Hill. — The Histology of the Sieve-Tubes of Angio sperms. 277 
Companion cells, either one or two, accompany each sieve-cell and 
illustrate clearly the way in which both they and the sieve-tube are formed 
from the same mother cell. The wall between sieve-tube and companion 
cell is crowded with threads as is the case in the phloem proper, and their 
distribution and character in such walls suggest that the threads are closely 
connected with the processes of cell division. 1 
Fig. 13. Vitis, Tangential section showing a string of sieve-tubes crossing a medullary ray. 
The connecting slime-strings crossing the callused sieve-plates are well seen. The short sieve-tubes 
are accompanied by their companion cells. (Copied from Wilhelm, PI. VI, Fig. 69.) 
The Callus. 
An examination of a callus-pad which is being reopened, and from which 
the slime-strings have been removed, throws some light on the way in which 
these masses are built up. For owing to the swelling action of the ferment 
a very regular and definite stratification is disclosed, which lends support to 
the view that the callus-cushions or pads are laid down by the protoplasm 
in definite layers in a manner similar to that of the formation by apposition 
of ordinary cell-walls. It seems highly probable, therefore, that the proto- 
plasm has the power to deposit callus as it would cellulose in accordance 
with the demands of the plant (Fig. 58, PI. XVIII). Additional evidence 
is afforded by those rare cases in which a layer of callus is laid down all 
over the wall of the sieve-tube apparently in place of a layer of cellulose 
such as that seen and figured in Phaseolus (Fig. 28, PI. XVII). 
1 Cf. Gardiner, Roy. Soc. Proc., 1900, p. 186, and Proc. Camb. Phil. Soc., xiv, 1907^. 209. 
