585 
Sieve-Tubes of Times. 
plasm, arises in the secondary pits on each side of the pit- 
closing membrane and grows towards the middle lamella, after 
which the heads of the rods enlarge to form the callus-masses. 
Russow thus differs from Wilhelm and Janczewski in his view 
of the formation of the callus, since they considered it was due 
to a change of the cellulose. According to Strasburger (1884) 
the fine strings enclosed in the callus-rods are ‘ plasma-filled 
canals,’ and the median nodules correspond to swollen places 
of the middle lamella which remain after the solution of the 
callus ; otherwise his account, which does not include develop- 
mental questions, corresponds with that of Russow. 
The views recently put forward by Strasburger differ con- 
siderably from the foregoing. Protoplasmic threads are seen 
to cross the young pit-closing membrane, and then at a later 
stage they are transformed bodily into callus-rods ; and the 
highly refractive, median nodules are found to be portions of 
the middle-lamella membrane between the rods of callus 
which persist in the old and empty sieve-plates. The callus- 
cushions are formed by the fusion of the heads of the rods, 
and are added to by the activity of the protoplasm. 
Thus, according to Strasburger x , it is the callus which is 
all-important for intercommunication between adjoining sieve- 
tubes, whereas Russow considers the slime-strings to be the 
necessary elements of the sieve- plate. 
The existence of these different views must, I think, be 
attributed to the employment of Russow’s callus reagent and 
water-blue, which stain both callus and protoplasm; and also 
to the use of alcohol material. It is impossible to study the 
development of the sieve-plate with these reagents, and it is 
only after the callus has been formed that any definite results 
can be obtained by their aid. 
The Present Research. 
The present research was accordingly undertaken, firstly, 
with the view of finding out which of the views already 
mentioned agreed with the observed facts ; and secondly, 
1 Strasburger, 1891 and 1901. 
