306 
Mangham. — On the Mechanism of 
equilibrium, waves which travel through the protoplasm by way of the 
connecting-threads, and finally reach the sieve-tubes, which act in part as 
sugar reservoirs, and in which such readjustments can be made more 
effectively than in other cells. 
The sugar requirements of the cambium cylinders, &c., throughout the 
plant will result in lateral withdrawal from the sieve-tubes and from such 
storage tissues as phloem parenchyma and medullary rays. 
In this process, as well as in the reverse one of storage, the companion- 
cells probably play the part of intermediaries. 
These cells do not, as a rule, form longitudinally connected series, but 
are frequently disposed somewhere between the sieve-tubes and the cells of 
the bundle-sheath in leaves, while in the stem they often occur between the 
sieve-tubes and the phloem parenchyma or the medullary ray tissue. 
It was shown by Gardiner and Hill 1 that the distribution of connecting- 
threads in the leaf of Pinus was such as to suggest that food material could 
most readily travel to the sieve-tubes by way of the albuminous cells, and 
Hill 2 demonstrated that the latter, in the stem, were very well connected 
with the medullary ray parenchyma. 
As already mentioned, the companion-cells of the Angiosperms studied 
in this way were found to have numerous protoplasmic connexions with the 
sieve-tubes on the one hand, and with the phloem parenchyma on the other. 
The albuminous cells of the Gymnosperms are probably physiologically 
comparable with the corftpanion-cells of the Angiosperms, and it would 
seem that the role in each case is to act as agents between the sieve-tubes 
and adjacent tissues during the lateral exchange of assimilates. With the 
present hypothesis the comparatively great development of connecting- 
threads in the walls of the companion -cells receives a rational interpretation. 
Experimental demonstration of this function of the cells under con- 
sideration is difficult, for at least two reasons. 
In the first place, owing to their small cross-section, and to the absence 
of longitudinal continuity, the companion-cells do not lend themselves 
readily to observation, as a little experience shows. 
In the second place, it is obvious that translocation might go on 
effectively by the rapid movement of sugar present in concentrations too low 
to permit of satisfactory detection by the microchemical methods available. 
It is, however, proposed to reserve further consideration of these and 
kindred points until opportunities permit of describing in some detail results 
obtained by the use of the osazone method of locating sugars in plant tissues. 
The foregoing considerations may be applied to all plants . 3 
For example, the sieve-tubes 4 of the larger Brown Algae, plants which 
1 Gardiner and Hill (1901). 2 Hill (1901). 
3 Saprophytes need further treatment beyond the scope of the present paper. 
4 Oliver (1887) and Sykes (1908). 
