578 Hill. — » The Histology of the 
rodlets [Callus- Stifle), which are in reality the slime-strings 
(Figs. 3 and 4, PI. XXXI, s.s). 
The rods with swollen heads (the true callus-rods of the 
sieve-plate (Fig. 3, PL XXXI, gs.) are usually grouped to- 
gether in twos and threes, and the portions of pit-closing 
membrane both between these groups and between the in- 
dividual rods are coloured blue-violet by the callus reagent 
(Fig. 3, PL XXXI, w.). 
The median nodules, which are seen to be equal to the rods 
in diameter, are not always sharply visible, but are usually 
stained yellow or brown. After the action of weak alcoholic 
potash the nodules may disappear or remain as irregular granules ; 
the two halves of the callus-rods become separated owing to 
the swelling, and in some rare cases a thin thread was seen to 
unite the two separated callus-corks (Fig. 6, PL XXXI). 
The nodules are still visible in sieves with a thick callus, 
but all traces of them disappear in sieves deprived of their 
callus. Russow asks, what do these nodules represent ? but 
cannot give a satisfactory explanation ; if they are portions of 
the middle lamella between the pit canals — 'the callus-corks, — 
then there is no communication, and yet a shutting of the 
sieves seems hardly likely. Janczewski also thought that 
they represented the middle lamella. 
In the older callus-cushions Russow mentions the appear- 
ance of darker striae which diverge from the sieve-pores, 
which he regards as continuations of the substance of the 
rodlets of slime-strings ( Stiftsubstanz ) situated in the pores 
(Fig. 7, PL XXXI, s.m.). 
He then mentions some interesting facts regarding the 
distribution of the callus-cushions, how that on the oblique 
terminal walls they are often one-sided and larger than else- 
where, whilst on the long walls each side of the plate may 
have a large cushion. With regard to sieves between medul- 
lary rays and sieve-tubes, he mentions their rarity, and that 
they are never perforated ; callus-rods, however, are seen reach- 
ing to the lamella, but median nodules are said to be absent. 
In a radial section of a sieve-plate, he sees the sieve-fields 
