590 
Hill . — The Histology of the 
is seen to travel as before from the surface to the lamella. 
As a result of this the portions of the threads on either side 
of the lamella are again similar to each other, but they now 
stain more deeply, and present a more glistening appearance 
than the protoplasmic threads in the embryonic sieve-plate, 
so that they appear rather to be of the nature of strings of 
mucus or slime (Fig. n, PI. XXXII). 
The Formation of the Callus-Rods, &c. 
If sieve-plates in this boundary region are examined in 
sections which have been stained with water blue after the 
previous safranin staining, a change is also seen to be taking 
place in the nature of the pit-closing membrane immediately 
in the neighbourhood of the thread-groups, for the ingrowths 
of the darkly staining portions of the threads are seen to be 
situated in a small patch or area of callus, which is stained 
a sky-blue colour by this reagent (Fig. 17, PI. XXXIII). This 
change of portions of the cellulose membrane into callus, 
which proceeds simultaneously with the alteration of the 
threads just mentioned, appears therefore to be due to the 
same cause as that which modifies their composition, and 
when these changes have, reached to the middle lamella the 
final result is that the cylindrical rods of callus enclosing the 
groups of slime-strings are formed which mark the active 
condition of the sieve-plate (Figs. 18 and 20, PI. XXXIII ; 
cf. Fig. ii, PI. XXXI). 
Accompanying the change of the protoplasmic threads into 
slime-strings, and the formation of the callus-rods from the 
cellulose layers of the pit-closing membrane, a change occurs 
at the pectic middle lamella, which is marked by the appear- 
ance of the median nodule. The structure of this nodule, 
which has always been an object of much difficulty, was by 
no means easy to make out, for the callus-rod appears to be 
divided by it into two halves, and by ordinary methods it 
cannot be seen that it is traversed by the slime-strings. If, 
however, the region of the middle lamella is examined after 
various treatments, so that in some cases the highly refractive 
