2 So Hill . — The Histology of the Sieve-Ttcbes of Angiosp'erms. 
protoplasmic thread in the wall of the companion cell (cf. Fig. 29, PL XVII, 
and Fig. 60, PI. XVIII). Some uncertainty also exists as to the presence 
or absence of a definite median node. 
The case of the thread groups between sieve-tubes and bast parenchyma 
cells appears to be similar to that of the companion cells just described. 
Here the thread groups show definite callus-cushions in the winter material on 
the sieve-tube side of the wall (Fig. 49, PI. XVIII), and in some cases there 
is clear indication that callus-rods extend through the sieve-tube portion of 
the wall as far as the lamella (Fig. 50, PI. XVIII). No callus is formed in 
the parenchymatous cell, so that here again the threads which appear homo- 
geneous are compounded of two distinct portions — the slime-string in its 
callus-rod in the sieve-tube portion of the wall, and the protoplasmic thread 
in the wall of the parenchymatous cell. 
At first, no doubt, the threads were protoplasmic throughout, as is the 
case with the connexions of the sieve- fields, but owing to ferment action 
the halves of the threads in the sieve-tube portions of the walls undergo 
secondary modifications and are converted into slime-strings enclosed in 
callus-rods. 
Owing to the difficulty experienced in the staining of the slime-strings 
in the sieve-tube side of the wall, it is not easy to determine the exact 
structure of the connexions between the sieve- tubes and these other ele- 
ments of the phloem ; in fact, in most cases the slime-strings appear to have 
been retracted from the sieve- tube portion of the wall in consequence of the 
shrinkage on fixation of the general protoplasm. Empty holes are thus 
left, and it is for this reason, no doubt, that the threads between sieve-tubes 
and companion cells usually appear to be so short and darkly stained, since, 
as a rule, only the portions of the threads in the companion-cell half of the 
wall are visible (Figs. 50-52, PL XVIII). 
The development of callus in connexion with companion-cell threads is 
nearly always very slight and difficult of observation, but conspicuous, though 
small callus-pads are frequently seen over the groups of threads between 
the sieve-tubes and bast parenchyma cells (Figs. 49 and 52, PL XVIII). 
The presence of nodes at the lamella, comparable to those found in the 
sieve-fields, has not been satisfactorily proved, though they appear to be 
present in the walls between sieve-tubes and bast parenchyma cells (Fig. 50, 
PL XVIII). 
General Considerations. 
These curious, unsym metrical thread groups, considered in connexion 
with the stages of development passed through by the threads of the sieve- 
fields, suggest questions of considerable interest, both as to the chemical com- 
position of the cell-walls in which they are found and as to the physiological 
