6o6 Hill . — The Histology of the 
nodule, which appears to be its homologue at the pectic 
middle lamella, has already been discussed, but the question 
of their physiological importance is a more difficult matter ; 
with regard to the latter two views seem to be possible, either 
that the callus-rod and median nodules are formed because 
they are of real value to the active sieve- tubes, or because on 
the contrary the ferment which is at work on the canals of the 
connecting threads cannot help affecting the cell-wall in their 
vicinity. 
As far as one can see, the first of these two suggestions is 
the correct view, for the action of the ferment, which gives 
rise to the sharp and clearly-defined callus-rods and nodules, 
is found to be so distinctly localized and circumscribed. It 
therefore seems likely that the callus, which swells so easily, 
may be a kind of spongy lining to the canals of each slime- 
string, and may regulate when necessary the dimensions of 
the pores of the active sieve-plates. 
The chief function of the callus-cushions, which are developed 
towards autumn, and through which the slime-strings are 
continued, is also apparently to bring about the diminution in 
size of the canals of the slime-strings, and thereby to gradually 
slow down and eventually stop the processes of translocation 
between adjoining sieve- tubes. With the disuse and death of 
the sieve-tubes the callus and nodules are dissolved away and 
open holes are left between the empty cavities of adjoining tubes. 
Summary. 
In conclusion it will perhaps be useful to give a brief 
summary of the results which have been arrived at during 
this research. 
In the earlier part of the paper a resume and criticism of 
the work of former investigators is given, and it is shown that 
the results obtained by Russow harmonize in the main with 
those which are brought forward in this paper, for he found 
that the mature sieve-plate is traversed by callus-rods which 
enclose strings of slime. 
Much of the confusion which has existed up till now about 
