Sieve-Tubes of Finns. 591 
nodule is disorganized, it can be seen that the slime-strings 
are not interrupted, but are continuous across the middle 
lamella, where they each possess a median node or swelling 1 , 
(Figs. 4 and 5, PI. XXXII) which is the somewhat enlarged 
node of the original protoplasmic thread 2 . At this stage the 
sieve-plates are completely developed and show the mature 
structure already described, which is characteristic of all the 
sieve-tubes in the active region of the phloem. 
The active sieve-tube region forms a broad zone of tissue 
some twelve to fourteen cells across, bounded internally by 
the ‘ boundary-cells * and externally by older sieve-tubes 
where the development of the callus-cushions is taking place 
(Fig. i, PI. XXXII). The relation between the slime-strings 
and the callus is not always easy to follow in the older sieves 
owing to the ease with which the callus swells with some 
reagents ; for the orange G, for instance, which is used to 
wash out the safranin often causes the callus to swell so 
strongly that an erroneous impression of the structure of the 
sieve-plate is liable to be given, unless a careful comparison 
of the double-stained sections is made with other sections 
stained by water blue alone. 
Both Russow 3 (cf. Fig. 7, PI. XXXI) and Strasburger 
mention the appearance of striae or threads in the large 
callus-cushions, and they are figured by Strasburger 4 , but 
I have found them very difficult to observe by their methods. 
As the callus-cushion increases in size owing to the fusion 
of the heads of the callus-rods, the slime-strings contained in 
the rods are lengthened to keep pace with the callus-growth, 
so that an appearance of dark-red radiating lines traversing 
a blue callus-cushion is obtained on staining with safranin 
and water blue. With further increase in size of the callus 
the threads become broken up into granules and' then can 
no longer be of any value for intercommunication (Fig. 14, 
1 It is clear from working through Russow’s methods, and from his accounts, that 
he saw both the median nodule and the nodes of the slime-strings, but he appar- 
ently did not attempt to harmonize the two effects. 
2 Cf. p. 588. 3 Russow, loc. cit., 1882, p. 281. 
4 Strasburger, ‘ Bot. Tract.,’ 1897, Fig. T03 B. 
