-'-' - ^ ^f^-?'^-'=-r'^-=^ ^ 



Fig. 2. — Cambium and young bast of Aristolochia macrophylla (Lam.) showing 

 each sieve tube and its companion cell formed from contiguous cells in same row; 

 X 1000. 



cell, the latter dividing longitudinally into a daughter cell which 

 becomes the sieve tube element, and another which becomes a 

 cambiform cell without further division or is divided by cross walls 

 into several of them." Strasburger (2) later made the definite 

 statement that companion cells and sieve tubes are sister cells. 



This statement is of course to be confirmed or disproved by 

 an examination of the method in which the bast develops in the 

 cambial region. The process as it occurs in Aristolochia is illus- 

 trated in fig. 2. As one traces the rows of cambial cells in the 



