6oi 
Sieve-Tubes of Pinus.. 
threads of the sieve-plate ; for ferments travel along the 
threads from the sieve-tube, converting them into slime- 
strings, and, at the same time, acting on the wall in their 
immediate vicinity form both the callus-rods and also the 
nodules at the middle lamella. The heads of the callus- 
rods swell with great ease or else are added to by deposi- 
tion from the protoplasm, for it is a common thing to find 
quite large cushions of callus covering the sieves communica- 
ting with the albuminous cells, in places where no callus- 
cushions are developed over the sieve-plates (Fig. 23, 
PL XXXIII). 
The difference between the character of the ‘ threads ’ on 
the two sides of the lamella is clearly shown by their staining 
reactions, for the short protoplasmic threads appear thick 
and stain deeply with safranin, whilst the longer slime-strings 
into which the sieve-tube portions of the threads have been 
converted appear thinner and stain only faintly with this dye 
(Fig. 12, PI. XXXI ; Fig. 12, PI. XXXII). The appearances 
produced are exactly like those found in the albuminous cells 
of the leaf, where a precisely similar orientation of the thread- 
groups is found to occur 1 . 
A striking difference, however, occurs between the albu- 
minous cell- sieves in the stem, and those in the leaves ; for 
in the stem the slime-strings of the broader portion of the 
membrane traverse callus-rods, and are in every way similar 
to the threads of the sieve-plate ; but in the leaf, although the 
longer threads appear to be of the nature of slime-strings, 
there is no callus-development associated with them 2 . 
1 Hill, loc. cit., Figs. 8 and 36. Fig. 36 is not a good reproduction of the 
original figure. 
Excellent examples of these unequally divided pit-closing membranes, showing 
the two kinds of thread, are found in Dammara australis ; they appear to agree in 
all respects with those described for Pinus . Cf. Strasburger, ‘ Leitungsbahnen,’ 
Fig- 35 , &c. 
2 Numerous experiments were tried to see if the pit-closing membranes between 
albuminous cells and sieve-tubes in the leaf gave any indication of callus. Leaves 
of all ages were examined (one, two, and three years old), and the following stains 
were used : Congo red, the albuminous cell-walls stained, but the thick pil-closing 
membrane stained less deeply than the general wall ; callus reagent, pit-closing 
membrane pale-violet j corallin soda and aniline blue, membrane unstained. 
