580 HilL — The Histology of the 
the slime-threads which perforate the callus-rods can be seen 
extremely clearly, after solution of the callus, to be the con- 
necting threads ; and the granules also occurring at the middle 
lamella are found to be nothing more than thickenings or 
swellings of the connecting threads 1 (Figs. 4 and 5, PI. XXXI). 
He also gives reasons why he considers the connecting 
threads of the sieve-tubes are not formed of protoplasm, 
because the protoplasmic threads appear finely granular in 
the parenchymatous cell-walls, whereas the connecting strings 
of the sieve-plate are throughout smooth and homogeneous. 
A comparison of Russow’s two sets of observations shows 
that what he formerly considered to be callus-rodlets ( Callus - 
Stifle ) must now be regarded as slime- threads or strings 
(, Schleimfaden), since with the latter method the callus had 
been dissolved away by the sulphuric acid (Figs. 4 and 5, PI. 
XXXII). He has also changed his views about the nature of 
the median granules, which he considered in the earlier paper 
to be portions of the undissolved pit-closing membrane, but 
now to be nothing more than swellings on the connecting 
threads. Thus, according to Russow, the sieve-plate is 
traversed by callus-rods and each rod encloses a group of 
slime-strings, which bear swellings or nodes at the middle 
lamella (Fig. 4, PI. XXXII). 
The various accounts of sieve-tubes given by Strasburger 
must now be considered. In the same year as Russow’s first 
paper, he attempted to solve the problem of the development 
of the sieve-plate 2 * , and a short abstract of his results now 
follows : — 4 In the pit-closing membranes of primordial pits, 
glistening points, which are capable of swelling, are visible ; 
the membrane between these spots then begins to thicken, 
thereby forming fine pores, which lead up to the swollen 
points and become filled with granular contents staining yellow 
in chlor. zinc, iodine solution ; at the entrance to the pores 
this substance, which is highly refractive, forms a knob-like 
1 Russow compares the median nodes on sieve-tube-threads with what he con- 
siders similar swellings on the threads in bast-parenchyma-cell walls of Fraxinus . 
a Strasburger, ‘ Bau und Wachstum,’ p. 57 et seq., Figs. 33-43, Taf. iv. 
