Sieve-Tubes of Pinus. 581 
swelling, and shortly afterwards the swollen places of the 
membrane are absorbed and the granular substance stretches 
across the membrane without a break.' 
Strasburger finds these rods react as albumen, but thinks 
they may differ at different seasons of the year ; Russow, 
however, who also describes this appearance of rods, considers 
them to be formed of callus. Strasburger then goes on to 
describe the formation of the callus-masses or cushions which, 
he states, arise as gelatinous-thickening layers of the thickened 
(that is the cellulose) portions of the pit-closing membrane, 
and are then added to by the protoplasm : fine radial lines 
are often seen in these cushions which appear to be fine 
canals. 
A further contribution to the subject is made by Stras- 
burger 1 in 1884 in his ‘Botanisches Practicum,’ where he de- 
scribes the structure of the mature sieve-plates of Pinus , and 
gives figures which have been reproduced in nearly every 
recent textbook. 
He used either Russow’s callus reagent or water-blue, and 
his descriptions agree mainly with Russow’s final results. He 
differs from Russow, however, since he considers the strings 
in the callus-rods to be formed of protoplasm, and the median 
nodules to be the swollen middle lamella of the sieve-plate. 
With regard to the formation of the callus-plates he seems 
to have reverted from his former position 2 to that of Russow 3 , 
for the head-like prominences of the sieve-fields are described 
as fusing together to form a plate 4 . The further development 
of the callus-plate is due to the protoplasm, and striae formed 
by the plasma-strings are often seen traversing the callus- 
cushions. 
In the disused sieve-tubes all the callus is dissolved away, 
but the primary wall of the tube is left separating one sieve- 
1 Strasburger, ‘ Bot. Pract.,’ 1884 (German ed.), pp. 147-151, Figs. 63 and 64. 
Similar figures without the explanatory text are given in the third edition of 
the Practicum, 1897, and in Strasburger’s recent textbook. 
2 ‘ Bau und Wachstum,’ p. 59. 
3 Sitzber. der Dorpater Nat.-Ges., 1882, p. 281. 
4 ‘ Botanisches Practicum/ p. 151. 
Q q 3 
