580 Hume . — The Histology of the Sieve Tubes of Pteridium 
droplets of each end of the slime string. The solution of the callus does 
not always go on quite at the same rate all over the same sieve plate, so that 
various stages of the sinking in of the granules may be observed. Sometimes 
the sinking in has not proceeded quite at the same rate (PL LIV, Fig. 19) on 
both sides of the plate ; and sometimes the granule becomes dislodged by 
cutting after it has sunk in some distance, and the deep pit into which it 
had sunk then becomes visible (PI. LIV, Fig. 1 4) ; the pit thus left empty has 
noticeably the same shape and size as the callus basin from which it was 
derived. Whether the callus always becomes completely dissolved away, 
or whether a thin shell or tubule is sometimes left, is not quite certain. 
Certainly stages were often seen in which no remaining trace of callus could 
be detected, but others were also observed in which a very faint blue tubule 
could be seen before the slime string was stained in safranin. Probably the 
solution is ultimately completed, and as far as could be seen no redeposition 
of callus ever takes place. Portions of the rhizome, which must have been 
many years old, were examined and the sieve tubes were found to be still 
functional ; thick slime strings, such as those described above, generally 
perforated the sieve plates ; in a few places traces of callus were observed 
still, but there is little doubt that these represent paths, which, for some 
reason or another, have not functioned very extensively. The whole aspect 
of the sieve plates suggests most strongly that it is as the result of use that 
much of the foregoing process of evolution is gone through. Thus the most 
rapidly developed slime strings represent the most frequented paths, and in 
support of this view it may be mentioned that in the terminal plates, which 
necessarily see the most service, the development is most rapid. 
In the pits connecting sieve tubes with phloem parenchyma cells 
(PL LIV, Figs. 27 and 38), callus was never observed, 1 even in elements in 
which it was fully developed in the sieve plate. The pits are simple depres- 
sions of considerable depth, usually containing a single connecting thread. 
The wall of the sieve tube being a great deal thicker than that of the adjoin- 
ing parenchyma cell, the middle lamella is asymmetrically placed, lying 
very much nearer to the lumen of the parenchyma cell than to that of 
the sieve tube. The connecting thread can be seen as a granular proto- 
plasmic filament running to the middle lamella, where there is a dark 
dot— the median node. The wall between the lumen of the parenchyma 
cell and the middle lamella is so extremely thin that the detection of 
any pit or thread on that side is very difficult, but in some cases they can be 
seen to exist. It would seem, therefore, as if there is no boring out of the 
thread even on the sieve tube side, such as Hill 2 has described for 
Pinus and the Angiosperms, and his view that the ferment action is initiated 
1 Contrast Hill (’01), PI. XXXIII, Fig. 23, and pp. 600-2, and Hill (’08), PI. XVIII, 
Figs. 50, 51, 52, and pp. 278-80. 
2 Hill (’08), p. 281. 
