272 Hill . — The Histology of the Sieve-Tubes of Angiosperuis. 
through the sieve-plate in the position of the original pit, and the group of 
fine threads is supplanted by the single large slime-string of the active sieve- 
tube (Text-fig. 8). The characteristic appearance of a sieve, with its more 
or less polygonal holes of varying size, is thus produced. Each hole has 
a thin lining of callus, and in section it is seen that the whole of the free 
surface of the sieve-plate mesh is coated with a thin film of callus 
(Figs. 8 and 9, PL XVII). 1 
By means of the short, thick slime-strings, with their accompanying 
protoplasm, the sieve-tubes are placed in direct continuity throughout the 
length of the plant, and may now be considered to be in the active con- 
dition (Text-fig. 8). Owing to the rapidity with which the developmental 
changes proceed, and to our ignorance of the periodicity of any given plant 
with regard to such changes, the earliest stages of the sieve-plate are un- 
fortunately very rarely met with. 
Throughout the summer the holes in the sieve-plate remain fully open, 
but towards autumn, and occasionally at an earlier period of the year, they 
become constricted owing to the enlarge- 
ment of the callus. This may be due 
partly to the swelling of the callus lining of 
the tubes, but mainly it is caused by the 
deposition of fresh callus by the protoplasm 
on the surface of the plate and of its pores. 
The slime-strings are thus narrowed and 
lengthened, with the result that the trans- 
location current through the sieve-plate is 
able accumulation of callus has been formed 
on either side of the sieve-plate. 2 After a 
time the slime-strings become disorganized, owing to the accumulation of 
callus, and the communication between the sieve-tubes ceases, but the much 
attenuated pores or canals may still be seen crossing the callus-pads, often 
enclosing disintegrated portions of the slime-strings, whilst at the middle 
lamella the original cellulose membrane is clearly visible (Fig. 11, PL XVII). 
In Wistaria the sieve-tubes function for one year only, so that the 
further history of the sieve-plate simply concerns its degradation. The 
gradually inhibited (Pig. 10, PL XVII). 
This process may continue until a consider- 
Fig. 9. An old sieve-plate of an 
empty sieve-tube, in surface view, to 
compare with Fig. 2. The protoplasm, 
slime, and callus have vanished, and 
a cellulose mesh-work (cl.) with empty 
holes ( m .) is left, 
1 Contrast Strasburger, 1. c., PI. XIV, Figs. 33 and 34. According to Strasburger the boring 
of the sieve-areas commences by a small hole after the fusion together of the callus-rods which have 
arisen by the transformation of the protoplasmic threads. 
The view here put forward, however, entirely differs from this, since there seems good evidence 
to prove that the largest possible hole is formed in each area immediately on the fusion or boring 
out of the group of small slime-strings, which are the transformed protoplasmic threads. 
2 As has been noticed by Lecomte, 1. c., p. 265, the callus-cushion may be unequally developed 
on the two sides of a sieve-plate.. 
